5958-9468 041007

5958-9468 041007
GC Inlets
An Introduction
Agilent Technologies
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© Agilent Technologies, Inc. 2005
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Manual Part Number
5958-9468
Edition
Second edition, July 2005
First edition, July 1991
Printed in USA
Agilent Technologies, Inc.
2850 Centerville Road
Wilmington, DE 19808-1610 USA
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Introduction to GC Inlets
Acknowledgements
I would like to acknowledge the efforts of several colleagues and cohorts who
went to considerable lengths to beat me into literary fluency. Key among these
is Beverly Bruns, technical editor, who—after developing the outline for this
book and spending an untold number of hours editing, including trying to
limit the size of my sentences (some of which exceeded the bounds of most
paragraphs)—finally succeeded in getting her point across; however, this was
not until Chapter 13 was in the works, at which point my retention of any new
information was relegated to short-term memory (which accounts for this
sentence, I am sure) and which will probably have to be relearned at the time
of my next writing effort, but only after this acknowledgement, which she has
vowed not to edit, has been completed.
My thanks to Cindy Haigh, Fred Reese, Charlotte Mastin, and the rest of the
Publications Department for their contributions to making this a readable
text. Special thanks go to Pat Sandra and Dale Snyder for input on technical
precision accuracy.
Matthew Klee
Introduction to GC Inlets
3
Preface
“If the column is described as the heart of chromatography, then sample
introduction may, with some justification, be referred to as the Achilles heel.”*
This sentiment reflects the reality of gas chromatography (GC): inlets and
other sampling devices are often the quality-limiting components of gas
chromatography.
Over the past few years, the accuracy and precision of sample introduction
have been greatly improved by advances in inlet and sampling technology.
Improvements in the capabilities of inlets, such as temperature programming,
coupled with addition of new techniques, such as cool on-column injection
and headspace sampling, have expanded the range of samples that can be
analyzed by gas chromatography and have improved the quality of results.
Certainly, the developments of GC sample introduction have kept pace with
the developments in capillary columns, detectors, and automation. Every
hurdle that is surmounted in sample introduction points to weaknesses in
column and detector technology; these are improved and point to further
weaknesses in sample introduction, and so on. Each iteration leads to a better
chromatographic system and a better tool for solving analytical problems.
Recent column and liner developments allow the use of packed column inlets
with wide-bore capillary columns, yielding faster, higher resolution analyses.
Advances in cool on-column inlets have helped to solve many of the problems
associated with vaporizing inlets. With cool on-column inlets, sample
degradation and discrimination have been virtually eliminated.
Programmed-temperature inlets permit tailoring of inlet conditions to the
sample and analysis needs.
Auxiliary sample introduction devices (valves, purge and trap samplers,
thermal desorbers, pyrolyzers, and headspace auto samplers) have also
expanded the types of samples and matrices that can be analyzed by GC.
This introduction to GC inlets is intended to present clearly the individual
capabilities, strengths, and weaknesses of the many inlets and auxiliary
sample introduction devices currently available.
Matthew S. Klee, Agilent Technologies, Inc., Wilmington, DE
Pat Sandra, State University of Ghent, Belgium
* V. Pretorius and W. Bertsch, HRC & CC, 6 (1983) 64
4
Introduction to GC Inlets
Contents
1
Introduction
2
Types of Inlets
Overview
16
Packed-Column Inlets
17
Capillary-Column Inlets 17
Capillary direct 17
Split/Splitless 17
Programmed temperature vaporizer (PTV)
Cool on-column 18
Auxiliary Sampling Devices 18
Valves 18
Headspace autosamplers 19
Purge and trap automatic samplers
Thermal desorbers 19
Pyrolyzers 19
3
18
19
Inlet Selection
Overview
22
Inlet Variables
22
Peak Broadening
24
Focusing Techniques 25
Stationary phase focusing
Solvent focusing 25
Thermal focusing 26
Retention Gap
28
Sample Introduction 29
Cool on-column injection
Introduction to GC Inlets
25
29
5
Flash vaporization 29
Fast autoinjection 30
Manual injection 31
Septa 34
Column Selection 35
Solute retention 35
Phase ratio 35
Column efficiency 35
Column capacity 36
Column temperature 37
Inlet Liners
4
37
Packed-Column Inlets
Overview 40
Inlet Design 41
Sample Considerations
Inserts/Liners
Temperature
44
44
44
Flow Rates 45
Troubleshooting 46
Decomposition 46
Flashback 46
Leaks 46
Summary 47
5
Capillary Direct Inlets
Overview
50
Inlet Design 52
Sample Considerations
Liners
6
53
53
Introduction to GC Inlets
Temperature
54
Flow Rates 54
Troubleshooting 55
Flashback 55
Band broadening in time 55
Band broadening in space 55
Summary 56
6
Split Inlets
Overview
60
Inlet Design
61
Sample Considerations
Liners
62
65
Temperature
Flow Rates
67
67
Troubleshooting 68
Needle discrimination 68
Inlet discrimination 68
Sample decomposition 69
Summary 70
7
Splitless Inlets
Overview
74
Inlet Design
74
Sample Considerations
Liners
76
78
Temperature
79
Flow Rates 79
Determining Purge Delay Time
Introduction to GC Inlets
80
7
Troubleshooting
Summary
8
82
84
Cool On-Column Inlets
Overview
88
Inlet Design 89
Secondary cooling
Sample Considerations
Retention Gaps
Temperature
92
94
95
Flow Rates 95
Troubleshooting
Summary 97
9
92
96
Programmed- Temperature Vaporizer (PTV) Inlets
Overview
100
Operating Modes 100
Cold split injection 100
Cold splitless injection 101
Solvent elimination split/splitless injection
Inlet Design
105
Sample Considerations
Liners
102
107
109
Temperature
109
Flow Rates 110
Troubleshooting 112
Summary 113
8
Introduction to GC Inlets
10
Valves
Overview
118
Valve Types 118
Rotary valves 118
Slider valves 118
Valve Design 119
Rotary gas sampling valves 119
Rotary liquid sampling valves 120
Slider valves 120
Valve connection to packed columns 121
Valve connection to capillary split inlets 123
Sample Considerations
123
Selecting Sample Loop Volume
Temperature 124
Gas valves 125
Liquid sampling valves
Transfer lines 125
Columns 125
124
125
Flow Rates 125
Troubleshooting 127
Peak broadening and tailing 127
Baseline shifts 127
Variation in peak area and retention time
Summary 129
11
127
Headspace Autosamplers
Overview
Design
132
133
Sample Considerations
Introduction to GC Inlets
137
9
Temperature
137
Flow Rates 138
Troubleshooting 140
Summary 141
12
Thermal Desorbers
Overview
Design
144
144
Sample Considerations 146
Air sampling 146
Volatiles in solids 147
Desorption temperature 147
Flow rates 147
Troubleshooting
Summary 149
13
148
Purge and Trap Samplers
Overview
Design
152
153
Sample Considerations
Temperature
154
154
Flow Rates 155
Troubleshooting 156
Summary 158
14
Analytical Pyrolyzers
Overview
162
Design 164
Resistively heated pyrolyzers
Curie-Point pyrolyzers 166
10
164
Introduction to GC Inlets
Microfurnace pyrolyzers
Sample Considerations
Temperature
167
168
169
Flow Rates 169
Troubleshooting 170
Summary 171
A
Gas Volumes of Solvents
B
Head Pressures for Capillary Columns
C
Determining Split Ratio
Calculating Split Ratio
177
Calculating Sample Reaching the Column
Introduction to GC Inlets
179
11
12
Introduction to GC Inlets
GC Inlets An Introduction
1
Introduction
Analytical gas chromatography methods involve a series of steps:
• Sample collection (sampling, transport, storage)
• Sample preparation (grinding, extraction, dissolution, derivatization)
• Sample introduction into the chromatographic system
• Chromatographic separation into individual components
• Detection of the components
• Data acquisition and reduction (integration, reporting)
For each step, the analyst must make appropriate choices among accepted
procedures and available instrumentation.Improper selection of the gas
chromatograph sample introduction system can dramatically limit
performance of the chromatography system and, therefore, the ultimate
performance of the analytical method.
Because of the great variety of columns and the diversity of samples that can
be analyzed with modern gas chromatography, several injection modes are
used; no single inlet can satisfy all analytical requirements.
This text provides a simple, concise introduction to the selection and use of
gas chromatography sample introduction devices.
Agilent Technologies
13
1
14
Introduction
Introduction to GC Inlets
GC Inlets An Introduction
2
Types of Inlets
Overview 16
Packed-Column Inlets 17
Capillary-Column Inlets 17
Capillary direct 17
Split/Splitless 17
Programmed temperature vaporizer (PTV) 18
Cool on-column 18
Auxiliary Sampling Devices 18
Valves 18
Headspace autosamplers 19
Purge and trap automatic samplers 19
Thermal desorbers 19
Pyrolyzers 19
Agilent Technologies
15
2
Types of Inlets
Overview
The main function of gas chromatograph (GC) inlets is to provide accurate,
reproducible, and predictable introduction of sample into the column. Usually,
the sample is a liquid that is injected into the inlet using a syringe, but samples
can also be introduced to the analytical column by auxiliary devices, such as
headspace automatic samplers and valves.
16
Introduction to GC Inlets
Types of Inlets
2
Packed-Column Inlets
Inlets are usually divided into two major categories—packed-column inlets and
capillary-column inlets. The most widely used type of GC inlet is one used
with packed columns. The packed-column inlet is simple (all flow goes to the
column), can protect the column from nonvolatile sample components, and
works with metal or glass columns. Packed-column inlets are most frequently
used for general analyses.
Capillary-Column Inlets
Types of capillary-column inlets include:
• Capillary Direct (vaporizing)
• Split/Splitless (vaporizing)
• Programmed Temperature Vaporizer (vaporizing)
• Cool On-Column (nonvaporizing)
Capillary direct
Packed-column inlets can be adapted to work with some capillary columns for
direct injections. Capillary direct inlets are used with wide-bore capillary
columns (id ≥0.5 mm) and are made by substituting a special insert inside a
packed-column inlet. They have essentially the same benefits and pitfalls as
packed-column direct inlets. Capillary direct inlets are vaporizing inlets and
are usually used as a transition between packed column and high-efficiency
capillary column analysis.
Split/Splitless
The first type of inlet designed for capillary analysis was the split inlet, which
is most commonly available now as a combination inlet for both split and
splitless injections. This is a vaporizing inlet which vents most of the sample in
the split mode and transfers most of it to the column in the splitless mode.
Because it is a vaporizing inlet, it is column protecting but can cause solute
discrimination and decomposition. Split injection is used for general analysis,
whereas, splitless injection is most frequently used for trace analysis.
Introduction to GC Inlets
17
2
Types of Inlets
Programmed temperature vaporizer (PTV)
The programmed temperature vaporizer (PTV) inlet offers a mixture of
injection possibilities, including cool sample introduction, split or splitless
modes, sample concentration (solvent elimination mode); and it is column
protecting. Due to this flexibility, PTV inlets are good inlets for both general
analysis and trace analysis.
Cool on-column
Cool on-column inlets give high accuracy and reproducibility, are sample
protecting, have the least solute discrimination among all the inlets, and work
by depositing the sample directly into the column. Unlike the other vaporizing
or “hot” sample introduction techniques, the sample is not exposed to high
temperatures during injection or transfer to the column. Cool on-column inlets
are used for the analysis of samples with a wide boiling-point range or those
that are thermally sensitive, and for trace analysis.
Auxiliary Sampling Devices
In addition to the inlets just described, there are also sample introduction
devices that introduce samples into the chromatographic column when
syringe injection is inappropriate (for example, with solid samples):
• Gas and liquid sampling valves
• Headspace autosamplers
• Thermal desorbers
• Purge and trap samplers
• Pyrolyzers
Depending on the sampling device and the GC column used, auxiliary
sampling devices can be connected directly to the column or to an existing
inlet.
Valves
Valves give very reproducible introduction of fixed volumes of gas or liquid
samples and are simple to automate. Valves are frequently used for sampling
gases and liquids in moving streams (process and online monitoring).
18
Introduction to GC Inlets
2
Types of Inlets
Headspace autosamplers
Headspace autosamplers are used to determine volatiles in liquids, solids, or
complex matrixes. Headspace autosamplers inject a portion of the gas that is
in equilibrium with a sample in a thermostatted, sealed vial. Headspace
analysis is used for the analysis of residual solvents, fragrances, and volatile
pollutants in soil and water.
Purge and trap automatic samplers
Purge and trap is a combination of dynamic headspace, trapping, and then
thermal desorption. Volatile components are continuously purged out of a
water sample (dynamic headspace), trapped on an adsorbent, and then
desorbed quickly for introduction into the GC (thermal desorption). This is
used mainly for analysis of environmental pollutants in water and for analysis
of volatiles in beverages.
Thermal desorbers
Thermal desorbers are used in environmental sampling, and are
complementary to headspace analysis and analytical pyrolysis. Volatile sample
components, which are contained in a solid sample or which have been
adsorbed onto a solid adsorbent, are thermally liberated in the sampler in a
stream of carrier gas and carried to the column. This technique is used for
monitoring hazardous gases in the workplace and for environmental air
analysis.
Pyrolyzers
Pyrolyzers are used to thermally cleave nonvolatile samples into volatile
fragments that can then be analyzed by gas chromatography. Temperatures in
the 500 to 1000 °C range are normally used for the analysis of polymers, fibers,
microorganisms, and geological samples.
Introduction to GC Inlets
19
2
20
Types of Inlets
Introduction to GC Inlets
GC Inlets An Introduction
3
Inlet Selection
Overview 22
Inlet Variables 22
Peak Broadening 24
Focusing Techniques 25
Stationary phase focusing 25
Solvent focusing 25
Thermal focusing 26
Retention Gap 28
Sample Introduction 29
Cool on-column injection 29
Flash vaporization 29
Fast autoinjection 30
Manual injection 31
Septa 34
Column Selection 35
Solute retention 35
Phase ratio 35
Column efficiency 35
Column capacity 36
Column temperature 37
Inlet Liners 37
Agilent Technologies
21
3
Inlet Selection
Overview
Inlets must be selected carefully for each analysis and used to optimal benefit
to maximize chromatographic efficiency, analytical accuracy, and the
reproducibility of results. Because capillary columns have higher efficiency
and lower sample capacity than packed columns, inlet selection and inlet
performance are much more vital to obtaining accurate results than they are
with packed column systems.
Once an inlet has been selected based on sample, column type, and analysis
goals, all inlet variables must be set appropriately to achieve optimal results. It
is possible to come close to the optimum mix of inlet conditions before
injecting the first sample.
Inlet Variables
The type of analysis to be done and the composition of the sample itself are the
primary factors that must be considered when setting or determining the
following interrelated variables:
• Injection technique
• Injection volume
• Inlet temperature
• Column selection
• Column temperature
• Liner selection
One example of a variable affecting vaporizing inlets is inlet temperature. One
of the pitfalls of vaporizing inlets is that they can cause sample degradation.
Labile sample components can degrade when exposed to heat or catalytic
surfaces. Figure 1 shows chromatograms for on-column (reference) and
splitless injections at several inlet temperatures. Higher splitless temperatures
result in a higher percentage of degradation and more peaks.
Low inlet temperatures or cool on-column injection, deactivation of the inlet
and/or liner, and high split-vent flows are all techniques used to reduce the
potential for sample degradation. See Figure 1.
22
Introduction to GC Inlets
Inlet Selection
On-column injection
3
Column: 50 m × 0.31 mm
SP-2100
6 °C/min
290 °C
120 °C
µ (H2) = 55 cm/s
1.5 µL On-Column
6 °C/min
Splitless injection
Column: 50 m × 0.2 mm
SE-54
280 °C
120 °C (2 min)
µ (H2) = 41cm/s
Injection port 200 °C
Injection port 300 °C
Injection port 400 °C
Figure 1
Introduction to GC Inlets
Chromatograms of styrene impurities obtained using on-column and splitless
sampling, showing increasing numbers of artifact peaks with increasing inlet
temperature. (The splitless chromatograms were kindly supplied by
Mr. Roger Miller, Huntsman Chemical Corporation.)
23
3
Inlet Selection
Peak Broadening
A basic functional requirement of GC inlets is that they introduce the sample
into the column as a narrow band having a composition that is identical to the
original sample. The inlet should not produce peaks that are wider than the
peak width that will result from the column band broadening process. Peak
broadening in the column is a function of column efficiency. More efficient
columns require narrower initial peak widths. This can be accomplished by
using inlets that generate narrow peaks initially, or by using subsequent
focusing techniques.
Initial bandwidths are broadened by two mechanisms:
• Band (peak) broadening in time
• Band (peak) broadening in space
Band broadening in time and space are characteristics of particular inlet types
and injection techniques. Band broadening in time is caused by the slow
transfer of sample vapor from the inlet to the column. The initial peak width is
equal to the time it takes for the sample to be transferred to the column.
Band broadening in space is a direct consequence of migration and spreading
of liquid sample within the column, either after cool on-column injection of
sample into the column or after recondensation of sample in the column (for
example, after vaporizing injection such as splitless injection). The condensed
liquid, which starts by occupying only a few centimeters of column, becomes
too thick to be stable and spreads over a longer length. The carrier gas pushes
the plug farther into the column, creating a “flooded zone.” The solute
material is spread over the full length of the flooded zone, creating an initial
peak width that equals the length of the flooded zone.
When recondensed sample is not compatible with the stationary phase, it
beads up in the column (like water on a newly waxed car). The beads then
separate and spread out over a longer flooded zone, concentrating solutes
unevenly. This increases the length of the flooded zone, creates split peaks,
and increases peak widths.
Three focusing techniques are used to narrow peaks broadened by time and
space:
• Stationary phase focusing
• Solvent focusing
• Thermal focusing
24
Introduction to GC Inlets
Inlet Selection
3
Focusing Techniques
Stationary phase focusing
Stationary phase focusing is the most frequently used focusing technique and
is possible only in temperature-programmedanalysis. In gas chromatography,
retention of solutes is an exponential function of temperature; so, as the initial
temperature of the column is lowered, the speed at which solutes travel down
the column slows dramatically. As vaporized sample moves from the inlet to
the column, it comes in contact with the stationary phase and is trapped in a
narrow zone. The lower the temperature, the more effective the focusing.
Solvent focusing
As condensed solvent starts to evaporate, solutes with volatility similar to that
of the solvent tend to concentrate and focus on the solvent tail. This solvent
focusing, or the “solvent effect,” yields very narrow peaks for these early
eluting compounds and is shown in Figure 2.
High-volatility solute
Low-volatility solute
Capillary column
Needle
Carrier gas
Injection
Flooded zone develops
High-volatility solute focused by solvent
Figure 2
Introduction to GC Inlets
The solvent effect
25
3
Inlet Selection
Figure 3 compares splitless injections for a sample dissolved in two solvents. No
solvent focusing occurs in the first case, because the starting oven temperature
is above the boiling point of the solvent hexane. Solvent focusing does occur in
the second, when octane is used, because the boiling point of octane is above the
initial column temperature. Early peaks on the solvent tail are clearly evident,
and the peaks for C11and C12 are considerably sharper.
n-octane
bp 125 °C
n-hexane
bp 68 °C
n-C11
OV-101 Glass WCOT column
Isothermal 15 °C
2 µL injected
100 ng C11 and C12
Helium carrier 0.9 atm
(0.8 mL/min
n-C12
n-C11 n-C12
0
10
20
0
10
20
Time, min
Figure 3
No solvent on the left due to inappropriate selection of solvent versus starting
temperature. Right combination is okay and shows sharpened peaks on the
solvent tail and for C11 and C12.
Thermal focusing
Thermal focusing is the thermal condensation of gases in a tube or at the head
of the column. Peaks narrow as solute volume is reduced during condensation.
Solutes will not migrate into the chromatographic system until the
temperature is raised and they are vaporized again. Sometimes cryogenic
temperatures (cryogenic focusing) are used to focus peaks from inlets or
auxiliary sampling devices that generate peaks broadened in time.
Thermal focusing narrows bandwidths effectively only when the column
temperature is approximately 150 °C below the boiling points of the solutes. In
this sense, thermal focusing does not rely on chromatographic processes. It only
requires a surface on which vapors can condense. Thermal focusing in
chromatographic columns is often accompanied by stationary phase focusing.
26
Introduction to GC Inlets
Inlet Selection
3
An example of thermal and stationary phase focusing is shown in Figure 4. The
top chromatogram was generated by split injection which, initially, generates
very narrow peaks. The bottom chromatogram was generated by splitless
injection of the same sample and shows broad initial peaks. Most of the peaks
eluting after 100 °C, however, were focused and yielded as peaks just about as
narrow as is possible with split injection.
Column: 13 m × 0.30 mm × 0.5 µm SE-52
Temperature: 50 °C↑ 5 °C/min↑ 180 °C
A. Split injection
175°C
150°C
125°C
100°C
75°C
50°C
B. Splitless injection (pentane solvent , no solvent recondensation)
Figure 4
Introduction to GC Inlets
Thermal and stationary-phase focusing during a temperature-programmed
run. Split yields narrow peaks from injection; splitless has focused peaks after
~100 °C. (Reproduced from K. Grob, Jr., Classical Split and Splitless Injection
in Capillary GC, Huthig, Heidelburg (1986), with permission.)
27
3
Inlet Selection
Retention Gap
A retention gap is an empty piece of column that accommodates the
condensed sample but does not retain solvent or solutes once they have been
vaporized. The primary function of retention gaps is to reduce the length of
the flooded zone created whenever solvent is condensed in the column. An
equally important function is to protect the column from nonvolatile sample
components, especially when doing cool on-column injection.
When peak broadening and splitting phenomena are observed, a retention gap
is usually required. As the solvent evaporates, all solutes move freely at carrier
gas velocity to the head of the analytical column where they are focused by the
solvent effect and/or the stationary phase (Figure 5). In general, peaks with k´ <5 are
focused by the solvent effect, while solutes with k´ >5 are focused by the stationary
phase.
The retention gap must be deactivated properly to minimize the length of the
initial flooded zone and the possibility of peak tailing or degradation.
Nonpolar solvents (for example, hexane, isooctane) require nonpolar
deactivated retention gaps. Polar solvents (for example, methanol, water)
require polar deactivated retention gaps.
Solvent evaporates
Volatiles focused by solvent
High-boilers focused by stationary phase
Stationary phase
Figure 5
28
Visual representation of on-column injection into a retention gap. Both solvent
and stationary phase focusing occur.
Introduction to GC Inlets
3
Inlet Selection
Sample Introduction
Inlets introduce liquid samples to the column in one of two ways—either by
vaporization in the inlet or by cool on-column injection. All capillary inlets are
vaporizing, including on-column direct injection, except for cool on-column
injection which deposits condensed sample directly into the column.
Cool on-column injection
Cool on-column injection is a technique in which the liquid sample is
introduced directly into the column. The inlet is constantly cooled by cold air
and/or circulated water. Cool on-column injection systems include
programmed heating that permits complete independence of the inlet
temperature and the column oven temperature.
Cool on-column injection has extended capillary GC use to many applications
that were initially not practical. Advantages of cool on-column injection
include:
• Elimination of needle discrimination
• Reduction of sample decomposition
• High analytical precision
Problems associated with cool on-column injection include band broadening
in space, column overload, and column contamination.
Flash vaporization
The classic means of introducing a liquid sample into a chromatographic
system is to inject it by syringe into a hot inlet where it is quickly vaporized
(flash vaporization). Benefits of flash vaporization include: transfer of the
liquid sample to a gas so separation can proceed, quick transfer of sample into
the column, and protection of the column from nonvolatile sample
components which stay behind in the inlet.
Problems associated with vaporizing inlets include band broadening in time
and space (splitless injection), needle discrimination, inlet discrimination, and
sample decomposition. Soon after a syringe needle passes through the septum
of a hot inlet, the needle heats up to the same temperature as the inlet. The
sample immediately starts to evaporate inside the needle, selectively
Introduction to GC Inlets
29
3
Inlet Selection
concentrating higher boiling solutes and initiating decomposition reactions.
These problems are exacerbated by high inlet temperatures and low-boiling
solvents.
Fast autoinjection
Fast autoinjectors reduce the dwell time of the syringe needle in the inlet,
thereby preventing significant needle heating and associated needle
discrimination problems. Figure 6 shows the relationship of discrimination
and needle dwell time. Fast autoinjection with dwell times under 500 ms is
clearly more accurate than the slower techniques. What could be considered a
“fast” manual injection takes 1 to 2 seconds, which still is not fast enough to
prevent needle discrimination.
1.3
Column: 10 m × 0.53 mm, HP-1
Temperature: 60 °C (2 min)↑ 15 °C/min↑ 310 °C (5 min)
Packed port injection at 350 °C, HP5880A GC
7673A Automatic Injector
Sample: 1 µL, C10 to C40 in n-hexane
1.2
Area ratio Cx/C20
1.1
250 ms
500 ms
1.0
1000 ms
0.9
2000 ms
0.8
0.7
10
14 16
20
25
30 32 34 36
40
C-number of n-alkanes
Figure 6
30
Effect of needle dwell time on needle discrimination
Introduction to GC Inlets
Inlet Selection
3
Other benefits of fast autoinjection include increased precision and accuracy.
Problems associated with fast autoinjection include higher pressure pulse
accompanying injection and an increased risk of flashback.
Manual injection
The most reproducible manual injection technique is the “hot needle”
technique, but it still has significant discrimination compared with fast
autoinjection. Figure 7 shows discrimination with the manual hot-needle
technique in comparison to other injection techniques.
Peak area normalized
to C9 (= 100)
Injection 1 µL corresponding to the needle
volume, 1: 15 split injection at 350 °C; peak
areas normalized to the nonane peak.
For comparison: on-column injection
without significant discrimination
On-column
100
Hot needle
Solvent flush
Cold needle
75
50
25
Filled needle
9
12
16
20
26
32
36
40
44
C-number of n-alkanes
Figure 7
Discrimination of alkanes with different injection techniques. (Reprinted from
K. Grob, Jr. and H. P. Neukom, J HRC&CC, 2 (1979) 15–21, with permission.)
With manual hot-needle injection, the sample is taken into the syringe barrel
without leaving an air plug between sample and plunger. The sample is
withdrawn so that there is air in the needle (“filled needle” includes sample in
the needle). After insertion into the injection zone, the needle is allowed to
heat up for 3 to 5 s. This is sufficient for the needle to reach the inlet
temperature. The sample is then injected rapidly by pushing down the plunger
and the needle is withdrawn quickly from the inlet within 1 s.
Introduction to GC Inlets
31
3
Inlet Selection
In solvent flush injection, a small plug of solvent separates the sample from
the syringe plunger and an air plug separates the sample and solvent plugs (air
in the needle, sample, air, solvent plunger). Upon injection, solvent flushes
through the needle after the sample.
Solvent flush injections may provide an advantage for analyzing very viscous
samples when a second type of solvent is needed during injection or when
total removal of the sample from the syringe is important.
In cold-needle injection, the plunger is depressed as soon as the syringe needle
enters the inlet.
Figure 8 shows chromatograms for manual hot-needle and filled-needle
injections in comparison to cool on-column injection which does not
discriminate. Hot-needle injection clearly discriminates less than the
filled-needle technique.
32
Introduction to GC Inlets
Inlet Selection
3
Filled needle
Hot needle
Cold on-column injection
Needle discrimination for filled needle and hot needle split injections
compared to cool on-column.
Sample: n-alkane in n-hexane, split ratio 1:40
Figure 8
Introduction to GC Inlets
Discrimination of n-alkanes depending on the injection technique.
(Reproduced from K. Grob and G.Grob, J HRC&CC, 2 (1979) 109–117, with
permission.)
33
3
Inlet Selection
Septa
Another key component of sample introduction is the inlet septum. All
columns must have carrier gas head pressure to establish flow through the
column. Septa maintain the leak -free seal and exclude air from the inlet. They
come in many different sizes and are made from many different types of
material specific to inlet type and analysis needs.
Septa are usually available according to their recommended upper
temperature limits. Lower temperature septa are usually softer, they seal
better, and they can withstand more punctures (injections) than their
high-temperature counterparts. If used above their recommended
temperatures, however, they can leak or decompose. This causes sample
losses, lower column flow, and ghosting.
Ghosting can also occur when sample components adsorb to the inlet side of
the septum. Septum contaminants are then released randomly throughout the
chromatographic run or on injection of subsequent samples, creating artifact
peaks unrelated to sample (ghost peaks).
Ghosting can be reduced or eliminated using several combined techniques
including:
• An inlet with septum purge which continuously sweeps the exposed surface
of the septum to remove potential contaminants
• Septa which are designed to minimize ghosting (preconditioned, low bleed)
• Sample volumes and solvents consistent with inlet volume, temperature,
and injection technique
Smaller sample sizes, lower inlet temperatures, and larger liners all decrease the
potential for ghosting. High temperature silicones, alternate polymer materials,
and layered composites of different polymers extend septum utility to
temperatures up to 350 °C. Prepunched septa prevent coring during injection,
and can withstand many times as many injections as standard septa before
leaking.
34
Introduction to GC Inlets
3
Inlet Selection
Column Selection
Solute retention
When solutes (liquids or gases) are retained at the head of the analytical
column, stationary phase focusing occurs. This is a chromatographic process
that is a function of column temperature, stationary phase type, and phase
ratio.
Phase ratio
The phase ratio reflects the ability of a column to retain solutes. Once
vaporized, sample transfers to the head of the column where it is retained by
the stationary phase. As the phase ratio increases, the column retention
power, sample capacity, and capability to focus solutes at the inlet decrease.
Knowing the phase ratio is useful for selecting a column that will sufficiently
focus solutes; a column with a lower phase ratio is needed to retain and focus
lower boiling compounds. A column with a phase ratio that is too low will have
excessive run times and lower efficiency. A general-purpose capillary column
has a phase ratio of approximately 250.
The equation used to determine phase ratio (β) of WCOT (wall-coated open
tubular) columns is:
r = ---------------------------------------------------------------------------------radius of the column
β = ------2d f
2 × stationary phase film thickness
(1)
Column efficiency
The type of inlet and how it will be used is limited to a certain degree by the
efficiency of the column to be used. The more efficient the column, the
narrower the peaks from the inlet must be, or the more effective the focusing
techniques. Improperly selected inlet conditions can virtually eliminate the
inherent benefits of using high-resolution columns.
Introduction to GC Inlets
35
3
Inlet Selection
Column capacity
GC columns can be overloaded with sample, depending on the inlet used, the
amount of sample injected, and the concentration of the sample. Stationary
phases are solvents for sample solutes, and each solute has a finite solubility
in the stationary phase.
When the solubility limit of a sample component in the stationary phase is
approached, solutes begin to overload the column. This causes
chromatographic peaks to broaden and the efficiency of the column to
decrease (Figure 9). The first few peaks in Figure 9 have not overloaded the
column significantly at 10 ng; however, at 60 ng, the later eluting compounds
have overloaded the column and produce wider, distorted peaks.
Column: 25 m × 0.20 mm × 0.11 mL Ultra-2
Injection: 1 µL splitless
7.0E+6
Abundance
6.0E+6
10 ng
5.0E+6
4.0E+6
60 ng
3.0E+6
2.0E+6
1.0E+6
6
Figure 9
8
10
12
14
16
18
20
Early peaks are not overloaded and are symmetrical. Late-eluting peaks
overload the column at 60 ng and are skewed.
Small diameter columns with thin stationary-phase films have low capacity
and restrict the maximum sample amounts which should reach each column.
Low capacity columns require either split injection techniques, small injection
volumes, or sample dilution prior to injection.
36
Introduction to GC Inlets
Inlet Selection
3
Column temperature
The initial column temperature is critical to solute focusing. Stationary phase
focusing is exponentially related to column temperature. Solvent focusing
requires low enough column temperature for condensation of the solvent
quickly at the head of the column. This is usually 20 °C or more below the
boiling point of the solvent. Thermal focusing requires even lower initial
temperatures (150 °C below the boiling point of the solute, for example).
Inlet Liners
Inlet liners also have a direct effect on analysis results. When dirty samples
are analyzed routinely, replaceable inlet liners (usually glass) are used to
simplify cleaning of the inlet and to minimize influence of contaminants on
subsequent analyses. These liners are replaced or cleaned as soon as any loss
in performance is noted.
Figure 10 shows the effect a dirty liner can have on peak shape. Distorted
peaks, lower sample recovery (sensitivity), change in response factor, and
lower reproducibility are all characteristic of a contaminated liner.
Temperature-programmed analysis
Split injection
Compounds: 10—n-decane, ol—1-octanol, p—2,6-dimethylphenol,
S—ethylhexanoic acid, A—2,6-dimethylaniline,
12—n-dodecane
A. Dirty inlet liner
B. Clean inlet liner
10
10
ol
P
12
A
ol P
12
A
S
Figure 10
Introduction to GC Inlets
S
Influence of a dirty vaporizing inlet (200 °C). (Reproduced from K. Grob and
G. Grob, J HRC&CC, 2 (1979) 109–117, with permission.)
37
3
Inlet Selection
The volume, type, and activity of the liner are the most important variables to
consider when selecting a liner. Each type of inlet works best with a certain
type of liner. Splitless inlets may require straight liners with no packing,
whereas PTV inlets require baffled liners or packed liners to retain liquid
sample during cold sample introduction.
The volume of the liner must be at least as large as the volume of the sample,
or flashback and sample loss will occur. Appendix A can be used to estimate
the minimum liner volume based on the gas volume of several common
solvents under typical inlet conditions.
Most liners require deactivation to minimize degradation of labile solutes.
Deactivation usually involves a silanization procedure and may only be
effective for a few days, after which the liner must be cleaned and
redeactivated.
38
Introduction to GC Inlets
GC Inlets An Introduction
4
Packed-Column Inlets
Overview 40
Inlet Design 41
Sample Considerations 44
Inserts/Liners 44
Temperature 44
Flow Rates 45
Troubleshooting 46
Decomposition 46
Flashback 46
Leaks 46
Summary 47
Agilent Technologies
39
4
Packed-Column Inlets
Overview
Packed-column direct inlets are very popular. Packed-column analysis is
frequently done when high efficiency separations are not needed or when
gases are analyzed by gas-solid chromatography. Packed column inlets are
simple in both design and use. Few parameters need to be set, and all carrier
gas flow flushes through the inlet into the column in the standard
configuration.
Packed-column inlets are vaporizing, so they can protect columns from
nonvolatile sample components; however, these inlets can also cause sample
degradation, needle discrimination, and adsorption of polar solutes.
40
Introduction to GC Inlets
Packed-Column Inlets
4
Inlet Design
Figure 11 shows the typical configuration of a packed-column inlet with a
glass column installed, as well as several inserts that substitute for the column
head in the inlet. Carrier gas enters the side of the inlet body and heats to inlet
temperature as it flows up between the inlet body and the outside of the
column (or insert). Once at the top, hot carrier gas moves down to carry
vaporized sample to the column. Different inserts are used depending on the
type of column used.
Introduction to GC Inlets
41
4
Packed-Column Inlets
Septum retainer nut
Septum
Heater block
Carrier gas
Glass
liner
Glass
liner
Glass
liner
Glass wool plug
Graphite ferrule
1/4-inch packed
glass column
Wide-bore
capillary
insert
Figure 11
1/8-inch
SS
column
insert
1/4-inch
column
insert
Typical packed column inlet with glass column installed for on-column direct
injection
Figures 12 and 13 are flow diagrams for standard packed-column inlets and
septum-purged packed column inlets. Typical packed-column inlets use
mass-flow controllers (differential-pressure controllers) to maintain a
constant flow rate of carrier gas through the column during temperature
42
Introduction to GC Inlets
Packed-Column Inlets
4
programming. A pressure gauge is usually provided to monitor pressure at the
head of the column. This is helpful for diagnosing leaks and column
degradation (increasing restriction). Septum purges reduce ghosting and
baseline perturbations caused by flashback and septum bleed.
External
plumbing
Internal
plumbing
Packed column
inlet
Pressure
gauge
Trap(s)
Mass flow
controller
Carrier
gas
To
detector
Electronic
flow sensor
(optional)
Column
Figure 12
Flow diagram, packed column inlet
External Internal
plumbing plumbing
Septum purged
packed column
inlet
Pressure
gauge
Trap(s)
Mass flow
controller
Carrier
gas
Electronic
flow sensor
(optional)
To
detector
Forward
pressure
regulator
Septum
purge vent
Column
Out
In
Fixed
restrictor
Figure 13
Flow diagram, septum-purged packed column inlet
Introduction to GC Inlets
43
4
Packed-Column Inlets
Sample Considerations
Thermally labile compounds are most effectively analyzed by intracolumn
(on-column) direct injection with glass packed columns. Packing should be
removed from the inlet section so that activity in the hot inlet is minimized,
and the packing material does not get contaminated quickly.
On-column direct injection is done by inserting the column (usually glass)
through the inlet (as in Figure 11) so the syringe enters the empty head of the
column during injection.
With this technique, the sample is exposed to reduced activity in the heated
zone, and the problems associated with hot injection into stainless steel inlets
are eliminated. Nonvolatile sample components are more apt to affect
subsequent analyses with on-column direct injection, since they can reach the
column packing and degrade chromatographic performance.
Dirty samples are most effectively introduced by extra-column injection into
inserts with replaceable glass liners.
Inserts/Liners
Variables used to select packed-column inserts include column dimensions
and sample composition. Figure 11 shows 1/8-inch and 1/4-inch od inserts,
which are inserted into the inlet in place of the 1/4-inch od glass column.
These inserts are stainless steel and are available with or without glass liners.
The column attaches to the base of the insert.
Inserts with glass liners are always preferred because they shield the sample
from the hot stainless steel surface which increases the adsorption of polar
sample components and promotes sample decomposition. Replaceable glass
liners also facilitate cleaning of the inlet if separation efficiency starts to
degrade because of inlet contamination.
Temperature
The inlet temperature should be at or above the boiling point of the solvent, or
major solutes of interest, to guarantee that they are efficiently transported
from the inlet zone into the column. Problems caused by excessive inlet
44
Introduction to GC Inlets
Packed-Column Inlets
4
temperatures include sample degradation, flashback, and increased syringe
discrimination. If degradation is possible, the inlet should be set at the lowest
temperature that does not broaden the peak or reduce the area of the
highest-boiling component.
If later eluting peaks are distorted or show less area than expected, the sample
is probably not evaporating completely or fast enough. Increase the inlet
temperature by 50 degrees and try again. If there is evidence of sample
degradation or flashback, then the inlet temperature could be too high. Reduce
the inlet temperature by 50 degrees and try again.
Flow Rates
Column flow is adjusted with the mass-flow controller and is measured by a
bubble flow meter at the detector end. Column flow for packed columns is
usually 30 mL/min when using He carrier gas. Consult the literature supplied
with specific columns for the manufacturer’s recommended flow rate.
Introduction to GC Inlets
45
4
Packed-Column Inlets
Troubleshooting
Most problems with packed-column inlets involve sample decomposition,
flashback, or leaks.
Decomposition
Since packed-column inlets are active, especially if glass liners are not used,
polar sample components will often tail or degrade in the inlet. Sample
decomposition caused by the inlet is easily diagnosed; the decomposition
products will have peaks at the same retention times as standards for the
decomposition product.
When inlet-caused decomposition is suspected, try intracolumn direct
injection, deactivated glass liners, or lower inlet temperatures, and remove
any column packing in the inlet zone.
The inherent activity of packed-column inlets is somewhat mediated by the
fact that they usually have low internal volume. When this is coupled with the
relatively fast flow rates used with packed columns, the residence time of
sample in the inlet is short and decomposition is reduced in comparison to the
decomposition that occurs with some capillary inlets (for example, splitless
inlets).
Flashback
The negative side of low inlet volume, however, means that excessively large
sample injections will easily exceed the capacity of the liner and will flash
back into gas supply lines and onto the septum. This can cause several
maladies, including ghost peaks, sample losses, irreproducible peak areas, and
decomposition.
Leaks
Since packed-column inlets are usually flow controlled, septum and column
leaks will have a direct impact on retention times and peak areas. Sample can
be lost through the leak holes, and air can diffuse back into the inlet to cause
column degradation. Change the septum on a regular basis and check column
connections at the first sign of problems. To prevent stationary phase
decomposition, make sure that the oven and inlet are at room temperature
when not in use and when changing the septum.
46
Introduction to GC Inlets
Packed-Column Inlets
4
Summary
Table 1
Standard packed column inlet procedures and practices
Parameters
Selection/Setting
Rationale
Inlet temperature
BP of solvent +50 °C
Ensures flash vaporization
BP of major solute(s)
Use for neat samples
1/8-inch stainless steel
For stainless steel columns only
1/4-inch stainless steel
Inserts permit use of columns up to 1/4-inch od
Liner
Glass
Use to lower activity (replaceable)
Initial column temperature
Temperature programming
Sharpen peaks and reduce runtime
Column type
1/8-inch packed stainless
Will not break
1/4-inch packed glass
Better for polar or labile compounds
20 to 40 mL/min
Use with nitrogen carrier gas
30 to 60 mL/min
Use with helium or hydrogen
Insert type
Carrier gas flow
Introduction to GC Inlets
47
4
Packed-Column Inlets
Table 2
Factors affecting packed column inlet accuracy and reproducibility
Symptom
Possible cause
Solution
Sample degradation
Temperature too high
Reduce inlet temperature
Inlet dirty
Replace the liner
Contact with metal
Use glass columns and liners
Residence time too high
Increase the flow rate
Compounds too labile
Derivatize sample
Use cool on-column injection
Activity in inlet
Use glass liner
Deactivate the liner
Use glass columns
Temperature too low
Increase inlet temperature
System voids
Check column installation
Injection technique
Use autoinjector
Use hot-needle injection
Septum leak
Replace the septum
Sample flashback
Reduce injection volume
Use larger volume liner
Lower inlet temperature
Increase the flow rate
Retention time irreproducibility
Septum leak
Replace the septum
Broad peaks
Insufficient focusing
Lower the initial oven temperature
Carrier flow too high or too low
Measure the flow and correct rate
Sample flashback
Reduce injection volume
Use larger volume liner
Septum degradation
Lower the inlet temperature
Change to high-temperature septum
Replace septum
Reduce inlet temperature
Peak tailing
Area irreproducibility
Ghost peaks rolling baseline
48
Introduction to GC Inlets
GC Inlets An Introduction
5
Capillary Direct Inlets
Overview 50
Inlet Design 52
Liners 53
Sample Considerations 53
Temperature 54
Flow Rates 54
Troubleshooting 55
Flashback 55
Band broadening in time 55
Band broadening in space 55
Summary 56
Agilent Technologies
49
5
Capillary Direct Inlets
Overview
Packed-column inlets can be modified easily for use with wide-bore capillary
columns by using appropriate inserts. Packed column inlets were designed for
use with packed columns at flow rates around 30 mL/min; however, wide-bore
capillary columns can be used successfully at packed-column flow rates with
only minor modification of the inlet. Figure 14 compares wide- and
narrow-bore capillary column analyses of an essential oil and demonstrates
the capabilities of a wide-bore column and direct injection.
50
Introduction to GC Inlets
Capillary Direct Inlets
A
,%•8
&&%•8
&*%•8
B
,%•8
Figure 14
&&%•8
&*%•8
5
Column: 25 m × 0.25 mm id
PEG-HMW
Temperature-programmed
70 to 190 °C at 2 °C/min
Injection in an all-glass
inlet splitter (1:20)
&.%•8
Column: 50 m × 0.5 mm id
PEG-HMW
Temperature-programmed
70 to 190 °C at 2 °C/min
Direct injection
&.%•8
Chromatogram of the oxygenated fraction of the essential oil of hops on a
narrow-bore (A) and wide-bore (B) capillary.
Capillary-column direct injection is often confused or even identified with
“on-column” injection. Just like packed-column direct injection, capillary
direct injection relies on flash vaporization, so sample decomposition and
Introduction to GC Inlets
51
5
Capillary Direct Inlets
needle discrimination can still be a problem. Cool on-column injection is done
with the inlet temperature below the boiling point of the solvent, so these
problems are prevented. The injection speed (fast-slow), the boiling point and
nature of the solvent, the injector temperature, and the oven temperature
must be carefully selected for each application that uses direct injection.
Inlet Design
A capillary direct inlet is basically a packed column inlet with a different type
of liner or inlet base (column connection), as is shown in Figure 15. All carrier
gas flow travels to the capillary direct column just as it does for packed
columns.
A
A
B
C
D
B
C
D
Open liner with septum purge
Tapered liner with expansion volume
Liner oriented for direct injection
Liner oriented for on-column direct injection
A Pressure regulation only
B, C, D Pressure or flow regulation
Figure 15
52
Glass liners for capillary direct injection
Introduction to GC Inlets
5
Capillary Direct Inlets
Sample Considerations
Analyses of labile samples are usually more accurate using intracolumn
injection, since the sample is exposed to a less active surface in the column
than it would be in an inlet liner. For intracolumn direct injection into
capillary columns, keep injection volumes as small as possible (<1 µL) and
flow rates as high as possible to prevent overloading the column and to reduce
flashback. Injecting slowly (over 1 to 2 s) will also reduce flashback, decrease
peak widths, and improve resolution of early eluting components.
Dirty samples are best analyzed using extra-column injection so that
nonvolatile sample components are trapped in the liner and do not degrade
column performance. Again, low injection volumes and slow injection speeds
are preferred, especially if early eluting peaks are of interest.
Liners
For capillary column direct injection, the volume of the liner should be at least
as large as the volume of sample vapors resulting from injection. Refer to
Appendix A to find the approximate gas volume per microliter injected
sample.
In addition to the inlet and liner design shown in Figure 11 on page 42, several
others are shown in Figure 15 on page 52. The configuration with a straight
glass liner (Figure 11 and Figure 15A) is effective for many analyses at high
column flow rates, but it is susceptible to sample flashback caused by low
internal volume.
Figure 15B shows a larger volume liner (in comparison to straight glass) with
a taper at the top and the bottom, which is also used for extra-column direct
injections. The tapers may help contain sample vapors and reduce flashback.
Some liners can be used for either extra- or intracolumn injections, depending
on the orientation of the liner in the inlet. When the liner is installed so the
large expansion volume is at the top of the inlet and the column is installed at
the bottom (Figure 15C), extra-column injection will occur. When the liner is
flipped so the column extends to the top of the inlet (Figure 15D) and is sealed
against the taper, the syringe needle enters into the hot column during
injection and intracolumn (or on-column) direct injection occurs.
Introduction to GC Inlets
53
5
Capillary Direct Inlets
Temperature
For direct injections, the inlet temperature must be high enough to
flash-vaporize the sample. With most samples this is dependent on the boiling
point of the solvent and/or major components and should be set 10 to 25 °C
above those values. Excessively high inlet temperatures should be avoided due
to the possibility of flashback and sample degradation.
Flow Rates
Since capillary direct inlets are usually converted packed-column inlets,
column flow rate is usually set by adjusting a mass-flow controller; however,
packed column flow controllers are usually configured to stabilize at flow
rates greater than 15 mL/min, which is near the upper usable flow range for
wide-bore capillary columns. When flow rates closer to 3.5 mL/min (optimum
for 0.53-mm capillary columns) are required, a flow restrictor (manufacturer
supplied) should be added to the GC pneumatics to increase flow stability.
54
Introduction to GC Inlets
5
Capillary Direct Inlets
Troubleshooting
Most of the problems associated with capillary-direct inlets relate to flashback
and band broadening in time or space.
Flashback
Capillary-direct inlet liners can easily be overloaded by: injecting excessive
sample volumes (>1 µL), using solvents with low boiling points, injecting too
quickly, using excessive inlet temperatures, and/or using low column flow
rates. Flashback is more of a problem with capillary direct inlets than it is
with packed columns, since large-bore capillary columns are usually used at
lower flow rates to maximize separation efficiency.
Flashback is of greatest concern when doing intracolumn injections, because
the volume of the analytical column is even smaller than that of the inlet
liners.
Band broadening in time
Band broadening in time can be a problem because of the lower flow rates
with capillary columns compared with packed columns. Stationary phase
focusing can help to narrow broad peaks, provided the initial oven
temperature is low enough.
Solvent focusing can be used with capillary direct injection and yields an
analysis similar to that of splitless injection without an inlet purge.
Band broadening in space
When using solvent focusing, column temperature must be set low enough to
ensure recondensation of the solvent (more than 25 °C below the boiling point
of the solvent). Band broadening in space can be a problem, however, if injection
volumes are too large and/or if the solvent is not compatible with the stationary
phase. Should broad or split peaks be observed in this mode, a retention gap
could be used to help recombine and focus the peaks.
Introduction to GC Inlets
55
5
Capillary Direct Inlets
Summary
Table 3
Standard capillary direct procedures and practiced
Parameter
Typical choice
Rationale
Inlet temperature
BP of solvent + 50 °C
Ensure flash vaporization
BP of major solute(s)
Use for neat samples
Straight glass
Readily available, inert
Expanded volume
Decreased flashback problems
Tapered end(s)
Flexible: down for extra-column
injection, up for intra-column injection
Initially low, then program up for analysis
Focuses solutes and reduces runtime
BP of solvent –25 °C
Use for solvent focusing
Column type
>0.5 mm id
Can use column flow rate and injection
parameters close to those for packed
columns
Carrier gas flow
10 to 20 mL/min
Produces results similar to packed
columns
3 to 10 mL/min
Provides higher efficiency and better
separation
Liner type
Initial column temperature
56
Introduction to GC Inlets
Capillary Direct Inlets
Table 4
5
Factors affecting capillary direct inlet accuracy and reproducibility
Symptom
Possible cause
Solution
Low peak areas, lost peaks, generation
of new peaks
Temperature too high
Reduce inlet temperature by 50 °C,
reevaluate
Dirty inlet
Clean or replace liner
Contact with metal
Use glass columns and liners
Residence time too high
Increase flow rate
Compounds too labile
Derivitize sample, use cool on-column
injection
Activity in inlet
Use glass liner and column
Deactivate liner
Improper carrier flow
Check and correct
Temperature too low
Increase temperature by 50 °C, retry
Temperature too high
Reduce temperature by 50 °C, retry
System voids
Check column installation
Poor injection technique
Use autoinjector
Use hot needle slow injection
Septum leak
Replace septum
Sample flashback
Inject less
Use larger volume liner
Reduce temperature
Increase flow rate
Retention time irreproducibility
Septum leak
Replace septum
Broad peaks
Incorrect column flow
Correct column flow
Insufficient focusing
Lower oven starting temperature
Sample flashback
Inject less
Use larger volume liner
Reduce inlet temperature
Septum degradation
Change septum type
Replace septum
Reduce inlet temperature
Peak tailing
Area irreproducibility
Ghost peaks rolling baseline
Introduction to GC Inlets
57
5
58
Capillary Direct Inlets
Introduction to GC Inlets
GC Inlets An Introduction
6
Split Inlets
Overview 60
Inlet Design 61
Sample Considerations 62
Liners 65
Temperature 67
Flow Rates 67
Troubleshooting 68
Needle discrimination 68
Inlet discrimination 68
Sample decomposition 69
Summary 70
Agilent Technologies
59
6
Split Inlets
Overview
The combined “split/splitless” inlet is the most popular inlet for capillary
column gas chromatography. Because it can be used in either split or splitless
mode, it provides a very effective combination that can cover most analysis
requirements. Split and splitless injection techniques have been studied
exhaustively, and several deficiencies have been identified for each. Since the
proper use of inlet temperature, liner type, oven temperature, injection
technique, and purge events are different for these two techniques, they will
be discussed separately.
The split inlet was the first sample introduction system developed for
capillary gas chromatography. In split injection, liquid sample is introduced
with a syringe into the hot inlet where it is vaporized immediately. A small
fraction of the resulting vapor enters the column while the major portion is
vented out an open fitting on the GC. Split injection guarantees narrow inlet
bands because there is a high gas flow through the inlet, and sample is
removed quickly.
60
Introduction to GC Inlets
Split Inlets
6
Inlet Design
A schematic diagram of a split inlet is shown in Figure 16.
Septum purge
flow controller
Total flow
controller
Purge vent
3 mL/min septum purge flow
104 mL/min
3 mL/min
101 mL/min
100 mL/min
Split vent
100 mL/min
Purge control
solenoid
Column head
pressure control
(back-pressure
regulator)
To detector
in
m
L/
1m
Figure 16
Flow diagram for a split inlet. This example shows a 1:100 split ratio (100 mL/min split flow,
1 mL/min column flow).
Carrier gas, controlled by a pressure regulator or a combination of a flow
controller and a back -pressure regulator, enters the injector at the top. The
flow is divided into three streams:
• One stream of carrier gas purges the septum to minimize ghosting and is
controlled by a needle valve or flow controller.
• The other stream of carrier gas enters the vaporization chamber, which has
a glass or quartz liner, and is mixed with vaporized sample.
• The mixed stream is split between the column inlet and the split vent.
Introduction to GC Inlets
61
6
Split Inlets
In Figure 16, a back -pressure regulator controls the column head pressure
and, therefore, the flow through the column. The remainder of the total flow is
vented out the split vent.
Sample Considerations
Split injection is required for samples that:
• Are very volatile
• Cannot be diluted for analysis (for example, solvents)
• Are gases that cannot be focused, or that have long injection times (valve
injections)
• Have important minor peaks eluting directly before the solvent peak (as in
solvent analysis).
Split injection is also good for screening samples of which little is known or for
those that have widely differing concentrations, since the split ratio can be
adjusted easily.
Figure 17 shows a chromatogram of impurities in styrene. Split injection was
used to guarantee a narrow peak for the main component, styrene. If the
analysis had been run using splitless injection, the early eluting peaks would
have been obscured by the much broader styrene peak.
62
Introduction to GC Inlets
6
Split Inlets
Column: 20 m × 0.25 mm, FSOT, DB Wax, d = 1 µm
Temperature: 40 °C↑ 2 °C/min↑ 150 °C
Carrier gas: Hydrogen, 35 cm/s
Sample: 1 µL styrene monomer, split 1:170
Peaks:
1. ethylbenzene
2. p-xylene
3. m-xylene
4. cumene
5. o-xylene
6. n-propylbenzene
7. p-ethyltoluene
8. m-ethyltoluene
9. o-ethyltoluene
10. α-methylstyrene
11. p+m-methylstyrene
12. phenylacetylene
13. trans -β-methylstyrene
14. benzalaldehyde
styrene
3
1
5
10
4
9
6
12
2
14
78
Figure 17
11
13
Analysis of styrene monomer. (Courtesy of R. Miller, Huntsman Chemical
Corporation.)
Split inlets are also a good choice for dirty samples, provided the liner is
cleaned or replaced at the onset of loss of performance (see Figure 10 on
page 37). This is because much of the sample is vented out of the split vent
during the vaporization process; and, over time, high-boiling sample residues
slowly leach out the split vent rather than onto the column.
Very complex samples are often analyzed with ultra-high resolution capillary
columns. These columns require very narrow initial peak widths and,
therefore, split injection. Figure 18 shows an ultra-high resolution
chromatogram of diesel oil obtained using a 100-µm capillary column and split
injection with a 300:1 split ratio.
Introduction to GC Inlets
63
6
Split Inlets
Column: 100 m × 100 µm, OV-1, d 0.2 µm
Temperature: 60 °C, 0.1 °C/min, 185 °C
Carrier gas: Hydrogen, 10 atm
Split ratio 300:1, 0.1 µL injected
C9
C10
C11
C12
C13
0
60
120
180
C15
240
C14
300
360
420
C16
420
480
540
600
C18
660
phytaan
pristaan
norpristaan
C17
720
780
C18
phytaan
C19
780
Figure 18
64
840
900
C20
960
C21
1020
1080
C22
1140
C23
11200
Ultra-high resolution (n = 106) analysis of diesel oil
Introduction to GC Inlets
6
Split Inlets
Liners
Figure 19 shows several types of liners that can be used with split inlets. Frits,
cups, baffles, glass beads, and glass wool are used to trap particulates and
nonvolatile sample components. These also increase reproducibility and
decrease discrimination by ensuring complete vaporization of the sample before
it reaches the column entrance.
Figure 19
Several liners used with split inlets
Glass wool is the most active of the liner packings; it increases the possibility
of adsorption and decomposition of the sample, so it should not be used when
analyzing polar or degradable samples. For these samples, deactivated glass
beads or baffled liners usually give better results.
Correct placement of packing in the liner is critical for optimal results. There
is always a temperature gradient down the length of the inlet, as shown in
Figure 20. The magnitude of the gradient depends on inlet design, the
Introduction to GC Inlets
65
6
Split Inlets
difference between inlet and oven temperatures, the quality of the inlet
insulation, and whether or not an insulation cup is used on the oven side of
the inlet.
Injection port temperature
setpoint, 350 °C
Bottom of
septum
10
20
30
40
Syringe tip
50
60
70
80
35 °C
Oven
90
300 °C
Oven
150 °C
Oven
Base of
injection
port
0
50
100
150
200
250
300
350
400
Temperature of gas stream, °C
Figure 20
Thermal profiles of a typical inlet
The hottest point in the inlet, and the point which is closest to the set point
temperature, is in the middle. Inlet packing (such as glass wool) should,
therefore, be placed in the center of the liner. An insulation cup should be
used at the base of the inlet to further reduce the magnitude of thermal
gradients.
Split liners are sealed in the inlet with O-rings or graphite seals. O-ring seals are
easier to remove and to replace than graphite which deforms and flakes apart.
The graphite seals should be used when inlet temperatures exceed 300 °C.
66
Introduction to GC Inlets
Split Inlets
6
Temperature
Complete evaporation of the sample is necessary to minimize discrimination
and to maximize the accuracy and reproducibility of split injections. For a new
sample, inlet temperature should be set close to but above the expected boiling
point of the highest-boiling major sample component (often the solvent). This
temperature can then be adjusted up or down in response to analysis
deficiencies such as decomposition and discrimination.
Flow Rates
After the column head pressure is set by adjusting the pressure controller (see
appendix B for starting points), the split and septum purge flows should be set
and measured. The septum purge is usually set between 3 and 5 mL/min and is
measured by a flow meter at the purge vent fitting on the GC. The split flow is
also measured at a union on the outside of the GC and is adjusted according to
desired split ratio. The split ratio is the ratio of the column flow relative to the
split vent flow.
Split ratio = column flow (mL/min) : split vent flow (mL/min)
For the most accurate calculation of split ratio, both values must be corrected
to the same temperature and pressure (see appendix C). For most analyses,
split ratios in the range of 1:50 to 1:500 should be used for conventional
capillary columns (0.20 to 0.32 mm id). Lower split ratios (1:5 to 1:15) can be
used with dilute samples, gaseous samples, and wide-bore columns, but the
resulting decrease in total inlet flow yields wider initial peak widths.
Therefore, focusing may be required for acceptable results.
Very high split ratios are used with high-speed capillary gas chromatography
(using 50 to 100-µm id columns), where split ratios in excess of 1:1000 may be
required to minimize initial peak widths. High split ratios are also used with
low capacity columns and with very concentrated samples. Inlet
discrimination is increased at high split ratios, whereas sample decomposition
is reduced.
Introduction to GC Inlets
67
6
Split Inlets
Troubleshooting
Split inlets are spared from most band-broadening phenomena, since narrow
peaks are generated as part of the splitting process. Therefore, any peak
broadening or tailing observed with split injection is usually due to improper
column installation, low split flow, or low inlet temperature. If you suspect that
the inlet temperature may be too low, increase it by 50 °C and compare the
results to the lower temperature analysis. Repeat if results are positive until no
further improvement is seen.
A majority of the problems encountered with split inlets are related to
discrimination and decomposition. Both analytical accuracy and
reproducibility decrease with the increases in discrimination and
decomposition. Split inlets suffer from both needle discrimination and inlet
discrimination.
Needle discrimination
Loss of high-boiling solutes or a decrease in areas of late eluters relative to
early eluters are symptoms of needle discrimination.
Needle discrimination can be reduced by using fast autoinjectors, by
minimizing inlet temperature, by injecting larger volumes, and/or by using a
solvent with higher boiling point.
Inlet discrimination
The different mechanisms that can cause split inlet discrimination are:
• Different diffusion speeds of the sample components from point of injection
to column inlet.
• Incomplete evaporation of some sample components.
Inlet discrimination of high boilers can be reduced by increasing inlet
temperature, by injecting less sample (the reverse of the actions taken to
counteract needle discrimination), by switching liner type, or by lowering the
split ratio (thereby increasing residence time in the inlet).
Inlet discrimination with low boilers occurs less frequently than
discrimination with high boilers and can be more difficult to isolate. Loss of
low boilers is influenced by liner design, flashback, sample loss via inlet leaks,
68
Introduction to GC Inlets
Split Inlets
6
and the pressure pulse following injection. Lower inlet temperature, lower
injection volume, and a change to higherboiling solvent may help reduce the
problem.
Sample decomposition
Sample decomposition is indicated by lost or misshapen peaks, and/or by the
generation of new ones. Decomposition is exacerbated by high inlet
temperature, long residence time of the sample in the inlet (low split flow,
large sample sizes), and activity in the inlet (high surface-area liners,
active/unsilanized packings).
Introduction to GC Inlets
69
6
Split Inlets
Summary
Table 5
Standard split procedures and practices
Parameter
Typical choice
Rationale
Inlet temperature
BP of last eluting compound
Ensures flash vaporization
Minimize inlet discrimination
Inlet liner
Large volume, deactivated
Minimize flashback and degradation
Inlet packing
Silanized glass wool
Retains nonvolatiles, stops flow of droplets
Glass beads or frit
Less active than wool
None
Least active
0.5 to 3 µL liquid
Split easily adjusted
0.1 to 10 mL gas
Split adjusted accordingly
Fast autoinjection
Less needle discrimination
Hot needle fast manual injection
Reproducible discrimination
Split ratio
50:1 to 500:1
Depends on sample and injection volume
Initial column temperature
Not critical
Narrow initial peaks
Septum purge
3 to 5 mL/min
Minimizes ghosting
Injection volume
Injection technique
70
Introduction to GC Inlets
6
Split Inlets
Table 6
Factors affecting split accuracy and reproducibility
Symptom
Possible cause
Solution
Low peaks, lost peaks, generation of
new peaks
Inlet too hot
Reduce temperature 50 °C
Dirty inlet
Clean or replace liner
Contact with metal
Use glass columns and liners
Compounds too labile
Derivitize sample
Use cool on-column injection
Active packing
Remove packing
Active liner
Change liner type
Deactivate liner
Residence time too long
Increase split flow
Increase column flow
Low area for late eluters
Solvent BP too low
Use higher-boiling solvent
Needle discrimination
Inlet temperature too low
Increase inlet temperature 50 °C
Needle dwell time too long
Use fast autoinjector
Inlet temperature too high
Decrease by 50 °C
Inlet dwell time too short
Reduce split flow
No glass wool or in wrong place
Center in the liner
Split flow too high
Decrease split flow
Injection volume too big
Decrease injection volume
Split flow too low
Increase split flow
Adsorption in inlet
Change liner
Remove packing
Increase temperature
Column overloaded
Increase the split flow
Fluctuation in split ratio
Check the flow controllers
Check for leaks (septum, liner, column)
Inlet discrimination
Wide peaks
Area irreproducibility
Introduction to GC Inlets
71
6
Split Inlets
Table 6
Factors affecting split accuracy and reproducibility (continued)
Symptom
Retention time reproducibility
72
Possible cause
Solution
Sample flashback
Reduce the sample size
Reduce the inlet temperature
Use a larger liner
Variable injection volume
Check the injection technique
Use an autoinjector
Decomposition
Remove liner packing
Decrease liner temperature
Overload
Increase split ratio
Inject less
Column degradation
Cut 0.5 m of inlet end
Replace column
Introduction to GC Inlets
GC Inlets An Introduction
7
Splitless Inlets
Overview 74
Inlet Design 74
Sample Considerations 76
Liners 78
Temperature 79
Flow Rates 79
Determining Purge Delay Time 80
Troubleshooting 82
Summary 84
For
of the
splitless
run. injection, a conventional split injector is operated in a nonsplitting mode by closing the split valve during injection. The sample is flash-vaporized in the liner, and sample vapors are carried into the column by the carrier gas where they are recondensed at temperatures below the boiling point of the solvent. After most of the sample has been transferred into the column, vapors remaining in the liner are cleared by opening the split vent which remains open for the duration
Agilent Technologies
73
7
Splitless Inlets
Overview
The most important benefit of splitless injection is that a majority of the
injected sample is introduced into the column; this results in much higher
sensitivity than that achieved using split injection.
Splitless injection is routinely used in areas such as environmental analysis,
pesticide monitoring of foods, and drug screening. In these applications,
sample preparation requirements are significant, and it is not always possible
or economically justifiable to clean up a sample extensively. So column
protection becomes as important as sensitivity. Also, samples with trace
quantities of important solutes that elute on the solvent tail may be focused by
the solvent to yield more sensitive analyses.
Inlet Design
A schematic for a purged splitless inlet is shown in Figure 21. The
pneumatic configuration is usually the same as that for split injectors. The
septum is continuously purged (approximately 3 mL/min) to maintain a
contamination-free system, while a flow of 30 to 60 mL/min is vented via
the split vent.
Prior to injection, a solenoid valve is activated so that the split flow is either
closed off or diverted to the top of the inlet with the septum purge flow (top
diagram). After injection of liquid sample into the liner, the sample
vaporizes and is transferred into the column very slowly (at the column
flow rate). After 30 to 80 seconds, the solenoid valve is deactivated, and
residual vapors in the inlet are vented to waste via the purge vent (or split
vent) as shown in the bottom diagram of Figure 21.
74
Introduction to GC Inlets
Splitless Inlets
Total flow
control
7
Septum purge
control
PURGE OFF
Purge vent
54 mL/min
53 mL/min
3 mL/min
3 mL/min
50 mL/min
1 mL/min
50 mL/min purge flow
Purge control
solenoid
Split vent
Column head
pressure
control
To detector
1 mL/min
in
m
L/
1m
Total flow
control
PURGE ON
Septum purge
control
Purge vent
54 mL/min
3 mL/min
3 mL/min septum purge flow
51 mL/min
50 mL/min
50 mL/min inlet purge flow
50 mL/min
Split vent
Purge control
solenoid
Column head
pressure
control
To detector
1 mL/min
1m
in
m
L/
Figure 21
Flow diagram for splitless inlet during injection (PURGE OFF) and after sample transfer (PURGE ON).
Introduction to GC Inlets
75
7
Splitless Inlets
Figure 22 shows the effect of inlet purging on the tail of the solvent peak.
Less than 5%
Recorder
trace
Without purge
Figure 22
With purge
Influence of inlet purge on solvent peak
Sample Considerations
The boiling point of the solvent restricts many splitless inlet parameters, such
as inlet and initial oven temperatures, purge time, and injection volume. In
splitless injection, higher boiling solvents have several advantages over
low-boiling solvents including: lower syringe discrimination, lower pressure
pulses concurrent with sample evaporation, easier solvent focusing, and
higher initial oven temperatures. Table 7 lists the boiling points and
recommended starting oven temperatures for several common solvents.
Table 7
76
Suggested initial oven temperatures for using the solvent effect
Solvent
Boiling point, °C
Suggested initial column
temperature, °C
Diethyl ether
36
10 to ambient
n-Pentane
36
10 to ambient
Methylene chloride
40
10 to ambient
Carbon disulfide
46
10 to ambient
Chloroform*
61
25
Methanol*
65
35
n-Hexane
69
40
Introduction to GC Inlets
Splitless Inlets
Table 7
7
Suggested initial oven temperatures for using the solvent effect (continued)
Solvent
Boiling point, °C
Suggested initial column
temperature, °C
Ethyl acetate*
77
45
Acetonitrile
82
50
n-Heptane
98
70
iso-Octane
99
70
Toluene
111
80
* Should be used ONLY with crosslinked stationary phases
Solvent selection must complement sample polarity. In addition, solvents must
not elute after sample components of interest or they will be obscured by the
large solvent peak. Polar solvents are usually required to dissolve polar
solutes; nonpolar solvents are used to dissolve nonpolar solutes. Since the
solvent is recondensed in the column during solvent focusing, solvent
compatibility with the column is important to minimize the length of the
flooded zone (peak broadening and splitting), or a retention gap must be used.
When analyzing only high-boiling sample components, splitless analyses are
independent of solvent boiling point. Figure 23 illustrates the splitless analysis
of a sample with high-boiling solutes. Even though the starting oven
temperature is above the boiling point of the solvents, the peak widths are all
narrow because stationary phase focusing predominates. In this case,
conditions conducive to solvent focusing are unnecessary and would lead to
longer run times. Analysis and recycle times, therefore, can be reduced by
starting at a higher initial oven temperature while still achieving high
sensitivity from splitless injection.
Introduction to GC Inlets
77
7
Splitless Inlets
Figure 23
C14
C15
C14
C16
C17
C15
diethyl ether, bp 35 °C
iso-octane, bp 99 °C
C17
C16
methylene chloride, bp 41 °C
Initial column temperature 100 °C
C16 C17
C14
C15
Stationary phase focusing with splitless injection and temperature
programmed analysis
To find a good initial column temperature for samples with only late eluting
peaks, increase the initial temperature by 25 °C increments until peak
broadening is seen for the earliest eluting peaks of interest, then lower that
temperature by 15 to 20 °C.
Liners
An important factor in the effective use of a splitless inlet is the size of the
vaporizing chamber. Liners with internal volumes between 0.25 mL and 1 mL
are common. Long and narrow inserts are preferred to obtain minimal sample
dilution during slow manual injection. Larger-volume liners are required for
autoinjection because the sample is injected and vaporized much more
quickly. See Appendix A for appropriate liner volumes based on solvent and
inlet temperature. When in doubt it is better to use oversized rather than
undersized liners.
Since the contact time between the sample and the liner is very long in
splitless injections, liner activity can cause decomposition of labile
compounds. Liners and packing material should be deactivated (for example,
78
Introduction to GC Inlets
7
Splitless Inlets
silanized). Even deactivated liners and packings will become more active with
time and use. So cleaning, redeactivation, or replacement of the liner on a
regular basis is highly recommended.
Splitless liners can be used without packing for manual injection. Some glass
wool is required when doing fast autoinjection to maximize reproducibility.
When dirty samples are analyzed, deactivated glass wool or glass beads help to
retain nonvolatile sample components.
Temperature
Inlet temperature must be high enough to completely vaporize the sample and
minimize its residence time in the inlet. However, the lowest temperature that
accomplishes this is preferred because it will reduce sample decomposition
and minimize flashback. In comparison to split injection, lower inlet
temperatures can be used because sample transfer to the column is slower.
Slow evaporation of the sample is compensated for by focusing techniques
(solvent and stationary phase focusing).
An inlet temperature that is too low will prevent higher-boiling solutes from
reaching the column, and inlet discrimination will occur. The proportion of
high-boiling solutes reaching the column will decrease as a function of their
boiling points. Peak areas for late eluting peaks will be progressively smaller
than expected. In this case, a higher inlet temperature must be used, and a
new optimal purge delay time must be determined at the new temperature.
Flow Rates
In comparison to split injection, faster column flows are generally preferred
with splitless injection since this decreases the time that the sample is in the
inlet. Column flow is set by adjusting column head pressure, just as is done
with split injection. See Appendix B for flow rate versus column head pressure
examples.
Purge flow (measured at the split vent) is typically 30 to 60 mL/min. Since the
purge flow is turned off or diverted automatically during the initial injection
period (purge delay time), it does not affect sampling unless purge time is set
too short or flashback occurs (Figure 24).
Introduction to GC Inlets
79
7
Splitless Inlets
Normalized area
16000
0.7, 1.7 µL injected ±2.7%
14000
12000
Liner overloaded
2.7 µL injected
4.7 µL injected
10000
8000
6000
4000
2000
2
4
6
8 10
20
30
Septum purge flow rate, mL/min
Figure 24
The quantitative recovery of n-C11 is a function of the septum purge flow rate
because of flashback with sample volumes larger than 1.7 µL.
Determining Purge Delay Time
Appropriate purge delay time is a compromise between the amount of sample
transferred to the column and the sharpness of the tail of the solvent peak.
Optimal purge time is dependent on all other injection variables and
corresponds to transfer of 95% to 99% of the sample to the column. Figure 22
on page 76 shows the sharpening achieved by optimal purging of the inlet.
Purging is only important when there are peaks of interest eluting near the
solvent tail, because these peaks would be hidden under the tail
The relationship of purge delay and the amount of sample reaching the column
is shown in Figure 25. The shape of the curves in Figure 25 is a function of the
of solvent and solutes, the volume of the vaporizing chamber, the sample size,
the injection speed, and the carrier gas velocity.
80
Introduction to GC Inlets
Splitless Inlets
7
Area counts
±1.2% deviation
~20 ppm n-C14 (stationary phase focused)
±1.2% deviation
~10 ppm n-C11 (solvent effect)
Solvent: isooctane
Column: 16.5 m × 0.25 mm SE-54
80 °C (0.5 min) →170 °C @ 15 °C/min
Sample size: 1.3 µL, (manual injection)
Injection rate: 0.1 µL/s
Purge flows: 5 mL/min septum, 60 mL/min inlet
10
Figure 25
20
30
40
50
Purge activation time, s
60
Effect of iinlet purge activation time on area counts
Purge delay time should be determined after all other inlet parameters have
been set. A simple empirical approach is as follows:
1 Start by injecting a sample using a long purge time (90 to 120 s) and measure
the area of a solute that elutes at ki´>5. This should correspond to 100% of the
solute reaching the column.
2 Next, reduce the purge time in large decrements (for example, by 30 s, 20 s,
etc.) and reinject the sample until a lower peak area results.
3 Compare the areas for the solute peak and adjust the purge time up or
down in smaller increments until the area is between 95% and 99% of the
original area.
For analyses of solutes eluting on the solvent tail, it is better to err in favor of
the short purge delays to ensure sufficient sharpening of the solvent tail. For
analyses of late eluting compounds, it is better to err in favor of long purge
delay times to maximize analytical sensitivity. Unnecessarily long purge
delays, however, will increase the amounts of contaminants transferring from
the liner to the column and will increase total run time. (Usually, the oven
temperature ramp does not start until after the purge delay time.)
Introduction to GC Inlets
81
7
Splitless Inlets
Troubleshooting
Most problems encountered with splitless injection are related to incorrect
purge time, degradation, improper focusing, and flashback.
Solvent compatibility with the stationary phase is important to avoid peak
distortion and splitting. Figure 26D shows peak distortion due to
incompatibility of solvent and stationary phase. Retention gaps should always
be considered when peak distortion occurs.
Appropriate initial column temperature is critical for effective solvent
focusing.
Figure 26B shows the result of proper solvent focusing with an initial oven
temperature of 25 °C. There are many narrow peaks on the solvent tail, and the
peaks after the solvent tail are sharp. In contrast, the early peaks in fFigure 26C
are hidden by the solvent and are broader because the initial column
temperature of 60 °C prevented recondensation of the solvent and solvent
focusing.
Sample vapors can be lost through the septum purge line if the insert is
overfilled with sample vapor (either too large injection volume or too small
liner volume). This leads to irreproducibility and nonlinearity of peak areas.
Match inlet temperature, liner volume, and injection volume carefully.
Decomposition, as indicated by loss of peak area or generation of new peaks,
can sometimes be dramatically reduced by changing liner type or by
deactivating the liner and inlet with silanizing reagents. Removing or reducing
the amount of liner packing can also decrease inlet activity.
82
Introduction to GC Inlets
Splitless Inlets
A
Split (reference)
18
16
15 14
12
10
8
6
25 °C
40 °C
B
7 °C/min
210 °C
Splitless with hexane recondensation
25 °C
40 °C
C
7 °C/min
210 °C
Splitless without hexane recondensation
D
210 °C
Methanol - solvent
7 °C/min
7 °C/min
210 °C
Figure 26
Introduction to GC Inlets
7
60 °C
25 °C
40 °C
Band broadening in space (C and D) in splitless injection. (Reproduced with
permission from K. Grob, Jr., Journal of Chromatography, 324 (1985) 251–259).
83
7
Splitless Inlets
Summary
Table 8
Standard splitless practices and procedures
Parameter
Selection/Setting
Rationale
Inlet temperature
200 to 280 °C
Ensure flash vaporization
Reduce if degradation occurs
Use higher for dirty samples and higher-boiling solutes
Inlet liner
Large volume, >0.8 mL
Use with autoinjectors
Small volume, <0.7 mL
Use only for slow manual injections
None
Use only with slow manual injections
Decreases degradation
Silanized glass wool
Use for fast autoinjection and dirty samples
Injection volume
0.5 to 3 µL liquid
Depends on solvent, liner, and conditions
Refer to Appendix A
Injection technique
Fast autoinjection
Most reproducible
Less needle discrimination
Hot needle slow manual
Inject 1 to 2 µL/s if narrow liner used and >1 µL injection
Hot needle fast manual
Use for <1 µL injections
Purge flow
20 to 50 mL/min
Not critical
Purge delay time
20 to 80 s
Adjust according to inlet and sample conditions
Oven temperature
BP solvent –25 °C
Necessary for solvent focusing
See Table 7 on page 76
Column flow
> 2 mL/min when
possible
Clears inlet fast
Reduces backflash and decomposition
Septum purge
1 to 5 mL/min
Reduce ghosting
Quantification
Internal standard
Maximizes reproducibility
Standard addition
Use only with constant injection volume
1 to 3 m, deactivated
Reduces peak distortion
(1 to 2 m per µL injected)
Promotes solvent and stationary phase focusing
Inlet packing
Retention gap
84
Introduction to GC Inlets
Splitless Inlets
Table 9
7
Factors affecting splitless accuracy and reproducibility
Symptom
Possible cause
Solution
Lost peaks, Skewed peaks,
Artifact peaks (degradation)
Inlet too hot
Reduce temperature 50 °C
Active packing
Remove or minimize packing
Active liner
Change liner
Deactivate liner
Liner too small
Use larger volume liner
Long residence time
Increase column flow rate
No solvent effect
Reduce oven temperature
Use higher-boiling solvent
No stationary phase focusing
Reduce initial oven temperature
Split peaks
Solvent/column not compatible
Use different solvent
Use retention gap
Area reproducibility
Flashback
Reduce injection volume
Use higher-boiling solvent
Use larger liner
Purge time or flow variability
Check purge on/off times
Inaccurate purge delay
Check and correct
Incompatible solvent
Use retention gap
Wide peaks
Retention time reproducibility
Introduction to GC Inlets
85
7
86
Splitless Inlets
Introduction to GC Inlets
GC Inlets An Introduction
8
Cool On-Column Inlets
Overview 88
Inlet Design 89
Sample Considerations 92
Secondary cooling 92
Retention Gaps 94
Temperature 95
Flow Rates 95
Troubleshooting 96
Summary 97
Agilent Technologies
87
8
Cool On-Column Inlets
Overview
Cool on-column injection is superior in many ways to other sample
introduction techniques. Its advantages can be summarized as follows:
• Elimination of sample discrimination
• Elimination of sample alteration
• Solvent focusing of early eluting solutes
• High analytical precision
If done properly, cool on-column injection provides the most accurate and
precise results of the available inlets. Syringe discrimination is completely
eliminated. Moreover, inlet-related discrimination does not occur, since the
liquid is introduced directly into the column. Automated on-column injection
provides even higher analytical precision. Add to this the elimination of
thermal decomposition and rearrangement reactions, and it becomes apparent
that cool on-column injection should be considered whenever high precision
and accurate results are required.
Even though cool on-column inlets offer very precise and accurate sample
introduction there are several important restrictions associated with this
technique:
• Maximum sample volumes are smaller compared with other inlets.
• Solute peaks eluting just before the solvent cannot be focused and are
difficult to determine.
• Capillary columns (especially those with a large phase ratio or small inner
diameter) can easily be overloaded with sample.
• Parameters such as initial column temperature, solvent nature, and
injection rate must often be optimized.
In addition, since the sample is directly deposited into the column, nonvolatile
sample components can accumulate at the head of the column and will
degrade efficiency and/or interact with subsequent injections. Another
disadvantage of on-column injection is the potential awkwardness of sample
introduction (varies with inlet design).
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Introduction to GC Inlets
Cool On-Column Inlets
8
Inlet Design
Cool on-column injection may be done manually into most capillary columns
with internal diameters greater than 0.2 mm, or automatically into wide-bore
(>0.3 mm id) capillary columns. Inlet selection and use depend, therefore, on
whether automatic injection is required.
For manual injections, a syringe with a fused-silica needle (essentially a
narrow od capillary column) is used to introduce the sample into the
analytical column. The basic requirements of a manual cool on-column inlet
are that it guide the delicate needle into the capillary column, provide a
pressure seal around the needle during injection, and have good thermal
control for heating and cooling.
A simple manual on-column inlet design is shown in Figure 27. The inlet has a
low thermal mass which facilitates cooling and heating. A duckbill valve
provides the pneumatic seal, and septum purge minimizes ghosting. The
duckbill valve, made of a soft elastomer, is a passive element that consists of
two surfaces pressed together and sealed by the column inlet pressure
(Figure 28).
Introduction to GC Inlets
89
8
Cool On-Column Inlets
Cool tower
needle guide
Duckbill valve
(isolation valve)
Septum
Spring
Septum purge out
GC
Insert
Cryogenic cooling
(optional)
Heater block
Carrier in
Column
Figure 27
90
Cross-section of cool on-column injector applying a duckbill valve. For automated injections the duckbill valve is replaced by a disc septum and septum
nut.
Introduction to GC Inlets
Cool On-Column Inlets
Figure 28
8
A cross-section of a “duckbill” valve used to isolate the chromatographic
system. The arrows show how the inlet pressure serves to seal the valve.
To inject a sample manually with the inlet shown in Figure 27, the required
amount of sample is withdrawn from a sample vial using a syringe equipped
with a fused silica needle (105 mm long, 0.14 mm od). Excess sample is wiped
from the outside of the needle prior to injection. A needle guide is depressed
and parts the surfaces of the duckbill valve, preventing contact between the
fused-silica needle and the valve. The needle is then pushed through the
needle guide and into the column. The needle guide prevents contamination of
the duckbill valve by the syringe.
Once the needle is well into the column, the needle guide is released; this
withdraws the needle guide from the duckbill valve and tightens the valve
against the syringe needle. The syringe plunger is then rapidly depressed,
injecting sample, and the syringe is immediately withdrawn. For several
seconds after injection, the liquid migrates and forms a stable film (flooded
zone). Then the inlet and oven temperatures are increased to initiate
chromatography.
When automatic cool on-column injection is required (multiple samples,
higher precision), an autoinjector and a standard syringe (stainless steel
needle) are used. Injection into wide-bore capillaries (id >0.5 mm) can be done
easily with a 26-gauge syringe needle, whereas injection into a 0.3-mm column
requires a 32-gauge needle. The narrower needles are more sensitive to
injector alignment and may require special sample-vial septa and inlet septa to
prevent bending.
Introduction to GC Inlets
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8
Cool On-Column Inlets
For automatic injection with the inlet shown in Figure 27, the duckbill valve is
replaced by a septum and nut. The required pressure seal is maintained by the
septum, as with other septum-equipped inlets. The sequence of events for
automated cool on-column injection is autosampler dependent but usually is
the same as with other inlet techniques. Autoinjection into narrow-bore
columns is accomplished by using wide-bore retention gaps which are
butt-connected to the narrow columns with reducing unions.
Secondary cooling
An alternative cool on-column inlet design to the one shown in Figure 27
extends cooling of the column outside the inlet into the GC oven. To
accomplish this, the first 30 to 100 cm of the analytical column run through a
metal sleeve through which cool gas is purged (secondary cooling). The sample
is injected manually through the inlet into the cooled portion of the column.
Immediately following injection, secondary cooling is shut off automatically
and the column heats to oven temperature.
With secondary cooling, a higher oven temperature can be maintained than
with other inlet designs. As long as the sample flooded zone does not extend
into the hot portion of the column, oven temperature can be maintained above
the boiling point of the solvent. This capability decreases analysis time and
increases sample throughput for appropriate applications because
temperature programs and instrument cool-down periods are decreased. This
benefit, however, becomes less significant as the variance diminishes between
the boiling point of the solvent and the elution temperature for the first peak
of interest.
Sample Considerations
Sample preparation is important for on-column injection because of the
potential for column overload, column contamination, incompatibility of the
solvent with the stationary phase, and the dependence of the initial column
temperature on the boiling point of the solvent. Many of the problems
associated with these variables can be resolved by using a retention gap ahead
of the analytical column.
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Introduction to GC Inlets
Cool On-Column Inlets
8
Cool on-column injection is restricted to small sample sizes in the range of 0.5
to 2 µL. The ideal volume depends on the column id, the compatibility of the
sample solvent and the stationary phase, sample concentration, stationary
phase film thickness, and column flow rate. Usually the smaller the sample the
better, providing that sensitivity requirements are met.
To introduce sample properly by cool on-column injection, the syringe plunger
should be pressed as fast as possible to prevent sample from adhering to the
needle (Figure 29). With fast injection (most effectively accomplished using
fast autoinjectors), the sample is sprayed into the column, away from the
needle, so reproducibility is increased and no discrimination or loss occurs.
Slow injection
Fast injection
Needle
Oven wall
Column
Figure 29
This diagram shows that by rapidly injecting the sample, the possibility of
sample “coating” the outside of the needle is eliminated. The sample is
condensed on the column at a point well away from the needle.
If injection volume is too large, or if the column flow rate is too slow, sample
may back out of the column and be lost through septum purge lines or around
the syringe needle. Excessive sample volume can also lead to peak distortion
or splitting (Figure 30). This can sometimes be corrected by using a retention
gap.
Introduction to GC Inlets
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8
Cool On-Column Inlets
Volume, µL
0.5
Peak width, s 1.26
Figure 30
1
1.20
2
1.50
4
–
8
–
Peak width as a function of injection volume showing the effects of band
broadening in space. The solute is dodecane and the solvent hexane. Column:
25 m × 0.31 mm SE-54. Hydrogen carrier gas at 44 cm/s. Oven profile: 60 to
320 °C at 15 °C/min.
Retention Gaps
There are no liners for cool on-column inlets because the sample is deposited
directly into the column. However, when coupled with on-column injection,
retention gaps serve many useful functions, including:
• Protection of the column from nonvolatile or reactive sample components
• Peak narrowing by containment of the flooded zone
• Serving as an interface for coupling narrow-bore capillary columns to
automated on-column injectors
• Serving as an interface for multidimensional chromatographs (LC/GC)
Retention gaps should be deactivated to reduce decomposition and peak
broadening. The length of the retention gap is dependent on the type and
volume of the solvent being injected. The more compatible the solvent and
retention gap, the shorter the retention gap can be (for example, a 30-cm gap
may be required per µL of injected hexane versus a 2-m gap per µL of injected
methanol).
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Introduction to GC Inlets
Cool On-Column Inlets
8
Temperature
Selection of the appropriate temperature program for cool on-column inlets is
important for obtaining good results. The inlet temperature during injection
should be at or below the boiling point of the solvent and/or major sample
components. Since the sample will be dispersed in a flooded zone, which can
extend into the column oven zone, the temperature of the analytical column
(or the initial portion of the column) should be equal to or below the starting
inlet temperature.
After the time necessary to create a stable flooded zone, the inlet temperature
should be raised, at least as fast as the oven temperature, to ensure that the
sample is transferred out of the inlet zone to the oven zone in a narrow band.
In some cases, a faster inlet temperature program may be used to narrow
high-boiling components by quickly moving them from the inlet to the cooler
column where stationary phase focusing can occur. With cool on-column
injection, there is no benefit to inlet temperature programs that lag oven
temperature.
Flow Rates
Column flow is set by adjusting column head pressure. High flow velocities
(30 to 50 cm/s) are recommended with cool on-column injections to ensure
that the sample is quickly carried away from the syringe into the column.
There is no split (the inlet purge) flow to be set; however, in septum-equipped
inlets, there may be an adjustable septum purge that should be set between
5 and 10 mL/min.
Introduction to GC Inlets
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8
Cool On-Column Inlets
Troubleshooting
The major problems found with cool on-column injection are associated with
column overload, solvent/stationary phase incompatibility, and column
contamination.
If the flooded zone after injection is too long (large injections, poor
wettability), peaks will be broad or split. A retention gap usually will resolve
this problem.
Loss of column efficiency with on-column injection usually is caused by
contamination or degradation of the stationary phase at the head of the
column. Only columns with an immobilized stationary phase should be used
with cool on-column injection to prevent displacement of the stationary phase
by solvents.
Immobilized stationary phases can be washed to remove contaminants and
renew performance. The column should be removed from the GC and
backwashed with a series of solvents, finishing with a volatile solvent like
pentane. If column performance does not improve after washing, cut 0.5 m off
the inlet side of the column. If that does not return column performance, the
column must be replaced and a retention gap should be used for all further
injections of dirty samples.
Sample degradation can occur with cool on-column injection if column or
retention gap activity is high. Use only well-deactivated retention gaps and
good quality capillary columns.
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Introduction to GC Inlets
Cool On-Column Inlets
8
Summary
Table 10
Standard cool on-column practice and procedures
Parameter
Selection/Setting
Rationale
Initial inlet temperature
≤ BP of the solvent
Ensures liquid injection
Initial time
0.1 min
Allows formation of a stable film
Initial inlet temperature
ramp
Same as oven (oven track)
Simple and effective
Faster than oven
Narrows initial peak width
Injection volume
0.1 to 2.0 µL liquid
Use smaller injection for small id columns
Depends on column capacity
Injection technique
Fast autoinjection
Projects droplets away from syringe tip
Fused-silica needle
Use for manual injection into small id columns
Oven temperature
Inlet temperature or slightly lower
Prevent backflash
Column flow
50 to 80 cm/s
Use for hydrogen carrier gas
30 to 50 cm/s
Use for helium carrier gas
Septum purge
5 to 10 mL/min
Use if installed to prevent ghosting
Quantification
All methods
Inherently reproducible technique
Lack of discrimination
Retention gap requirements
1 to 3 m, deactivated
Corrects peak distortion
Protects column from nonvolatile components
Permits autoinjection with narrow-bore columns
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8
98
Cool On-Column Inlets
Introduction to GC Inlets
GC Inlets An Introduction
9
Programmed- Temperature Vaporizer
(PTV) Inlets
Overview 100
Operating Modes 100
Cold split injection 100
Cold splitless injection 101
Solvent elimination split/splitless injection 102
Inlet Design 105
Sample Considerations 107
Liners 109
Temperature 109
Flow Rates 110
Troubleshooting 112
Summary 113
Agilent Technologies
99
9
Programmed- Temperature Vaporizer (PTV) Inlets
Overview
PTV inlets combine the benefits of split, splitless, and on-column inlets. The
sample is usually injected into a cool liner, so no syringe needle discrimination
can occur. Then the inlet temperature is increased to vaporize the sample. The
user programs vent times and temperatures to achieve the equivalent of split
or splitless transfer of sample vapors to the column. PTV injection is
considered the most universal sample introduction system because of its
flexibility. Depending on the mode of injection, PTV advantages include:
• No syringe-needle discrimination
• Minimal inlet discrimination
• No special syringe needed
• Use of large injection volumes
• Removal of solvent and low boiling components
• Trapping of nonvolatile components in liner
• Split or splitless operation
• Retention time and area reproducibilities approaching cool on-column
injection
• Cold trapping of gas injections (for example, from valves, headspace, and
purge and trap autosamplers)
Operating Modes
The three most important modes of PTV operation are cold split injection, cold
splitless injection, and solvent elimination injection.
Cold split injection
Cold split injection is useful for general analysis and sample screening. In cold
split injection, the liquid plug is introduced into a cold vaporizing chamber.
This prevents syringe fractionation (discrimination), and the sample volume
can be more reproducibly introduced than in classical split injection.
After the syringe is withdrawn, the split vent is opened and the inlet is heated.
All sample vapors are then split between vent and column flow paths in a
manner similar to that which occurs with conventional split inlets with one
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Introduction to GC Inlets
Programmed- Temperature Vaporizer (PTV) Inlets
9
exception. The sample is not vaporized instantaneously; evaporation of
solvent and solutes occurs in the order of their boiling points. Therefore,
sample components reach the column sequentially and the amount of sample
at the head of the column directly after injection is smaller than the amount
found with flash vaporization inlets. This permits the injection of larger
sample volumes before loss in column efficiency is experienced. It also
provides more accurate and reproducible sample splitting since there is
minimum pressure and flow perturbation within the inlet during sample
transfer.
Cold splitless injection
Cold PTV splitless injection is used for trace analysis, as is conventional
splitless injection, but it has the advantage of lower sample discrimination and
decomposition. In cold splitless injection, the split vent is closed during
injection of the sample into the cool inlet liner. The inlet is then heated and the
sample is transferred to the column, which is maintained at a low temperature
(analogous to conventional splitless injection), to recondense the solvent for
solvent focusing. After a preselected time (30 to 90 s), the split line is activated
to vent residual vapors from the glass liner as is done with conventional
splitless inlets.
In general, larger sample volumes can be introduced with PTV splitless inlets
compared with conventional splitless inlets with similar liner volumes. During
the progressive evaporation of the sample, vapors are removed efficiently from
the liner, minimizing flashback.
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Programmed- Temperature Vaporizer (PTV) Inlets
Figure 31 compares the chromatograms obtained using cold splitless and cold
split PTV injections of the same sample. The relative peak heights are the same
for the two PTV modes, demonstrating that there is negligible discrimination
with these injection techniques.
Figure 31
Example 1
Gas Chromatogram of a Grob Test
in Diethyl Ether and Cyclohexane
Example 2
Gas Chromatogram of a Grob Test
in Diethyl Ether and Cyclohexane
25 m of OV 1701 glass, 0.6 bars of N2
Oven: 40 to 180 °C at 10 °C/min
Injector: 10 to 220 °C in 33 s
Detector: 260 °C
without splitting
25 m of OV 1701 glass, 0.6 bars of N2
Oven: 40 to 180 °C, 10 °C/min
Injector: 10 to 220 °C in 33 s
Detector: 260 °C
Splitting ratio: 1:35
Chromatogram of a Grob test in diethyl ether and cyclohexane with and without splitting, Courtesy of
Gerstel GmbH.
Solvent elimination split/splitless injection
Solvent elimination injection is used to selectively remove solvent from the
sample to allow injection of larger sample volumes or to concentrate dilute
samples for higher sensitivity. Liquid sample sizes in excess of 200 µL have
been injected with this technique with good analytical precision. The
procedure is equally effective using a single large injection or several smaller
injections. The maximum permissible sample volume is a function of liner
volume, inlet temperature, and flow rates.
For solvent elimination injection, the sample is introduced into the inlet under
the following conditions:
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Introduction to GC Inlets
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Programmed- Temperature Vaporizer (PTV) Inlets
• The split vent (plus solvent vent line, if present) is off.
• The inlet temperature is close to, but below, the boiling point of the solvent.
• The syringe plunger is depressed slowly to prevent flashback.
After injection, vent flows are turned on. Total vent flow rate is high (up to
1000 mL/min) to remove solvent vapors efficiently. Inlet temperature can be
increased slightly to aid in solvent evaporation. After most of the solvent
vapors have vented through the split line, the split line can be closed (solvent
elimination splitless mode) or open (solvent elimination split mode) as the
inlet temperature is raised.
Solvent-elimination splitless mode improves analytical sensitivity by
maximizing the amount of higher-boiling solutes reaching the column while
minimizing the initial sample load on the column.
Solvent-elimination split mode permits injection of larger sample volumes and
removal of solvent and low boiling sample components. However, this mode is
rarely used since the benefit of sample concentration by solvent elimination is
negated by splitting of the sample during inlet heat-up.
There is a significant pitfall to solvent elimination injection—loss of volatile
sample components that are vented with the solvent. The applicability of the
technique is usually restricted, therefore, to the analysis of compounds with
low volatility.
Figure 32 illustrates the loss of volatile sample components with the solvent
during solvent venting. In this example, the solvent was vented after six 1-µL
injections of a mixture of C13to C20 alkanes in hexane (concentration 5 ppm).
The resulting chromatogram shows that significant amounts of C13 through C16
are lost with the solvent, while negligible loss is observed for the later eluting
compounds.
Introduction to GC Inlets
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9
Programmed- Temperature Vaporizer (PTV) Inlets
25 m Ultra2, FS
Oven
0.7 bar helium
40 °C constant 0.5 min
40 to 250 °C at 50 °C/min
250 to 330 °C at 15 °C/min
330 °C constant 1.5 min
No. of injections
before heat-up
6
Heat-up only takes place after last injection
Injector
10 to 330 °C at 12 °C/s
Solvent venting
30 s
Detector
FID, 300 °C
Split ratio
x:30, 30 s
splitless
Figure 32
The multiple injection technique allows concenmtration of the compounds above C16 uniformly
without distortion.
The analysis of compounds with medium volatility is feasible but requires
packing of the insert with adsorbents such as Tenax, activated charcoal, or a
porous polymer. With adsorbents, however, the temperatures required to desorb
the sample to the column after solvent elimination can be very high (300 to
350 °C). At these temperatures, decomposition has been observed, limiting the
applicability of this approach.
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Introduction to GC Inlets
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Programmed- Temperature Vaporizer (PTV) Inlets
Excellent results already have been demonstrated using the various PTV
modes, but more investigation is necessary to establish the full range of
possibilities for this technique. Its power lies mainly in the capability to
“program” the injection port temperature in conjunction with liner selection
and split vent timing.
Inlet Design
Different configurations of PTV inlets are commercially available and offer
varying degrees of flexibility in injection modes and use.
Figure 33 is a schematic of one PTV injector that looks very similar to a
conventional split/splitless inlet. In comparison to a conventional
split/splitless inlet, the PTV has:
• Lower thermal mass
• Rapid heating and cooling capabilities
• Lower internal volume
Introduction to GC Inlets
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9
Programmed- Temperature Vaporizer (PTV) Inlets
• Multiple timing of split-vent and inlet heating cycles
Septum
Septum purge
Carrier gas
Seal
Heating coil
Glass wool
Cooling device
Insert
(vaporization chamber)
Split line
Capillary column
Figure 33
Schematic diagram of a PTV injector
The inlet diagrammed in Figure 33 consists of a 5 to 8 cm-long, 0.2 cm od,
0.15-cm id glass liner, packed with silanized glass wool. The carrier gas flow
into the inlet and around the liner is similar to that of a conventional split
inlet (Chapter 6).
PTV inlets are actively cooled before and during injection by Peltier devices or
by forced gases (air, liquid N2, or liquid CO2). Cryogenic cooling of the inlet can
reduce inlet temperature enough to thermally focus gas injections from other
sampling devices in the liner. This is a distinct advantage of using PTV inlets in
comparison to conventional inlets for coupling auxiliary sampling devices to
capillary columns.
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Introduction to GC Inlets
9
Programmed- Temperature Vaporizer (PTV) Inlets
Postinjection, PTV inlets are heated using electrical heaters or preheated
compressed air. Depending on design, inlet temperature ramps are either
ballistic (ramped to the maximum temperature at an uncontrolled maximum
rate) or programmable.
Sample Considerations
Sample screening and general analyses are best done using cold split PTV
injection.
Trace analysis is best done by cold splitless PTV injection, unless only late
eluting compounds are important. In that case, solvent elimination splitless
injection may be useful since larger sample volumes can be injected without
overloading the column. Solvent elimination modes should not be considered
for samples that have early-eluting peaks of interest, since they will be vented
with the solvent.
PTV sampling techniques are very useful for analyzing dirty samples. Cold
injection prevents needle discrimination, the PTV liner protects the column
from nonvolatile sample components, and more analyses can be done before
loss in performance is seen than with conventional split, splitless, or cool
on-column techniques.
Figure 34 compares repeated analyses of a milk extract by conventional
splitless and cold PTV splitless inlets. The PTV analysis still shows good peak
shape and sensitivity for the peak of interest (nitrenedipine) after
50 injections (Figure 34D); however, using the conventional splitless inlet
(Figure 34B), the peak height is significantly reduced after only five injections.
The applicability of PTV techniques to analyze thermally labile samples is
somewhere between that of vaporizing and cool on-column inlets. Since the
sample is introduced at low temperatures and then heated to a maximum
value, few sample components are exposed to the high ending temperature,
Introduction to GC Inlets
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9
Programmed- Temperature Vaporizer (PTV) Inlets
and decomposition is reduced. However, PTV inlets are still more active than
capillary columns, so the very sensitive samples should be analyzed by cool
on-column injection.
Conventional splitless injection
Injection 1
Injection 5
X
X
B
A
Cold splitless PTV injection
Injection 1
C
Figure 34
108
X
Injection 50
X
D
Comparison of repeated injections of a milk extract by conventional splitless (A, B) and cold PTV
splitless (C, D) inlets. Peak X is nitrenedipine. Courtesy of Gerstel GmbH.
Introduction to GC Inlets
9
Programmed- Temperature Vaporizer (PTV) Inlets
Liners
There are few choices in liner design for PTV inlets. However, liner volume and
activity are still key issues to be considered when selecting among the few
available PTV liners. PTV liners require packing or a modified surface to hold
the liquid sample in place before and during the vaporizing process. For labile
samples where glass wool may cause sample degradation, a baffled liner (as
shown in Figure 35) should be used to minimize activity. Deactivation of the
liner may also be required.
(Courtesy Gerstel GmbH)
Figure 35
PTV inlet tube with deformation of the cross-section
Liners should be cleaned or replaced on a regular basis and whenever a loss in
performance is seen.
The volume of liners for PTV inlets does not have to be as large as that for
conventional vaporizing inlets for the same volume of injected sample, since
the sample is not flash vaporized. Liner volume, however, is related to the
maximum volume of liquid that can be introduced and the speed of liquid
sample introduction during solvent-elimination injection.
Temperature
PTV inlet temperature is dependent upon the mode of injection. Cold
split/splitless injection requires that the inlet temperature be below the
boiling point of the solvent. Inlet temperature during solvent elimination
injection should be set nearer to the boiling point of the solvent so that solvent
is selectively evaporated.
After injection of the sample into the liner, the inlet temperature program
should be fast enough to transfer sharp peaks to the column and should end at
a temperature high enough to evaporate all sample components of interest.
Introduction to GC Inlets
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Programmed- Temperature Vaporizer (PTV) Inlets
The inlet temperature should always be above the column temperature.
Excessively fast inlet temperature ramp rates, however, can lead to flashback
or column overload if the sample volume is large.
Flow Rates
The mode of PTV injection also dictates inlet timing and flow rates. During
cold split and splitless injection, the split flow is off when sample is injected
into the liner. For split injection, the split vent is then turned on and the inlet
is heated. Split flow is adjusted to give the desired split ratio, just as it is for
conventional split injection.
For cold splitless injection, the split flow remains off until all the sample has
been transferred to the column. This can take longer than it does with
conventional splitless injection, because it is necessary to wait until the inlet
reaches its final temperature. If a fast inlet temperature ramp is used, sample
transfer can also take less time than it does with conventional splitless
injection, since PTV liners have smaller volumes and are cleared of sample
vapors quickly.
When important sample components elute near the solvent tail, purge delay
should be optimized in a conventional manner (see Chapter 7). This will
ensure that 95% to 99% of the sample is transferred to the column and that the
solvent tail is sharp.
In PTV solvent elimination mode, the split vent is on during the sample
introduction step. The split flow should be high enough to clear the inlet of
solvent vapors (up to 1000 mL/min) but should be optimized with respect to
solvent boiling point and inlet temperature to prevent excessive loss of sample
components.
Table 11 summarizes the typical inlet temperature and flow conditions for the
various modes of PTV sampling.
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Introduction to GC Inlets
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Programmed- Temperature Vaporizer (PTV) Inlets
Table 11
Inlet conditions for PTV injection modes
PTV inlet mode
Cold split
Cold splitless
Solvent elimination
split
Solvent elimination
splitless
• Liner temperature
<< Solvent BP
<< Solvent BP
< Solvent BP (vapor
pressure dependent)
< Solvent BP (vapor
pressure dependent)
• Purge/Split flow
OFF
OFF
100 to 1000 mL/min
100 to 1000 mL/min
• Delay before inlet heating
NONE
NONE
5 to 30 s
5 to 30 s
Inlet condition during injection
Inlet conditions during inlet heating
• Liner temperature program
Maximum rate
To max temp in
<80 s
Maximum rate
To max temp in <80 s
• Column temperature
Solute dependent
< Solvent BP
Solute dependent
< Solvent BP
• Purge/Split flow
Split ratio
dependent
Off until final inlet
temp reached
Split ratio dependent
Off until final inlet
temp reached
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Programmed- Temperature Vaporizer (PTV) Inlets
Troubleshooting
Most problems with PTV inlets involve improper setting of inlet temperature
and/or flow parameters, and sample decomposition. The selection of liner, and
the setting of inlet temperatures and purge timing, are all critical for optimal
use of an inlet. Liner configuration (volume, packing, style) and extent of
deactivation have a major effect on sample decomposition.
In cold split PTV injection, large sample volumes and slow inlet heating will
cause slow transfer of sample from the inlet to the column and will result in
broad peak widths. However, sample degradation is more likely as the
temperature ramp rate is increased, so optimization of inlet conditions
becomes more important with labile samples.
In cold splitless PTV injection, sample degradation is more likely to occur than
with cold split injection because the sample is in contact with the liner and
liner packing material for a longer time. Therefore, high column flow rates,
slower temperature ramp rates, and deactivated liners should be used when
analyzing labile samples.
Loss of volatile sample components and flashback are the two most common
problems with solvent elimination modes of PTV injection. The solvent can be
eliminated efficiently even when the inlet temperature is below the boiling
point of the solvent. The solvent still has significant vapor pressure and is
removed slowly, preventing flash vaporization. Injecting sample slowly (1 to
2 µL/s) also reduces flashback and loss of volatiles.
All PTV injection modes can yield wide initial peak widths, so solvent and/or
stationary phase focusing is usually required. Retention gaps should also be
used to prevent peak splitting and distortion in splitless PTV injection modes.
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Programmed- Temperature Vaporizer (PTV) Inlets
Summary
Table 12
Standard PTV practice and procedures (Cold split/splitless modes)
Parameter
Selection/Setting
Rationale
Injection mode
Cold split
For general use and sample screening
Cold splitless
For trace analysis
Adjustable (2 to 12 °C/s)
Use slower ramp rates for labile, complex, or large volume
samples
Use faster ramp rates for most samples
Use faster ramp rates to shorten splitless purge delay time
Ballistic
Simple, less expensive instrumentation
Straight with silanized wool
For general use
Baffled
For labile samples
Packed with an adsorbent
For focusing gaseous injections from auxiliary sampling
devices
Injection volume
0.1 to 1.5 µL
Use lower volumes for volatile solvents and fast ramp rates
Use volumes >1.5 µL only in solvent-elimination mode
Sample injection technique
Autosampler, manual, fast
or slow
Not critical for cold split or splitless modes
Oven temperature
BP solvent – 25 °C
For proper solvent effect in splitless mode
Sample dependent
For split mode
Column flow
30 to 50 cm/s
Vlears inlet faster
Less backflash
Septum purge
1 to 5 mL/min
Minimize ghosting
Quantification
Any method
Inherently reproducible
Low discrimination in cold injection modes
Retention gap
1 to 3 m, deactivated
Compensates for extended flooded zone and
solvent-column incompatibility
Inlet temperature ramp rate
Inlet liner
Introduction to GC Inlets
113
9
Programmed- Temperature Vaporizer (PTV) Inlets
Table 13
Standard PTV practice and procedures (Solvent-elimination split/splitless modes)
Parameter
Selection/Setting
Rationale
Injection mode
Split
Accomodates large sample volumes
Eliminates low boilers
Splitless
For trace analysis where large sample volumes are
necessary
Straight with silanized wool
For general use
Packed with adsorbent
For retaining medium volatility solutes during solvent
venting
Injection volume
5 to 250 µL
Large sample volumes require repeat injections of smaller
individual volumes
Largest volumes inlet and liner dependent
Sample injection technique
Slow manual
Minimize flashback
Inlet temperature
Below solvent BP
Ensure gentle evaporation
Minimize loss of medium volatility solutes
Ramp delay
5 to 30 s after last injection
Depends on solvent volume and inlet temperature
Inlet temperature ramp rate
12 °C/s
For most samples
Ballistic
Simpler, less expensive instrumentation
Inlet liner
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Introduction to GC Inlets
9
Programmed- Temperature Vaporizer (PTV) Inlets
Table 14
Factors affecting PTV accuracy and reproducibility
Symptom
Possible cause
Solution
Lost peaks, artifact peaks
(degradation)
Active packing
Remove the packing
Active liner
Change or deactivate the liner
Liner too small
Use larger liner
Ramp temperature slower
Residence time too large
Increase column flow rate
No solvent effect
Reduce oven temperature
Use higher-boiling solvent
No stationary phase focusing
Reduce the initial column temperature
Slow sample transfer from inlet
Increase the inlet temperature ramp
Split peaks
Solvent/Column not compatible
Use different solvent
Use a retention gap
Try solvent-elimination mode if early
peaks are not important
Area reproducibility
Sample too big
Reduce injection volume
Purge time or flow variability
Check instrument and correct
Wide peaks
Introduction to GC Inlets
115
9
116
Programmed- Temperature Vaporizer (PTV) Inlets
Introduction to GC Inlets
GC Inlets An Introduction
10
Valves
Overview 118
Valve Types 118
Rotary valves 118
Slider valves 118
Valve Design 119
Rotary gas sampling valves 119
Rotary liquid sampling valves 120
Slider valves 120
Valve connection to packed columns 121
Valve connection to capillary split inlets 123
Sample Considerations 123
Selecting Sample Loop Volume 124
Temperature 124
Gas valves 125
Liquid sampling valves 125
Transfer lines 125
Columns 125
Flow Rates 125
Troubleshooting 127
Summary 129
Agilent Technologies
117
10 Valves
Overview
Sampling valves are simple mechanical devices that introduce a sample of
fixed size into the carrier gas stream. Sampling valves are most frequently
used to sample gases or liquids in constant-flowing streams such as those in
chemical reactors, chemical transfer lines, high-pressure liquid natural gas
processing, waste effluent streams, fermenters, and petroleum distillation
towers. Valves can be coupled directly to the chromatographic column or in
series with a packed-column direct inlet or a capillary split inlet.
Gas sampling valves must be appropriately thermostated to achieve accurate
and reproducible injection volumes and to prevent condensation of gas
samples. For thermostating, heated valve ovens or “boxes” are used; or the
valve can be positioned inside the GC oven with the column.
Liquid sampling valves have lower sample volumes than gas sampling valves
and require that the sample:
• Remain a liquid while filling the sample loop
• Expand quickly into a gas when switched into the carrier gas stream
Valve Types
Rotary valves
Rotary valves are the type of sampling valves used most frequently for gas and
liquid sampling. They rotate in one direction to load the sample and then in
the opposite direction to inject the sample. Rotary valves are very robust and
can withstand high pressures and temperatures. These valves are rotated by
hand, air-pressure, or electric actuators. The actuators are activated by an
electrical signal from a timing device such as an integrator, a data system, a
timer, or the GC itself.
Slider valves
Another type of valve frequently used for gas sampling is the slider (diaphram)
valve, which tends to switch faster than the rotary valve, has lower internal
volume, and may have longer lifetime. These features are desirable for
high-resolution capillary chro- matography; however, use of slider valves is
somewhat limited because they cannot be used reliably above 150 °C.
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Introduction to GC Inlets
Valves
10
Valve Design
With sampling valves, the sample is contained in a “loop,” which is either an
attached length of tubing (gas sampling valves) or an etched groove in the
valve rotor itself (liquid sampling valves).
Rotary gas sampling valves
Figure 36A is a diagram of a rotary gas sampling valve in the load position.
Gaseous sample flows through the inlet line into the valve, through the sample
loop, and out again through the sample drain line.
For injection (Figure 36B), the valve rotor is turned and the sample becomes
part of the carrier gas stream flowing into the column.
To vary the sample volume, the sample loop is replaced with tubing of
appropriate volume (achieved by varying tubing id and length). Typical gas
sample volumes are between 0.25 and 1.0 mL.
A
B
Carrier
4
3
Column
Carrier
or
2nd valve
4
5
5
2
6
1
Column
or
2nd valve
6
3
2
1
Loop
Sample in
Drain
Load
Figure 36
Inject
Rotary gas sampling valve
Introduction to GC Inlets
119
10 Valves
Rotary liquid sampling valves
Figure 37A is a diagram of a rotary liquid sampling valve in the load position.
In a manner analogous to gas sampling valves, liquid sample flows through the
sample inlet line into the valve and out through the drain line. The liquid
sample volume is usually less than 5 µL. In contrast to gas sampling valves,
however, the sample is contained in a groove in the valve rotor (internal loop);
and a restrictor is attached to the sample drain line. The restrictor keeps the
sample liquified within the valve. When the valve is rotated (Figure 37B), the
compressed liquid expands into a gas and is swept into the column by carrier
gas.
A
B
Carrier
Column
Sample in
Drain
S
Restrictor
P
W
Column
S
C
Load
Figure 37
Carrier
P
W
C
Inject
Rotary liquid sampling valve
Slider valves
Slider (diaphram) valves provide the same function and generally operate in
the same manner as rotary sample valves; however, they are designed to give
narrower initial peak widths and faster switching times than rotary valves.
There are fewer slider valves in use than rotary valves because they have not
been available as long and cannot be used at temperatures and pressures as
high as rotary valves can.
Slider sampling valves operate on the same principle as rotary valves, except
the active hardware component slides instead of turns. The distance the slider
travels is usually small compared with rotary valves, and the energy required
to move the slider from one position to the other is low, so that these valves
switch very quickly. In addition, slider valves have lower internal volumes
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Introduction to GC Inlets
10
Valves
than rotary valves, so that injected peaks are not broadened while passing
through channels in the valve. The combination of fast switching speed and
low internal volume makes slider valves more suitable for high-resolution
analyses than rotary valves.
Valve connection to packed columns
Valves can be connected directly to packed columns or connected via an
intermediate transfer line. The carrier gas flow to the valve comes from a mass
flow controller or from a packed-column direct inlet. The main benefit of
connecting valves in series with GC inlets is that the analyst can use both the
inlets and the valves without replumbing the chromatographic system.
Figure 38A illustrates a typical series-type connection of a gas sampling valve
to a packed column direct inlet. The connection of a liquid sampling valve
would be the same except that the valve would not be mounted in a heated
valve box. Carrier gas is diverted from the base of the inlet to the valve with
small id empty tubing. Another piece of empty tubing connects the outlet of
the valve to the column in the GC oven. This tubing should either be traced
with heating tape or enclosed in a larger piece of aluminum tubing (to conduct
heat from the valve box) to prevent condensation of sample on cold spots
between the valve and the GC oven.
Introduction to GC Inlets
121
10 Valves
A
Sample
Carrier in,
30 mL/min
30 mL/min
Drain
Heated valve box
GC oven
30 mL/min
B
Packed column
30 mL/min
carrier
Capillary inlet
Sample
Septum purge,
3 mL/min
Split vent,
25 mL/min
Drain
Heated valve box
GC oven
2 mL/min
Capillary column
Figure 38
122
In-series connection of gas sampling valves and inlets to packed column inlet (A) and split inlet (B).
Introduction to GC Inlets
10
Valves
Valve connection to capillary split inlets
When connecting valves to capillary columns (Figure 38B), a split inlet must
be used to keep initial peak widths narrow. In this configuration, carrier gas
flows to the valve first and then to the split inlet. There are several advantages
to this:
• Liquid samples can be injected directly into the inlet without going through
the valve and transfer lines.
• A high total flow rate (split flow + column flow) from the valve is
maintained so that initial peak widths are narrow.
• The sample is split to avoid capillary column overload.
• The split ratio can be adjusted easily in response to sample load, column
changes, and sensitivity requirements.
The transfer line from the valve to the inlet should be heated to prevent
sample condensation and ghosting.
Sample Considerations
The physical state of a sample dictates whether a liquid or a gas sampling valve
should be used. If a gas stream is to be monitored, a gas sampling valve is used,
and all transfer lines and the valve are heated as required to prevent
condensation or adsorption of the sample on the tubing.
If the stream to be sampled is a liquid, a liquid sampling valve is frequently
used, and valve temperature and outlet restrictor are selected to maintain the
sample in the liquid state while loading the sample loop. A prerequisite for the
use of liquid sampling valves is that all sample components are quickly
vaporized when the valve rotates and the loop pressure drops to the column
head pressure. Sample components that do not flash vaporize will flood
transfer lines, will cause ghosting, and will interfere with subsequent
injections.
If the liquid sample does not vaporize quickly when reduced to column
pressure, the sample should be vaporized before reaching the valve. In this
case, a gas sampling valve is used. The valve and transfer lines (in and out) are
heated enough to evaporate the liquid sample before it reaches the valve and
to keep it gaseous all the way to the column.
Introduction to GC Inlets
123
10 Valves
Higher boiling or very polar solutes (acidic or basic) may stick to sample lines
and valve rotor (polyimide) surfaces. For those samples, nickel or Hastelloy
connecting tubing should be used and the sample lines heated to reduce tailing
of solutes. Also, it would be helpful to use either a valve with a PTFE rotor
(low temperature valve), which is less adsorptive, or a slider valve, which has
low surface area exposed to the sample.
Selecting Sample Loop Volume
One microliter of liquid expands several hundred times when evaporated.
Therefore, gas sample volumes are typically 200 to 1000 times the volume used
for liquid injections. Sample volumes are usually 250 µL for gases and 0.25 µL
for liquids when using a split inlet and capillary columns. For packed columns,
a 1-mL gas sample or 1-µL liquid sample size is typical.
Changing sample loops (or rotors, in the case of liquid sampling valves) is time
consuming, so initial experiments are usually done to determine the required
sample volume for the column, inlet, and sample being used. After the sample
size has been fixed, the split ratio or loop pressure can be adjusted to alter
sample amount as analysis requirements change.
Temperature
Elevated valve temperature is important to guarantee total transfer of the
sample to the column, to minimize adsorption of sample to transfer lines and
valve components, and to maximize reproducibility.
Rotary gas sampling valves are available in both low- and high-temperature
versions (175 °C versus 325 °C), depending on the temperature necessary for
the sample. The rotors of low-temperature valves are primarily PTFE; they
rotate freely and have low adsorptivity toward polar compounds but will leak
and ghost at high temperatures. The rotors in high-temperature valves are
usually polyimide or graphite/polyimide and can withstand higher
temperatures. However, they are more adsorptive toward polar compounds and
may leak or seize at temperatures below 150 °C.
Liquid sampling valves are designed to operate below 75 °C.
Slider valves usually have PTFE sliders (the counterpart to rotors) and are,
therefore, limited to temperatures below 175 °C.
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Introduction to GC Inlets
Valves
10
Gas valves
Heated gas sampling valves are usually placed in valve boxes (compartments)
outside the GC oven so that they can be independently thermostated. Valve
temperature should be high enough to ensure rapid and complete transfer of
sample to the column or inlet, but not so high that sample decomposition
occurs.
The amount of sample contained in the sample loop is inversely proportional
to the valve temperature. As the temperature increases, the sample amount
decreases, as does sample load on the column and analysis sensitivity. This is
why thermostating of the valve is so important for reproducible analysis.
Liquid sampling valves
Liquid sampling valves are usually mounted on the outside of the GC and are
not thermostated. If there is problem with sample vaporization, liquid
sampling valves can also be mounted in a heated valve box.
Transfer lines
Sample lines going in and out of the valve should be heated above the boiling
point of the highest boiling sample component to prevent sample loss,
adsorption, and ghosting.
Columns
The initial peak widths from valve injections are often broad, and they are
difficult to focus because the compounds are volatile. To get appropriate
focusing of these volatile solutes, both stationary phase focusing, using low
phase ratio columns and low initial column temperature, and thermal focusing
are used.
Flow Rates
Peak broadening increases with increases in the following:
• Sample volume
• Volume of the connecting tubing
• Inlet volume
Introduction to GC Inlets
125
10 Valves
The larger the volume between the valve and the column, the more the sample
will be diluted and the wider the initial peak width. This phenomenon
becomes less important as the carrier gas flow through the valve increases.
Most rotary valves are designed to transfer sample to the column efficiently if
carrier flow rate is above 20 mL/min, which is why sampling valves work so
well for packed-column analyses.
For maximum efficiency using capillary columns, the sampling valve should be
coupled to a split inlet so that a high carrier gas flow rate can be maintained
through the valve while providing the reduced flow rate required by capillary
columns. The total flow through the valve should be at least 20 mL/min, which
is then split between column, split vent, and septum purge flow paths in the
split inlet.
Depending on the chromatographic column used, split ratio will change as the
column flow rate changes. When using wide-bore columns, most of the flow
from the valve will go to the column (column flows of 10 to 15 mL/min), and
when using narrow-bore columns (0.2 mm), most of the flow will be vented out
the split vent.
Whenever sampling valves are coupled through inlets, it is helpful to use inlet
liners with the smallest internal volume. This minimizes peak broadening
caused by dilution of the sample as it passes through the inlet. Remember,
however, that these inlets are then prone to flashback if liquid samples are
introduced by syringe.
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Introduction to GC Inlets
10
Valves
Troubleshooting
Most of the problems associated with sampling valves are related to peak
broadening in transfer lines and inlets, sample adsorption to the valve or
transfer lines, leaks, and perturbations in the baseline.
Peak broadening and tailing
Voids in the flow system (valve and connecting tubing) cause tailing and peak
broadening. Use inlets and liners with small internal diameters and connect
the valve to the inlet or column with short lengths of connecting tubing of
narrow inner diameter.
If the width of early-eluting peaks is too broad, stationary phase or thermal
focusing effects should be used with packed-column ports or increased split
flow when capillary split inlets are used. Inlets should be equipped with
narrow inner diameter liners, and narrow-bore connecting tubing should be
used between the valve and inlet.
Baseline shifts
Baseline perturbations are caused by changes in column flow as the valve is
rotated and as the sample loop equilibrates to system pressure. Slow valve
rotation momentarily stops carrier gas flow; and when the valve stops
rotating, a sudden increase in flow occurs which slowly returns to the set
point. Check actuator pressure (usually 40 to 75 psi), valve rotor tension, and
valve temperature to ensure that the valve rotates as quickly as possible. A
restrictor or backpressure regulator can be added to the sample vent line to
maintain the sample loop at system pressure. This will reduce the time it takes
for the flow to stabilize after the valve is switched.
Variation in peak area and retention time
The amount of sample contained in the loop and, therefore, the amount
injected onto the column is proportional to loop pressure and temperature.
Variations in pressure and temperature leads to variability in peak areas. Flow
restrictors or back -pressure regulators help to maintain constant loop
pressure, and valve boxes help maintain temperature.
Introduction to GC Inlets
127
10 Valves
Leaks can occur in the valve itself or at any of the connecting points with
transfer lines. Leaks usually cause area irreproducibility, retention time
changes, and increases in the area of air peaks (with thermal conductivity
detectors). Leaks in rotors can sometimes be fixed by tightening the nuts
holding the rotor in the valve body. Leaks in connections are usually found
with an electronic leak detector or with a liquid leak detection fluid such as
Snoop.
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Introduction to GC Inlets
Valves
10
Summary
Table 15
Standard valve practice and procedures
Parameter
Selection/Setting
Rationale
Gas sample volume
0.25 to 1 mL
Use larger loops for larger columns
Liquid sample volume
0.25 to 5 µL
Use larger volumes for larger columns
Actuator pressure
45 psi, air
Depends on valve design
Valve rotor
PTFE
Use for temperatures <150 °C
Graphite/Polyimide
Use for temperatures 150 to 325 °C
BP of sample + 50 °C
Prevent condensation and tailing of gas samples
> BP of heavy component
Ensures vaporization of liquid samples
After inlet
Use for packed columns (use small liner)
Through split inlet
Use for capillary and PLOT columns
Direct to column
Use for packed columns
Use with small internal volumes (microvalves) or capillary
columns
Valve temperature
Valve connection to column
Introduction to GC Inlets
129
10 Valves
Table 16
Factors affecting valve accuracy and reproducibility
Symptom
Possible cause
Solution
Lost peaks (degradation)
Valve or transfer line too hot
Reduce temperature 50 °C, reevaluate
Transfer line reactivity
Use nickel or Hastalloy tubing
Lost or tailing peaks
Valve or transfer line too cold
Increase temperature 50 °C, reevaluate
Baseline perturbation
Valve rotation slow
Increase actuator pressure
Rotor distorted
Replace rotor
Sample/Column pressure too different
Add back-pressure regulator to sample drain
Column overload
Use smaller sample loop
Increase split flow
Flow too slow
Increase column and/or split flow
System voids
Check connections
Reduce volume of connecting tubing
Peak tailing broad peaks
130
Introduction to GC Inlets
GC Inlets An Introduction
11
Headspace Autosamplers
Overview 132
Design 133
Sample Considerations 137
Temperature 137
Flow Rates 138
Troubleshooting 140
Summary 141
Agilent Technologies
131
11 Headspace Autosamplers
Overview
Headspace autosamplers are used in gas chromatography to inject a portion of
the gas (headspace) which is in equilibrium with a sample in a sealed,
thermostated vial. Headspace sampling is useful for analyzing volatiles in dirty
samples, solid samples, samples that contain high boilers that are not of
interest, and samples with high water content. These types of samples are
encountered in most analytical fields including environmental (volatiles in soil
or water), polymer (monomers and residual solvents), foods and flavors
(aromas in foods and beverages), and pharmaceutical (residual solvents in
precursors and formulations).
Headspace analysis is a valid substitute for many of the analyses currently
being done by purge and trap autosamplers. Advantages of headspace
sampling in comparison with purge and trap include:
• Headspace instruments are less complex.
• High concentration of water in samples does not affect analyses as much.
• Adsorbent traps are not required.
• There is no chance of breakthrough (loss) of volatile sample components.
• Samples can be heated to increase the volatility of analytes.
• Repeated sampling can be done.
The basic advantage of purge and trap in comparison to headspace analysis is
slightly higher sensitivity; purgeables are quantitatively removed from the
sample during purging.
The objective in headspace (HS) analysis is to drive the desirable sample
components into the headspace for sampling while leaving the undesirable
components behind in the vial. Temperature, valve switching times, and
sample treatment are manipulated to get as close as possible to this goal.
A qualitative HS technique which makes use of repeated sampling of the same
vial is called “multiple headspace extraction.” This technique can compensate
for some of the matrix effects encountered with HS analysis and circumvents
the need to have a standard in the same matrix as the sample.
Repeated sampling of a single vial can give added information on the relative
volatility of sample components. The first sampling of a vial will be high in the
most volatile components (early eluting peaks). As these are removed, the
distribution of components in the sample changes, and the chromatograms for
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Introduction to GC Inlets
Headspace Autosamplers
11
subsequent samplings of the same vial will shift toward late eluting
components. Overall sample concentration will be lower with each repeated
sampling. This limits the number of times sampling can be repeated. However,
concentration decrease is inversely proportional to sample component
solubility (the more soluble the sample component, the smaller the decrease in
concentration per sampling). This phenomenon can be used to help
differentiate polar and nonpolar solutes in a sample.
When using a headspace autosampler with packed columns or with wide bore
capillaries and packed column inlets, the headspace sampling valve and most
of the system voids are cleared quickly with column flow rates above
10 mL/min, and the initial widths of peaks are narrow. When using columns at
flow rates less than 10 mL/min, the HS unit is coupled to the column through
a split inlet so that the flow through the HS is greater than 10 mL/min.
Since most of the compounds determined by headspace analysis are very
volatile, packed columns, or thick -film capillary columns are usually required
to achieve stationary-phase focusing. If stationary-phase focusing is not
possible, and the sensitivity requirements of the analysis prevent higher flows
from the headspace unit, thermal focusing is required (cryogenic cooling).
Design
Vial temperature, equilibration time, vial pressure, pressurization time, vent
time to the sampling valve, and flow rate influence the concentration and
distribution of volatile sample components that will reach the GC. For this
reason, quantitative headspace analyzers must provide automated control of
all of these variables. Autosamplers are usually controlled by a dedicated
control module, although control through the GC or data system is sometimes
possible.
The usual procedure for quantitative static headspace analysis is to weigh or
measure a sample, and often an internal standard, and seal it in a vial. The vial
is then:
1 Thermostated
2 Pressurized with inert gas
3 Vented through the sample loop of a gas sampling valve
Introduction to GC Inlets
133
11 Headspace Autosamplers
The gas-sampling valve is activated to inject the sample into the carrier gas
stream and into the GC column. The time at each step is programmed by the
user before the start of sampling.
Figures 39 through 42 show flow diagrams of the headspace autosampling
process. Figure 39 shows that carrier gas flow is split between a bypass to the
column and the sample loop through the probe needle. Sample equilibrates in
the vial at a preset temperature and time during this first stage of the process.
After the sample has equilibrated, the probe needle enters the vial and the vial
is pressurized up to 4 bar (Figure 40). This usually takes less than 30 s.
Carrier
gas
Loop
Vent
Figure 39
134
Standby mode—A small flow of carrier gas is purging the loop
Introduction to GC Inlets
11
Headspace Autosamplers
Carrier
gas
Loop
Vent
Figure 40
Carrier
gas
Pressurixation mode—The probe is down and the vial is pressurized to about
1.3 bar
Loop
Vent
Figure 41
Introduction to GC Inlets
Vent mode—Headspace vapor flows through the sample loop as the vial is
depressurized to atmosphere (operator-selected timing)
135
11 Headspace Autosamplers
Carrier
gas
Loop
Vent
Figure 42
Injection mode—The loop contents are swept into the GC injection port. After
a preselected time, the system returns to the standby mode (Figure 39)
As shown in Figure 41, when two valves are simultaneously switched under
automatic control, the pressurized headspace sample flows out of the vial
through the probe, the sample loop, and the vent. The loop-filling (vent) time
should be long enough so the loop is filled with sample vapors but not so long
that the vapors diffuse out of the loop. Typical vent times are between two and
ten seconds. To inject sample, flows are switched again with valves so that all
of the carrier gas flow goes through the sample loop to the column.
Headspace autosamplers can either be stand-alone units, which are physically
separate from the GC, or units integrated with the GC. In stand-alone
headspace autosamplers, the sample usually flows from the headspace unit to
the GC through a heated transfer line with a syringe needle at the end. The
syringe needle is inserted into the GC inlet just as liquid sample syringes are,
although it remains in the inlet throughout the full sampling process.
With headspace units that are integrated with the GC, the transfer line from
the sample valve can be coupled directly with packed columns. It also can be
coupled indirectly to capillary columns via a splitting device in the GC oven.
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Introduction to GC Inlets
Headspace Autosamplers
11
Sample Considerations
To achieve the most reproducible headspace data, sample amount (volume or
mass) must be measured accurately and reproducibly and sample matrix
effects (varying amounts of water, presence or absence of other components in
the sample) must be minimized. Matrix effects may be reduced by:
• Grinding solid samples
• Adjusting pH
• Saturating the water sample with salt
• Saturating an organic or solid sample with water
Sample pretreatment can often help increase the sensitivity of analysis also.
Increasing the exposed surface area of solids increases diffusion of volatiles out
of the matrix. Saturating a water sample with a salt such as Na2SO4 reduces the
solubility of organics in the water and increases their concentration in the
headspace. This is sometimes called “salting out.” Changing sample pH can
neutralize organic acids or bases in the sample and drive them into the
headspace for easier analysis. Saturating organics with water can also drive
some components into the headspace. Wetting solids, such as soils, with water
or aqueous acids or bases can decrease adsorption interactions and increase
the concentration of volatiles into the headspace.
Temperature
Temperature stability and reproducibility are critical for reproducible
analyses. Headspace analyzers control vial temperatures by a constant
temperature bath (oil, water) or an oven.
The concentration of volatile sample components in the headspace is directly
related to the temperature of the sample. Increasing the sample temperature
drives more volatiles into the headspace and increases sensitivity; however,
excessive temperature can increase interferences and can cause sample
degradation. When little is known about a sample a priori, it is best to set the
initial vial temperature low (for example, 40 °C) and run a test chromatogram. If
sensitivity is a problem, increase vial temperature by 25 °C and try again.
Transfer line and valve temperatures should be higher than the boiling point
of the highest boiling headspace component to prevent adsorption or
condensation between the autosampler and the GC.
Introduction to GC Inlets
137
11 Headspace Autosamplers
Flow Rates
When adjusting the flows of gases to the headspace unit and columns, some
flow comes from the HS unit and some from the GC. As is illustrated in
Figure 43A, for a 0.53-mm id column and packed-column direct inlet at
20 mL/min total flow, 10 to 15 mL/min should come from the HS unit and 5 to
10 mL/min from the GC flow controller (set the GC flow with the standard
mass flow controller first, then add headspace autosampler flow).
When using narrow-bore columns and split inlets, a total flow rate of
20 mL/min is typical, with 10 mL/min coming from the headspace unit and
10 mL/min from the GC (Figure 43B). The split ratio is adjusted to change
sample load on the column in response to sample and analysis requirements.
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Introduction to GC Inlets
11
Headspace Autosamplers
A
15 mL/min
5 mL/min
20 mL/min
Controller
10 mL/min
B
Septum 3 mL/min
10 mL/min
Split 15 mL/min
2 mL/min
Controller
Figure 43
Introduction to GC Inlets
Flow paths for connection of a headspace autosampler to a packed column
direct inlet (A) and a split inlet (B)
139
11 Headspace Autosamplers
Troubleshooting
The main problems with headspace sampling involve irreproducibility, lack of
sensitivity, and decomposition. Peak broadening is not much of a problem with
headspace analysis, because it can be prevented by properly setting flow rates
and using solute focusing techniques.
Since the timing of each of the sampling events in headspace analysis affects
the quantity of sample reaching the column, appropriate selection of
conditions is critical for maximum sensitivity and reproducibility. If the vial
equilibration time is too short, the concentration of volatiles in the headspace
is decreased, and sensitivity and reproducibility decrease.
If the vial pressurization time is too short, there will not be enough force to
move the sample to the valve, and sensitivity and reproducibility decrease.
Short vent times do not permit the sample to move from the vial to the sample
loop. Vent times that are too long permit sample to diffuse out of the sample
loop, reducing sensitivity and reproducibility. Injection times that are too
short result in partial injections and in lower sensitivity and reproducibility.
Ghosting and baseline perturbations can result from contamination of septum
or autosampler flow lines and from carryover of liquid sample into the sampling
needle. Transfer lines should be clean and heated sufficiently to prevent
condensation or adsorption of sample components. Samples should not occupy
more than 75% of the vial volume to prevent contact with the autosampler
needle.
Sample contamination can occur if the sample is exposed to room (laboratory)
air. Ambient air contains many organics that will adsorb/partition in samples
and will be present in the vial headspace. Compare results for samples that
were exposed for different times to test for presence of contaminants and run
a “blank” sample of room air in a sealed vial to determined contributions from
ambient air. To prevent contamination, minimize the time that the sample is
exposed to room air and/or blanket the sample with argon before sealing the
vial. Contamination is also minimized by using a purged sample preparation
enclosure for preparation, weighing, and sealing the sample in the sample vial.
140
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Headspace Autosamplers
11
Summary
Table 17
Standard headspace practice and procedures
Parameter
Selection/Setting
Rationale
Connection to GC
Through split inlet
For capillary columns
Through direct inlet
For large-bore capillary and packed columns
Direct to column
For wide-bore packed columns
Packed-column flows
Headspace flow rate
20 to 35 mL/min
Clears sample lines
GC flow
5 to 10 mL/min
Sweep inlet of vapors
Capillary split flows
Headspace flow
10 to 20 mL/min
Clears sample lines
Lower flow yields less split flow
GC flow
5 to 10 mL/min
Sweeps inlet
Split flow
10 to 25 mL/min
For wide-bore capillary (5 to 10 mL/min column flow)
10 to 30 mL/min
For small-bore capillary (1 to 5 mL/min column flow)
Injection parameters
Vial temperature
Depends on sample
Valve temperature
Bath temperature + (5 to 10 °C)
Prevents condensation
Vial equilibration time
10 to 45 min
Depends on sample type and amount
Long time for solids
Vial pressurization time
20 s
Prepares vial for venting
Vial pressure
1.2 to 1.4 bar
Higher pressure dilutes sample
Vent time
2 to 10 s
Vents vial to sample valve
Injection time
10 to 30 s
Use longer times with slower HS flow
Introduction to GC Inlets
141
11 Headspace Autosamplers
Table 18
Factors affecting headspace accuracy and reproducibility
Symptom
Possible cause
Solution
Sample degradation
Transfer lines too hot
Reduce temperature
Vial temperature too hot
Reduce bath temperature
Valve resetting during run
Increase inject time
System contamination
Bake out valve and transfer line
Clean or replace sampling needle
System flow too slow
Increase headspace flow
Decrease GC flow
Increase split flow
System voids
Check connections
Reduce liner volume (id)
Reduce volume of connecting tubing
Insufficient focusing
Use column with lower ß
Lower initial column temperature
Equilibration time too short
Increase time
Vial temperature too low
Increase 20 °C, evaluate
Vent time too short or too long
Adjust
Vial cap leak
Use new sample
Reseal vial
Leaking inlet septum
Replace or tighten septum
Leaking connections
Inspect and reseal connections
Split flow too high
Reduce split and/or GC flow
Sample exposed too long before sealing
Seal immediately
Minimize transfer times
Ambient air contaminants
Purge vial with argon before sealing
Sample carryover
Clean sampling needle
Sample vial too full
Leaching from GC septum
Choose different septum type
Baseline perturbations
Peak tailing, broad peaks
Peak areas too small
Sample contamination
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GC Inlets An Introduction
12
Thermal Desorbers
Overview 144
Design 144
Sample Considerations 146
Air sampling 146
Volatiles in solids 147
Desorption temperature 147
Flow rates 147
Troubleshooting 148
Summary 149
Agilent Technologies
143
12 Thermal Desorbers
Overview
Thermal desorption units provide a quick, reproducible means of liberating
volatile analytes from solid samples or adsorbents. The types of samples for
which thermal desorption is often used include adsorbent tubes for
environmental air sampling, geological samples (dirt, rocks), and polymers.
Thermal desorption is an integral part of purge and trap sampling; volatile
compounds are concentrated on an adsorbent and then thermally desorbed to
the GC for analysis.
Thermal desorption is done by isolating the sample in a flowing stream of
carrier gas and rapidly increasing the temperature for a set period of time.
This can be accomplished using a dedicated instrument, an analytical
pyrolyzer, or a purge and trap sampler.
The time required for complete desorption of analytes is a function of sample
matrix, sample size, strength of the interaction between adsorbed analytes and
the solid, desorption temperature, and diffusion time of the analytes out of the
sample. Thermal desorption is a slow process and usually generates broad
peaks, for which solute focusing is required.
Design
Two typical thermal desorber designs are shown in Figure 44. They consist of
desorption and control modules. The desorption modules consist of a heater,
which rapidly heats the sample from room temperature to temperatures up to
400 °C, and carrier gas flow control. The control modules control the upper
desorption temperature and time, and sometimes, the temperature ramp profile.
The ease of connecting sample tubes (or containers) to the desorber depends
on the design; however, all designs must provide a leak -free seal to prevent
sample loss and exposure to air.
In Figure 44A, a heated transfer line connects the desorption unit to a
standard inlet on the GC. The sample line must be heated to prevent loss of
sample and peak broadening. As with headspace autosamplers, connection of
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Introduction to GC Inlets
12
Thermal Desorbers
the thermal desorber should be done through a split inlet for capillary
analyses. This helps minimize peak broadening and accommodates the high
flow rates necessary for efficient thermal desorption.
A
Heated transfer line
Sample
tube
Heater
Controller
Carrier gas
GC
Remote desorption
module
B
Carrier gas
Thermal desorber
Cryogenic coolant
Cryogenic trap
Desorption vent
Controller
GC
Figure 44
Typical thermal desorption system connected to GC
In Figure 44B, the desorption unit connects to the GC, and the analytical
column feeds directly into the desorber. With this design, a cryogenic trap is
provided to thermally focus the broad desorption peak under high flow rate.
The cryogenic trap is then heated rapidly to transfer the sample to the column
at normal column flow rates.
Introduction to GC Inlets
145
12 Thermal Desorbers
Sample Considerations
Air sampling
For environmental air sampling and ambient air monitoring in the work place,
the thermal desorption “sample” is actually a tube packed with a known
amount of adsorbent (carbon, Tenax, silica gel, or mixture), as shown in
Figure 45. A known amount of air is drawn through the tube using a calibrated
vacuum pump. Analytes of interest adsorb and concentrate on the adsorbent.
In some cases, several hundred liters of air must pass through the adsorbent
tube to concentrate analytes enough to be quantified. The tube is then sealed
and transported to the laboratory where it is installed in the thermal
desorption unit.
Single adsorbent
Glass
wool
Mixed adsorbent
Glass
wool
Solid sample
Figure 45
Typical configurations of adsorption tubes for thermal desorption
The sample can be contaminated or may decompose between the time it is
collected (adsorbed) and the time it is desorbed for analysis. Adsorbent tubes
should be sealed to prevent contamination and kept cool to reduce sample
loss.
Some air samples may contain components that are very hard to desorb due to
high polarity, activity, or reactivity (for example, some sulfur, nitrogen,
hydroxy- or carboxy-containing compounds); however, these analytes are often
even more difficult to sample by other means due to low analyte concentration
or high lability. In these cases, it is sometimes helpful to use a sampling tube
that contains a chemical that will selectively react with the compound of
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Thermal Desorbers
12
interest to form a more stable product. The adsorbed reaction product can
then be desorbed and analyzed; however, this approach requires considerable
development time and effort.
Volatiles in solids
Solid samples, such as polymers, in which residual solvents and monomers are
to be determined, may have to be ground up to increase surface area. This also
increases the diffusion rate of analytes out of the solid matrix and provides a
representative sample.
Solid sample size requirements depend on the instrument being used,
concentration of volatiles, and the type of gas chromatographic column used.
Small sample sizes are preferred because desorption speed is faster and initial
peak width is narrower compared with initial peak width for large sample
sizes. The minimum sample size is restricted by analytical sensitivity and
sample-handling requirements.
Desorption temperature
The key to successful thermal desorption is a fast, reproducible desorption
temperature ramp. The higher the ramp rate and the final temperature are, the
faster the desorption of analytes from the solid, and the narrower the initial
peak widths are; however, upper temperature is limited by sample and
adsorbent lability. Many of the common polymer adsorbents used for air
sampling degrade at temperatures above 300 °C. For most air analyses, however,
desorption temperatures do not need to exceed 200 °C.
Multiple temperature ramps can be used for selective or sequential desorption
of the solutes of interest. This is useful for samples that have very volatile
solutes on the surface of the solid (solvents), and less volatile material in the
bulk (plasticizers).
Flow rates
During the desorption process, flow rate through the desorber should be fast
enough to transfer sample efficiently to the GC inlet or column. When
desorbers with transfer lines are used, packed column flow rates (for example,
30 mL/min) are usually required to sweep sample through this extra
connection volume. When using a split inlet, most of this flow should be split
out the vent, as is done with headspace analyzers.
Introduction to GC Inlets
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12 Thermal Desorbers
With desorbers that have cryogenic traps between the desorption zone and the
column, high flow rates can be used during desorption into the trap. Capillary
column flow rates are then used during vaporization of the focused peak from
the trap to the column.
Troubleshooting
Most of the problems associated with thermal desorption relate to peak
broadening, incomplete sample desorption, or sample decomposition.
Reproducible desorption from air sampling tubes requires that the
appropriate adsorbent be selected for the analytes of interest, and that the
maximum recommended temperature limit of the adsorbent not be exceeded.
Appropriate adsorbents trap analytes efficiently and then thermally desorb
them quickly and quantitatively.
Degradation during thermal desorption leads to ghosting and interference with
subsequent analyses (system contamination). Degradation decreases with
decreases in desorption temperature and time.
Peak broadening can be minimized by reducing sample size, and by increasing
desorption temperature and ramp rate. The length and volume of the
connecting tubing and fittings should be as small as possible to prevent
additional peak broadening.
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Thermal Desorbers
12
Summary
Table 19
Factors affecting thermal desorption accuracy and reproducibility
Symptom
Possible cause
Solution
Sample degradation
Desorption too hot
Reduce desorption temperature
Baseline
System contamination
Clean transfer lines
Bake out system
Peak tailing, broad peaks
System flow too slow
Increase system flow
System voids
Check connections
Reduce liner volume
Reduce volume of connecting tubing
Desorption speed too slow
Increase temperature ramp
Decrease sample size
Increase flow rate
Column overload
Reduce sample size
Increase split flow
Leaking connections
Inspect and reseal connections
Desorption temperature too low
Increase temperature and/or time
Sample exposed too long before sealing
Seal immediately
Minimize transfer times
Peak areas too small
Sample contamination
Introduction to GC Inlets
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12 Thermal Desorbers
150
Introduction to GC Inlets
GC Inlets An Introduction
13
Purge and Trap Samplers
Overview 152
Design 153
Sample Considerations 154
Temperature 154
Flow Rates 155
Troubleshooting 156
Summary 158
Agilent Technologies
151
13 Purge and Trap Samplers
Overview
Purge and trap samplers are designed for the environmental analysis of
volatile components in water. They can be used for other samples if the minor
sample components are more volatile than the major component (for example,
fragrances in beverages). The system uses a combination of dynamic
headspace, adsorbent trapping, and thermal desorption.
In most purge and trap analyses, helium is purged through the sample in a
sealed system and the volatiles are continuously swept through an adsorbent
trap to concentrate and focus the purgeables. After a set time, the sample
purging is stopped, carrier gas is directed through the trap, and the trap is
rapidly heated to desorb sample to the gas chromatograph.
As is the case with thermal desorption, thermal and/or stationary phase
focusing is sometimes necessary with purge and trap analyses to achieve
chromatographic resolution of early eluting solutes due to the wide initial
peak widths.
Many of the volatile compounds analyzed by purge and trap methods also can
be analyzed using headspace analysis; however, in comparison to headspace
analyses, purge and trap sampling has several advantages, including:
• High sensitivity (purgeables are quantitatively removed from the sample and
concentrated on the trap)
• Well-established and widely used methods for environmental analysis
Since most samples analyzed by purge and trap units are aqueous, a large
quantity of water is collected on the adsorbent trap along with the volatiles of
interest. The water is liberated during thermal desorption and condenses in
the chromatographic column. Because water can reduce significantly the
accurate quantification of important sample components, silica gel is often
added to other adsorbents in the trap. Because silica gel has a much higher
affinity for water than it does for the sample components of interest, it
reduces the amount of water reaching the column while passing the important
analytes.
Placement of drying tubes or water-selective adsorbents in the flow path
between the purge and trap sampler and the GC column is more effective for
water removal than adding silica gel to the trap; however, it is more
complicated and expensive.
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Purge and Trap Samplers
13
Design
Figure 46 shows the flow diagram for a typical purge and trap system. A water
sample (5 to 20 mL) is injected into the sample tube and is equally distributed
(seeks a level) between the sample tube and reservoir. A purge gas valve then
is actuated to purge volatile sample components from the sample and carry
them to the adsorbent trap. The trap is typically a 30 cm × 3 mm stainless steel
tube filled with a mixture of Tenax, silica gel, and charcoal. During the purge
process, all the sample moves into the sample tube above the glass frit. The
purge gas (usually helium) is dispersed into small bubbles as it passes through
the frit.
Purge gas
inlet
Optional
water
remover
and/or
cryogenic
trap
To vent
Sample
tube
Reservoir
6-port
valve
Adsorbent
trap
GC
Glass
frit
GC carrier
gas
Figure 46
Introduction to GC Inlets
To detector
Flow diagram for a typical purge and trap autosampler
153
13 Purge and Trap Samplers
After a preselected purge time (10 to 15 min) the purge valve is closed, and a
6-port sampling valve is rotated to direct carrier gas through the trap in the
reverse direction. The trap then is heated rapidly (200 to 800 °C/min) to desorb
sample to the GC inlet or column. Purge and trap autosamplers usually are
connected to the GC via heated transfer lines.
A cryogenic trap can be placed between the purge and trap sample and the GC
to focus the sample peak. Also a water trap can be placed between the sampler
and the GC to reduce the amount of water reaching the column.
Sample Considerations
Samples may lose volatile components between the times of collection and
analysis, or they may be contaminated during handling; therefore, proper
sealing, cooling, and handling of samples is important for reproducible
analyses.
Standard addition or internal standard methods are preferred for quantitative
analyses to reduce the influence of variability in time, temperature, flow, and
effectiveness of the adsorbent.
Temperature
Sample purging is usually done at room temperature. Thermostating is not
considered important as long as the purge time exceeds the time necessary to
remove purgeables quantitatively from the sample.
The adsorbent trap is usually maintained at room temperature during purging.
The trap is heated rapidly (200 to 800 °C/min) to desorb analytes to the GC. The
temperature required to desorb analytes depends on the type of adsorbent and
analytes, but it is below 200 °C for standard environmental analyses.
The transfer line should be heated to prevent adsorption or condensation of
analytes. A transfer line temperature of 80 °C is typical for environmental
analyses.
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13
Flow Rates
Purge flow rates depend on sample type and analyte concentration, as well as
on the strength of interaction between the trap and the purgeables. Typical
purge flow rates range from 20 to 60 mL/min for helium. Even though nitrogen
can be used for purging samples, helium is better because it is less soluble in
water and displaces volatiles more efficiently.
The flow rate for carrier gas through the trap during desorption is from 5 to
10 mL/min for wide-bore capillary columns, and from 30 to 40 mL/min for
packed column analyses.
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13 Purge and Trap Samplers
Troubleshooting
Variability in purge and trap data can come from several sources:
• Temperature or time variation during purging
• Flow variation during purging
• Breakthrough of sample from the adsorbent trap
• Contamination of the trap
• Degradation of sample on the trap
• Slow desorption from the trap
• Influence of water on chromatographic peak shape and retention
Accuracy and reproducibility are improved significantly by using internal
standards which correct for variability in ambient conditions and flow rates.
Even when traps with silica gel adsorbents are used, water can still be
desorbed to the GC, causing peak distortion and shifting retention times.
Water traps (condensation tubes, semipermeable membranes) are very
effective for reducing the amount of water reaching the column.
Peak broadening due to slow desorption can be narrowed by optimizing
adsorbent type and amount, desorption temperature, and desorption flow
rate. Increasing stationary-phase focusing and adding a cryogenic trap
between the sampler and the GC are also helpful for minimizing peak
broadening.
Peak broadening also can be caused by diffusion, adsorption, and/or
condensation in connecting tubing and transfer lines. To mediate these
sources of peak broadening, the amount of connecting tubing should be
minimized, and all transfer lines between the trap and the GC should be
heated.
Since the trap is fixed in the purge and trap unit, it can be reused. If a sample
contaminates the trap, it will affect all subsequent samples by reducing trap
capacity (increasing breakthrough), changing desorption characteristics,
and/or by creating ghost peaks. Contaminated traps must be replaced.
All adsorbent traps lose capacity with use. As adsorbent capacity decreases, the
probability of sample breakthrough increases. Breakthrough is manifested by
decreased sensitivity and reproducibility. For this reason, most traps are
replaced on a regular basis.
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13
Sample degradation is a function of analyte, trap activity, transfer line
temperature, and desorption temperature. Sensitivity, reproducibility, and
accuracy decrease as degradation increases. Degradation can be minimized by
using trap adsorbents which have weaker interaction with the solute and by
lowering desorption and transfer line temperatures.
Introduction to GC Inlets
157
13 Purge and Trap Samplers
Summary
Table 20
Standard purge and trap practice and procedures
Parameter
Selection/Setting
Rationale
Sample size
5 to 25 mL
Depends on sample
Sample purge rate
20 to 60 mL/min helium
Depends on sample and adsorbent type
Purge time
10 to 15 min
Required to remove all purgeables
Purge temperature
Ambient
Does not usually require thermostatting
Solute trap
24 to 30 cm × 3 mm
Depends on instrument
Trap adsorbent
Tenax, charcoal, silica gel
Depends on sample
Transfer line temperature
80 °C
Prevents sample loss
Desorption ramp rate
Ballistic (200 to 800 °C/min)
Faster rates = narower initial peak width
Final desorption temperature
180 °C for 4 minutes
Depends on sample and adsorbent
Trap desorption flow
5 to 10 mL/min
Use for wide-bore capillaries
30 to 40 mL/min
Use for packed columns
Packed column inlet
Use for large-bore or packed columns
Connection to GC
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Purge and Trap Samplers
Table 21
13
Factors affecting purge and trap accuracy and reproducibility
Symptom
Possible cause
Solution
Baseline perturbations
System contamination
Clean transfer lines
High water background
Use a different trap
Use a water-removal device
Purge sample for a shorter time
Desorption flow too slow
Increase desorption flow
Decrease GC flow to inlet
Slow desorption
Reduce amount of adsorbent
Use a different adsorbent
Increase ramp rate
System voids
Check connections
Reduce inlet liner volume
Reduce volume of connecting tubing
Interference from water
Use a different trap
Use a water-removal device
Purge sample for a shorter time
Transfer line temperature low
Increase line temperature
Sampling time too short
Increase purge time
Adsorbent not working
Replace adsorbent tube
Leaking connections
Inspect and reseal connections
Sample exposed too long before sealing
Seal vial immediately
Minimize transfer times
Sample carryover
Clean sampling lines
Replace trap
Peak tailing, broad peaks
Peak areas too small
Sample contamination
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13 Purge and Trap Samplers
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Introduction to GC Inlets
GC Inlets An Introduction
14
Analytical Pyrolyzers
Overview 162
Design 164
Resistively heated pyrolyzers 164
Curie-Point pyrolyzers 166
Microfurnace pyrolyzers 167
Sample Considerations 168
Temperature 169
Flow Rates 169
Troubleshooting 170
Summary 171
Agilent Technologies
161
14 Analytical Pyrolyzers
Overview
Pyrolysis is useful for analyzing samples such as polymers (plastics, paints,
fibers), biological samples (bacteria, viruses, biopolymers), and geological
samples (coal, rocks). The primary disciplines that routinely use analytical
pyrolysis are polymer and forensic laboratories.
Analytical pyrolysis involves the thermal cleavage of large molecules into
smaller molecules (usually above 400 °C). A measured sample is heated in an
inert atmosphere to temperatures that cause thermal degradation. The resulting
fragments (pyrolysates) are more volatile than the starting sample and can be
separated by gas chromatography, yielding characteristic “fingerprints”
(pyrograms). The types and quantities of the pyrolysates are related to the
original sample components.
The higher the pyrolysis temperature, the greater the degree of fragmentation
and rearrangement reactions. This leads to more complex pyrograms with more
early eluting peaks. Figure 47 compares pyrograms of polyethylene at 700 and
1000 °C. The 1000 °C pyrolysis has a much larger proportion of small molecular
weight pyrolysates and a lower proportion of high molecular weight pyrolysates
compared to the 700 °C pyrolysis.
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Analytical Pyrolyzers
C2
14
A
C3
C4
C6
C10
C7
C5
C8 C9
B
C22
Figure 47
Total ion chromatograms (mass spectral) for the pyrolysis of 0.35 mg of
polyethylene at 1000 °C (A) and 0.45 mg at 700 °C (B). Numbers above the
clusters indicate the length of the carbon chain of the hydrocarbons in the
cluster.
The combined use of selective detectors (for example, mass-selective, atomic
emission, and infrared detectors) can often simplify data analysis
considerably. If selective detection is not used, standards of known molecular
structure are required to correlate fragmentation pattern to the original
molecular structure.
Because pyrograms can be very complex, mathematical techniques (pattern
recognition) are useful to help identify key pyrolysates that differentiate
sample types. Often only a small subset of the several hundred peaks in the
pyrogram is important to identify unequivocally an unknown within a class of
compounds. Once these peaks have been identified, an analytical method may
be developed to focus on them (by optimizing the chromatography or by using
selective detectors) and to simplify the analysis.
In those fields where it is necessary to characterize combustion products from a
sample, analogous information can be gained more simply by pyrolyzing the
sample in an oxidizing or reducing atmosphere (air or H2). The substitute
Introduction to GC Inlets
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14 Analytical Pyrolyzers
atmosphere is plumbed to the pyrolysis interface and flows only during the
pyrolysis. Inert carrier gas is then used during the chromatographic analysis of
pyrolysates.
Design
There are several types of pyrolyzers available for analytical pyrolysis. All
pyrolyzers provide the same function—high temperature pyrolysis of
samples—but they achieve it in different ways. Three of the most common
types are:
• Platinum resistively heated
• Curie point
• Microfurnace
The important features that differentiate pyrolyzers are their ease of use,
flexibility in temperature program and range, and cost. In general, the more
flexible and easier to use a pyrolyzer is, the more expensive it will be.
Resistively heated pyrolyzers
Resistively heated pyrolyzers use sample probes with platinum ribbons or coils
to heat the sample (Figure 48). These pyrolyzers are very flexible but can be
expensive relative to other styles. With resistance-heated devices, the maximum
pyrolysis temperature can be adjusted continuously up to 1400 °C and ramped
at rates up to 20 °C/ms.
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Analytical Pyrolyzers
14
Glass wool
Platinum coil
Solid sample
Heated interface
Liquid
sample
Carrier
gas
Inlet
GC
Platinum ribbon
Figure 48
Column
Example of a resistively-heated pyrolyzer
Liquid samples are aliquoted by syringe onto a ribbon probe. Solvent is then
flash vaporized at a temperature just above its boiling point. Solid sample is
placed in a quartz tube between plugs of quartz wool. The tube is then inserted
within the coil of a coil probe.
Before pyrolysis, the probe containing sample can be:
• Inserted directly into a packed column inlet (this usually requires
horizontal inlet orientation)
• Inserted into a heated interface that is coupled to the GC inlet through a
syringe needle assembly or with a special seal that replaces the septum cap
of the inlet
• Inserted into a stand-alone unit that has trapping and focusing capabilities.
These units are coupled to the GC with a transfer line as is done with some
headspace autosamplers.
A unique advantage of resistance pyrolyzers is that thermal desorption of
volatiles can be done at a low temperature prior to pyrolysis of the bulk solid.
This is useful in profiling solvents, dissolved gases, or residual monomers in a
polymer sample.
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14 Analytical Pyrolyzers
Curie-Point pyrolyzers
Curie-point pyrolyzers provide very fast temperature ramp rates and very
reproducible final temperatures. Curie-point pyrolyzers are usually less
expensive than resistively heated pyrolyzers; however, they are not nearly as
flexible.
Curie-point pyrolyzers work by ferromagnetic heating of a wire or foil to its
Curie point. The specific blend of metals comprising the wire or foil dictates
its Curie point and, therefore, the final sample temperature and extent of
pyrolysis.
A typical Curie-point pyrolyzer design is shown in Figure 49. Heating takes
place ballistically in an interface that couples radio frequency (RF) energy to
the ferromagnetic alloy. Each alloy has only one possible final temperature. A
variety of alloys must, therefore, be kept on hand when developing methods or
analyzing different sample types.
Solid sample
Sample tube
Foil
Wire
Carrier gas
Liquid sample
GC
Inlet
Column
Figure 49
Example of a Curie-point pyrolyzer
For Curie-point pyrolysis, liquid samples are placed onto a wire (the wire is
dipped into sample, or sample is aliquoted onto the wire with a syringe). Solid
samples are wrapped in pyrolysis foil. The sample wire or foil is placed into a
quartz tube which is then inserted into the pyrolysis interface.
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14
Microfurnace pyrolyzers
Microfurnace pyrolyzers are the least expensive and least flexible type of
pyrolyzer. The sample is placed in a boat or tube which is then quickly inserted
into the heated zone. Alternatively, the sample can be injected directly into the
heated quartz furnace tube (Figure 50) using a special syringe. Some units use
a sealed sample tube which is punctured in the furnace to release pyrolysates
after a preset equilibration time.
Carrier gas
Septum purge
Quartz wool
Solid or liquid sample
GC
Inlet
Column
Figure 50
Example of a microfurnace pyrolyzer
Furnace pyrolyzers are the least flexible type of pyrolyzer since equilibration
of the furnace at each new temperature takes considerable time. The
maximum possible pyrolysis temperature is lower than that with
resistance-heated and Curie-point pyrolyzers.
Introduction to GC Inlets
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14 Analytical Pyrolyzers
Sample Considerations
Samples selected for pyrolysis should be representative of the bulk sample.
Solid samples have a higher probability of being inhomogeneous than do
liquid and gas samples. Some form of sample treatment may be necessary to
get an accurate pyrogram.
If the sample is soluble or suspendable in a solvent (sonication helps), then
homogenizing becomes easier and the sample can be reproducibly aliquoted
directly onto a pyrolysis wire or foil. Afterward, the solvent can be
evaporated actively or passively, leaving a small representative solid sample
stuck to the foil.
Solid samples that cannot be suspended in solvent, or that will not stick to
the foil when the solvent is evaporated, must be placed within a quartz tube
bracketed by quartz wool (resistively heated pyrolyzers), sealed within
pyrolysis furnace boats (furnace pyrolyzers), or folded inside Curie-point
pyrolysis foil.
Sample sizes should be less than 50 mg, the smaller the better. Large
samples yield irreproducible results because of inconsistent secondary
reactions, problems with thermal gradients within the sample, and slow
diffusion of pyrolysates out of the sample. The minimum solid sample size
that can be used practically is often dictated by the analyst’s ability to
prepare and weigh the sample.
Small sample sizes are easier to measure if the sample is dissolved. A
syringe can then be used to deposit the dilute sample onto a wire or ribbon.
When the solvent is evaporated, a very small, reproducible amount of
sample remains. Concentration of dilute samples can be achieved by
repeating the sample deposition process.
Care must be taken when handling samples for pyrolysis; finger oils, lab
bench particulates, and other contaminates can cause spurious peaks in the
pyrogram. For maximum reducibility samples must be placed in sample
tubes, on wires, and on ribbons because there are temperature gradients
over the heated zone. Centering samples in the middle of the zone is best,
because that is usually the hottest part.
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14
Temperature
Reproducible pyrolysis temperature is important. This is dependent on
reproducibility in sample amount, location of sample in the hot zone,
temperature ramp rate, ending temperature, and final hold time. Some
pyrolyzers incorporate a feedback mechanism which adjusts heating power
during the pyrolysis to compensate for differences in sample conditions.
The appropriate pyrolysis temperature depends on the sample and analytical
requirements. Higher pyrolysis temperatures cause more complex pyrograms,
stressing lower molecular weight fragments and increasing overall sensitivity.
Lower pyrolysis temperatures reduce secondary reactions and
rearrangements and stress higher molecular weight fragments that can be
important for identifying original molecular structures.
The minimum temperature required for pyrolysis depends on the sample and
matrix. Excessive pyrolysis temperature fragments the sample into such small
molecules that they do not provide the information necessary to identify the
original sample or to differentiate it from similar samples.
The usual procedure to determine a suitable pyrolysis temperature is to screen
replicate samples at several temperatures (for example, 450 °C, 600 °C, 800 °C)
and then compare pyrograms. Selective detectors are very useful during the
screening process to help track secondary pyrolysis and to relate pyrolysates to
original sample components.
Flow Rates
Carrier gas flows over the sample during pyrolysis transferring pyrolysates to
the column and limiting the contact time of gaseous components in the
high-temperature zone. Most pyrolyzers are efficiently swept with packed
column flow rates (for example, 30 mL/ min). When capillary columns are
used for analysis, the pyrolyzer should be interfaced to the GC through a split
inlet, with a majority of the flow being vented (split ratio from 1:10 to 1:50).
When traps (cryogenic and/or adsorbent) are used to focus pyrolysates
between the pyrolyzer and the GC, higher flows can be used during the
pyrolysis and trapping. When desorbing sample from the trap to the GC,
carrier gas flows that are consistent with column requirements should be
used.
Introduction to GC Inlets
169
14 Analytical Pyrolyzers
Troubleshooting
Irreproducibility and peak broadening are the major sources of trouble with
analytical pyrolysis. The smaller the sample, the more reproducible is the
pyrolysis and the narrower the initial peak width.
Contamination of the pyrolysis interface can lead to ghosting and loss of
pyrolysates. Glass or quartz interface liners and fused silica transfer lines help
reduce adsorption and catalytic degradation of pyrolysates.
Catalytic activity of pyrolysis wires and foils, or of the lining of the interface,
can affect pyrolysis fragmentation reactions and the profile of the resulting
pyrogram. Consistency in the alloys and pyrolysis temperatures will increase
analytical reproducibility.
Peak tailing can result from adsorption in transfer lines and peak dilution in
interfaces and connections. Heating transfer lines and minimizing the volume
of connecting tubing and interfaces help to minimize tailing.
170
Introduction to GC Inlets
Analytical Pyrolyzers
14
Summary
Table 22
Standard pyrolysis practice and procedures
Parameter
Selection/Setting
Rationale
Sample size
< 10 mg
Maximize reproducibility
Temperature ramp rate
Ballistic
For Curie-point and furnace pyrolyzers
1 to 20 °C/ms
For restively-heated pyrolyzers
Final pyrolysis temperature
400 to 1000 °C
Depends on sample goals and type of pyrolyzer
Pyrolysis time
5 to 20 s
Depends on sample goals and type of pyrolyzer
Carrier gas flow
10 to 40 mL/min
Depends on sample goals and type of pyrolyzer
Table 23
Factors affecting pyrolysis accuracy and reproducibility
Symptom
Possible cause
Solution
Peak tailing
Pyrolyzer flow too slow
Increase the flow rate
Slow pyrolysis
Increase the temperature ramp rate
Reduce the sample size
System voids
Check connections
Reduce liner volume
Reduce volume of connecting tubing
Insufficient focusing
Reduce the initial column temperature
Use cryogenic or adsorbent trap
Insufficient flow
Increase the flow rate
Sample too large
Reduce the sample size
Sample too large
Reduce the sample size
Irreproducible sample placement in hot zone
Center the sample in hot zone
Irreproducible sampling
Homogenize the sample
Dissolve the sample first
Broad peaks
Area reproducibility
Introduction to GC Inlets
171
14 Analytical Pyrolyzers
172
Introduction to GC Inlets
GC Inlets An Introduction
A
Gas Volumes of Solvents
Table 24 Approximate gas volumes of common solvents per 1 µL injected at several inlet temperatures and
pressures
Head Pressure,
Inlet temperature, °C
Solvent
kPa
100
200
300
Ethyl acetate
69 (10 psig)
138 (20 psig)
207 (30 psig)
186
133
103
236
168
131
286
203
158
Hexane
69
138
207
139
100
77
177
126
98
214
152
119
Isooctane
69
138
207
110
79
61
140
99
78
170
121
94
Methanol
69
138
207
460
329
255
584
415
324
708
503
392
Methylene chloride
69
138
207
284
203
158
360
256
200
437
311
242
Methyl t-butyl ether
69
138
207
153
109
85
194
138
108
235
167
130
Water
69
138
207
1010
722
561
1282
910
710
1554
1105
860
Agilent Technologies
173
A
Gas Volumes of Solvents
These values were calculated using the equation:
Po
4 V 1 ⋅ D T + 273
V g = 2.24 × 10 × --------------- × ------------------ × -----------------273
Po + Pi
MW
where Vg
‘
V1
D
MW
T
Pi
P0
174
=
=
=
=
=
=
=
(2)
the resulting gas volume (µL)
the liquid volume injected (µL)
the density of the liquid (g/mL)
the molecular weight of the solvent
the temperature of the inlet (°C)
the inlet pressure
atmospheric pressure.
Introduction to GC Inlets
GC Inlets An Introduction
B
Head Pressures for Capillary Columns
Table 25
Suggested initial head pressures for capillary columns
Helium head pressure
Hydrogen head pressure
Column id, mm
Column length, m
kPa
psi
kPa
psi
0.20
0.20
0.20
12
25
50
85–140
145–235
235–360
12–21
21–34
34–52
48–84
87–145
145–230
7–12
13–21
21–34
0.32
0.32
0.32
12
25
50
29–53
55–95
95–160
4.2–7.7
7.9–14
14–23
17–32
33–60
60–105
2.5–4.7
4.8–8.7
8.7–15
0.53
0.53
10
30
8.5–16
24–44
1.2–2.4
3.5–6.3
5.0–9.7
14–27
0.7 –1.4
2.1–3.9
The lower pressures in each range are based on average linear velocities of
approximately 30 cm/s for He and 40 cm/s for H2. This yields higher column
efficienciescies for late-eluting compounds but longer analysis times compared
with the higher pressures.
The high pressures in each range are recommended as a starting point for
most analyses and yield a good compromise between efficiency and the speed
of analysis over a broad k range.
Agilent Technologies
175
B
176
Head Pressures for Capillary Columns
Introduction to GC Inlets
GC Inlets An Introduction
C
Determining Split Ratio
Split ratio is the relationship of column flow to split vent flow. This can be
column flow relative to split flow (for example, 1:100), or split flow relative to
column flow (such as 100:1). The larger number is always assumed to be that
for the split vent flow. Knowing the split ratio is necessary for:
• Documenting the analysis so that it can be repeated
• Calculating the amount of sample reaching the column
Calculating Split Ratio
To calculate the split ratio, both split flow and column flow need to be
determined accurately. The split flow is measured with a bubble flow meter at
the split vent, which is usually accessible at the front or top of the instrument.
The column flow can be measured at the detector with a bubble flow meter if
using wide-bore capillary columns at high flow rates (above 5 mL/min).
Two phenomena reduce the accuracy of the flow measured by bubble flow
meters:
• The diffusion of carrier gas through the bubble (decreases the apparent flow
rate)
• The vapor pressure of water (increases the apparent flow rate)
Both of these influences can be reduced or corrected to yield a corrected split
ratio (CSR):
CSR = Corrected split flow : Corrected column flow
(3)
Errors introduced by diffusion of carrier gas through the soap bubble can be
reduced by filling the bubble flow meter with carrier gas and bubbles prior to
and during flow measurement. This decreases the concentration gradient of
Agilent Technologies
177
C
Determining Split Ratio
gas across the last bubble and, therefore, the net flow of carrier gas through
the bubble to the low concentration side. For measurement of very low flow
rates, use a large-volume connecting tube that is well purged with air prior to
flow measurement.
To correct flow rates mathematically for the influence of water vapor pressure
on the measurement from a soap bubble flow meter, the following equations
are used:
a
a
w
SF ⋅ ( p – p )
SF = ------------------------------------a
p
(4)
and
a
a
w
CF ⋅ ( p – p )
CF = -------------------------------------a
p
(5)
Corrected split vent flow
Corrected column flow
Split vent flow determined by bubble flow meter (mL/min)
Column flow rate determined by bubble flow meter (mL/min)
Ambient pressure (approximately 100 kPa; 1 atm = 760 mm Hg =
14.7 psi = 101 kPa)
pw = Vapor pressure of water ( = 2.4 kPa @ 20 °C, 3.2 kPa @ 25 °C,
4.3 kPa @ 30 °C
where SF =
CF =
SFa =
CFa =
pa =
For small inner diameter columns and low flow rates, column flow rate is
more accurately determined by measuring the retention time of a solute that is
not retained by the column (t0). Air injections are useful for measuring t0 with
thermal conductivity detectors. Methane injections are useful for measuring t0
with flame ionization detectors. Once t0 is known, the volumetric flow rate can
be calculated using the column dimensions and the ratio of inlet and outlet
pressures:
c
3.14 ⋅ r 2 ⋅ L 2 ⋅ ( P 3 – 1 )
CF = --------------------------- × --------------------------t0
3 ⋅ ( P2 – 1 )
178
(6)
Introduction to GC Inlets
C
Determining Split Ratio
where CFc =
r =
L =
P =
t0 =
Column flow at column temperature
Column radius (cm)
Length of the column (cm)
(Absolute inlet pressure)/(Absolute outlet pressure)
Retention time of the nonretained peak (min)
The column flow rate should then be corrected to room temperature for direct
comparison with the split vent flow rate, which was measured at room
temperature:
a
c 273 + T
CF = CF × --------------------c
273 + T
where CF =
CFc =
Ta =
Tc =
(7)
Corrected column flow at room temperature
Column flow at column temperature
Ambient temperature, °C
Column temperature at which to was measured, °C
Calculating Sample Reaching the Column
Once CSR has been determined, the amount of sample reaching the column
after splitting (Amt) can be calculated. For this to be an accurate value, no
inlet or needle discrimination must occur during injection.
of sample or solute injectedAmt = Amount
-------------------------------------------------------------------------------------CSR + 1
Introduction to GC Inlets
(8)
179
C
180
Determining Split Ratio
Introduction to GC Inlets
Agilent Technologies
© Agilent Technologies, Inc.
Printed in USA, July 2005
5958-9468
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