schmitt11amt

schmitt11amt
Atmos. Meas. Tech., 4, 1445–1461, 2011
www.atmos-meas-tech.net/4/1445/2011/
doi:10.5194/amt-4-1445-2011
© Author(s) 2011. CC Attribution 3.0 License.
Atmospheric
Measurement
Techniques
A sublimation technique for high-precision measurements of δ 13CO2
and mixing ratios of CO2 and N2O from air trapped in ice cores
J. Schmitt1,2 , R. Schneider1 , and H. Fischer1,2
1 Climate
and Environmental Physics, Physics Institute, & Oeschger Centre for Climate Change Research, University of Bern,
Sidlerstrasse 5, 3012 Bern, Switzerland
2 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Received: 17 February 2011 – Published in Atmos. Meas. Tech. Discuss.: 16 March 2011
Revised: 6 July 2011 – Accepted: 11 July 2011 – Published: 19 July 2011
Abstract. In order to provide high precision stable carbon isotope ratios (δ 13 CO2 or δ 13 C of CO2 ) from small
bubbly, partially and fully clathrated ice core samples we
developed a new method based on sublimation coupled to
gas chromatography-isotope ratio mass spectrometry (GCIRMS). In a first step the trapped air is quantitatively released
from ∼30 g of ice and CO2 together with N2 O are separated from the bulk air components and stored in a miniature glass tube. In an off-line step, the extracted sample
is introduced into a helium carrier flow using a minimised
tube cracker device. Prior to measurement, N2 O and organic
sample contaminants are gas chromatographically separated
from CO2 . Pulses of a CO2 /N2 O mixture are admitted to
the tube cracker and follow the path of the sample through
the system. This allows an identical treatment and comparison of sample and standard peaks. The ability of the method
to reproduce δ 13 C from bubble and clathrate ice is verified
on different ice cores. We achieve reproducibilities for bubble ice between 0.05 ‰ and 0.07 ‰ and for clathrate ice between 0.05 ‰ and 0.09 ‰ (dependent on the ice core used).
A comparison of our data with measurements on bubble ice
from the same ice core but using a mechanical extraction device shows no significant systematic offset. In addition to
δ 13 C, the CO2 and N2 O mixing ratios can be volumetrically
derived with a precision of 2 ppmv and 8 ppbv, respectively.
Correspondence to: J. Schmitt
([email protected])
1
Introduction
CO2 concentration measurements on polar ice cores provide direct atmospheric information of past carbon dioxide
concentrations over up to the last 800 000 years (Fischer
et al., 1999; Petit et al., 1999; Monnin et al., 2001; Ahn
and Brook, 2008; Lüthi et al., 2008). Knowing the underlying natural causes of these CO2 changes is key to predict its future dynamics. Therefore, refining our quantitative understanding of the observed glacial/interglacial variations in atmospheric CO2 mixing ratio of about 100 ppmv,
but also smaller variations during the Holocene, is an ongoing task of outstanding importance for the paleo climate
community (Indermühle et al., 1999; Trudinger et al., 1999;
Brovkin et al., 2002; Broecker and Clark, 2003; Köhler and
Fischer, 2004; Elsig et al., 2009; Lourantou et al., 2010b).
An important constraint on past changes in the global carbon cycle is the carbon isotopic signature of CO2 (δ 13 C,
with δ 13 C = [(13 C/12 C)sample /(13 C/12 C)reference − 1] × 1000).
However, the data coverage of δ 13 C measurements is still
fragmentary due to methodological limitations; i.e. measurements were done on selected time intervals, using different
ice cores and different extraction devices, and in some cases
precision is not sufficient (Leuenberger et al., 1992; Francey
et al., 1999; Indermühle et al., 1999; Smith et al., 1999; Elsig
et al., 2009; Lourantou et al., 2010a,b; Schaefer et al., 2011).
CO2 and δ 13 C measurements usually use dry mechanical extraction (ball mills, needle crackers, cheese graters etc.) to
release the air from gas enclosures in the ice. However, wet
extraction methods, often used for other atmospheric trace
gases, such as CH4 , might lead to CO2 production within
Published by Copernicus Publications on behalf of the European Geosciences Union.
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
the melt water due to acid/carbonate reactions (Kawamura et
al., 2003). Also the high solubility of CO2 connected with a
strong isotopic fractionation during gas-liquid transfer from
the aqueous HCO−
3 /H2 CO3 system limits the applicability of
wet extraction methods for CO2 (Anklin et al., 1995; Zhang
et al., 1995). The dry extraction methods, however, suffer
from rather low extraction efficiencies, ranging from only
50 % for fully clathrated ice to up to 90 % for bubble ice
(Etheridge et al., 1988; Ahn et al., 2009; Lüthi et al., 2010;
Schaefer et al., 2011). Note within this paper the term extraction efficiency refers to the efficiency during the actual
extraction process, i.e. the release of enclosed gases from the
ice sample. It does not include “post-extraction steps”, like
removal of the water vapor or the transfer of gas from the
extraction line to the detection system. Extraction efficiency
becomes important for ice from the bubble/clathrate transition zone (BCTZ), where fractionation between different gas
species during the bubble clathrate phase transition has been
observed (Ikeda et al., 1999; Lüthi et al., 2010). It becomes
also crucial for high precision measurements of δ 13 C from
partly or fully clathrated ice. Here, dry extraction methods
are only able to extract the CO2 from opened bubbles and
decomposing clathrates, where fractionation processes may
come into play during the clathrate relaxation process. However, for CO2 it is currently not clear if the observed gas
fractionation during dry extraction is associated with a significant isotopic fractionation. A recent study using a ball
mill dry extraction technique compared δ 13 C values from
two ice cores with either bubbly ice or ice from the BCTZ
(Schaefer et al., 2011). While the authors also discuss other
effects like ice microfractures and uncertainty from the gravitational correction being the culprit, ice from the BCTZ resulted in a larger scatter for δ 13 C and showed a small but significant offset compared to bubbly ice of the same age interval. Accordingly, an extraction method achieving essentially
100 % extraction efficiency provides the best conditions to
reliably decipher the carbon isotopic composition of CO2 for
clathrated ice, which covers most of the time span archived
in deep Antarctic ice cores. In summary δ 13 C measurements
using dry extracted air from the BCTZ and clathrate ice are
potentially vulnerable to larger systematic and stochastic errors. The only extraction technique for CO2 from ice core
samples which enables 100 % extraction efficiency is sublimation under vacuum. Several attempts to apply such a sublimation technique for concentration measurements of greenhouse gases have been undertaken during the last decades
(Wilson and Long, 1997; Güllük et al., 1998). Although the
sublimation apparatus designed by Güllük et al. (1998) was
improved by Siegenthaler (2002), the resulting CO2 data still
showed a higher scatter and were less precise than the conventional mechanical crushing method (Siegenthaler, 2002).
No attempts to measure δ 13 C using sublimation extraction
have been conducted so far.
Another issue with respect to δ 13 C measurements regards
the limited sample size available from ice cores. In earlier
Atmos. Meas. Tech., 4, 1445–1461, 2011
attempts, large sample sizes of up to 1 kg were needed applying the dual inlet technique (Leuenberger et al., 1992;
Francey et al., 1999; Smith et al., 1999). This technique
allows for a precision of ∼0.05 ‰ but with the drawback
of high sample consumption. This poses serious limitations
in creating highly resolved records in deep ice cores with
thin annual layers and strongly limited ice core availability.
Furthermore, contamination with drilling fluid caused some
erroneous measurements as observed by Eyer (2004) and
observations by H. Fischer during measurements on Taylor
Dome ice. Here we present a new sublimation extractionGC-IRMS technique which enables high precision measurement of δ 13 C together with CO2 and N2 O mixing ratios on
30 g of both bubble and clathrate ice (equivalent to air samples of only 2–3 ml STP (standard temperature and pressure;
20 ◦ C and 1 atm)). In contrast to the dual-inlet analysis, sample consumption is considerably reduced by a factor of more
than 10 using the continuous flow inlet technique. In addition, an efficient gas chromatographic sample clean-up is
possible removing the drilling fluid contamination and the
isobaric component N2 O. Since the gas extraction from the
ice is by far more time consuming than the actual IRMS measurement, the system was split into two separate lines. This
allows to measure many extracted gas samples within a short
time span and, thus, to take advantage of identical measurement conditions in the IRMS for a large set of samples. This
is crucial as changes in the performance of the IRMS measurement, like source tuning, variations in the H2 O background are a common problem. Following this technical
partition, we first show the setup of the “sublimation system” and then the “tube cracker-GC” as the inlet system to
the mass spectrometer. Subsequently, the referencing strategy, systematic corrections applied to the data and the performance of the analytical method on air samples and different
kind of ice core sample types are discussed. In addition, we
compare our data with previous results from other methods
to evaluate the absolute accuracy of the measurements.
2
2.1
Experimental setup
General layout
The challenge to measure δ 13 C at high precision together
with the CO2 mixing ratio on small ice samples led to the development of two separate preparation systems (Figs. 1 and 2
show the sublimation system, Fig. 3 the “tube cracker-GC”
inlet). With the sublimation system (Figs. 1 and 2) an ice
sample of ∼30 g is continuously sublimated. CO2 (together
with N2 O and organic impurities) is cryogenically separated
from the major air components (N2 , O2 and Ar). After the
sublimation of the ice sample is finished, the trapped fraction
(CO2 , N2 O and organic impurities such as components of
the drill fluid) is transferred into a small glass tube (Fig. 4).
In parallel, the corresponding total air content is determined
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
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Fig. 1. Schematic view of the vacuum sublimation system. The sublimation vessel including the ice sample is cooled via a cold air stream
supplied by the “cooling device”. The released atmospheric gases from the sample are separated and collected in the CO2 trap and air
trap (right side). Reference air can be admitted during the sublimation of gas free blank ice via a fused silica capillary to provide identical
conditions for sample and reference (top left).
manometrically. From this, the mixing ratios of CO2 and
N2 O are calculated using the peak area provided by the mass
spectrometric measurement. Although the sublimation step
takes about one hour, the overall processing time is about 4 h,
which limits our sample throughput to two samples per day.
For the GC-IRMS measurement, the tubes are opened within
a miniature cracker and the gas sample (CO2 , N2 O and impurities) is transferred into a He carrier stream (Fig. 3). In a
first step, the helium stream is dried, then the sample gases
are cryofocused, finally a pulse of gas is transferred to a GC
column, where the components are separated. The purified
CO2 is admitted to the IRMS via an open split. Both systems
are equipped with “reference devices” to either introduce air
standards or a CO2 /N2 O mixture in helium, thus, to mimic
the ice sample’s way through the various analytic steps.
2.2
2.2.1
Sublimation system
was constructed to minimize the total surface of the system. Moreover, to reduce out-gassing further, polymer based
O-rings or valves were excluded and only metal seals and
valves are used. All valves which are in contact with sample CO2 are all-metal valves (1/400 , Fujikin, FUDDFM-71G6.35, Japan).
The improvements of the original Güllük apparatus by
Siegenthaler reduced the total volume of this sublimation line
to 1730 ml, whereas our design allowed a further reduction
to ∼200 ml (for convenience, volume is used as a rough estimate for surface area). Since their sample size is comparable
to ours, we achieved a considerable reduction of the active
surface area of the sublimation line exposed to the sample
volume. The main improvement was to merge the sublimation vessel and the bulky outer water trap into one vessel.
Figure 2 shows the combined sublimation-water trap vessel.
This single vessel design required a new cooling strategy using cold air as cooling agent.
General remarks
2.2.2
Degassing of CO2 from O-rings, polymer based valves and
seals and from glass and metal surfaces from the apparatus itself, are the main sources of contamination, which reduce measurement precision (Güllük et al., 1998; Siegenthaler, 2002; Elsig, 2009). As a consequence, our system
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Sublimation apparatus
The high vacuum within the sublimation line is provided by a
turbo molecular pump backed by a rotary vane vacuum pump
(both Leybold Vacuum, Germany). A large, 1/200 diameter
water trap (Fig. 1, “water trap 1”) held at liquid nitrogen
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
Fig. 2. Photo showing the sublimation vessel with a loaded ice sample and the infrared source. The cooling jacket starts just above the
ice sample and a cold air stream is fed in from both sides. The
infrared source consists of 18 bulbs mounted on a holder, and the
red UV filter visible in the central part. For the sublimation of the
ice sample the infrared source is moved upwards, hence the section
with sample is surrounded by the bulbs.
temperature is inserted between the sublimation line and the
vacuum pumps to prevent H2 O from entering the high vacuum side. The main advantage of our approach lies in the
compact design combining the sublimation of the ice and the
close-by removal of the bulk water vapour into one single
vessel (33 mm o.d., 121 mm length, CF flange, Caburn, UK)
shown in Figs. 1 and 2. The cooled upper part of the sublimation vessel is termed internal water trap. The compact design
results in two benefits: First, the total surface area is reduced
and the number of potentially leaky seals or connections is
kept at a minimum. Secondly, it allows fast sublimation rates
Atmos. Meas. Tech., 4, 1445–1461, 2011
at low temperature and, thus, low water vapour pressure due
to the large cross section of the sublimation vessel providing
a high conductance. Consequently, the pressure and temperature difference between the ice sample and the condensed
ice in the internal water trap a few cm above is small. This
is mandatory to achieve a high sublimation rate at low temperature to keep the ice surface well below −20 ◦ C. Above
this temperature, a quasi-liquid layer might form on the ice
surface allowing chemical reactions to take place (Güllük et
al., 1998; Barnes et al., 2003).
An air stream of −115 ◦ C directed by a cooling jacket allows the upper zone of the sublimation vessel, i.e. the internal water trap, to be cooled to the desired temperature.
The sublimation is run at around 0.25 mbar pH2 O or −34 ◦ C.
The pressurised air is cooled via a copper heat exchanger
mounted in a 2-l Dewar (“cooling device” in Fig. 1). Liquid nitrogen (LN2) is automatically pumped from a reservoir
into this heat exchange Dewar until the temperature set-point
is reached, with the temperature of the air flow regulated by
a thermocouple and a proportional-integral-derivative (PID)
controller (West 2300, UK). Although the design of this LN2
pump is basic the resulting temperature of the cooling air
can be adjusted fairly constant with changes being <2 ◦ C
(Schmitt, 2006). The temperature of the cooled air stream
can be continuously adjusted from ambient temperature to a
lower limit of −150 ◦ C, where the system becomes unstable
due to condensation of O2 . Via a flow regulator valve and
a manometer (Vair and Pair , Fig. 1) the flow is adjusted to
the required demand of coolant from 0 to ∼50 l min−1 , corresponding to a maximum cooling capacity of ∼100 W (with
a 1T of ∼100 ◦ C between the cold air stream and the glass
vessel). Primarily, the air stream has to remove the latent heat
released during the deposition of the water vapour on the wall
of the sublimation vessel (∼30 W) and secondly to cool the
lower part of the glass vessel which absorbs long-wave radiation. Infrared light from halogen bulbs, which surround the
sublimation vessel (18 bulbs each with max 30 W at 12 V,
Microstar, Osram, Germany) provides the energy for sublimation (Figs. 2 and 3). As the absorption coefficient of ice
below 600 nm is small, only the long wave part (λ > 600 nm)
of the emitted spectrum is used for the sublimation. Additionally, the emitted light is passed through a special foil filter
(Colour Foil, No. 105 orange, LEE, Germany), absorbing the
short wave light (<600 nm). Removing the short wave part
is a precautionary measure to get rid of the UV part, which
could potentially interfere with organic impurities within the
ice to produce in-situ CO2 .
The flange head has two 1/400 feedthroughs, which are connected to the 1/400 tubing (Fig. 1). The left feedthrough is
connected to a pressure transducer, “PS ” (1 Torr max, Leybold Vacuum, Germany), to control the H2 O vapour pressure
and, thus, temperature during the sublimation. Via valve
V2s, the sublimation vessel can be connected to the forevacuum to evacuate the sublimation vessel after opening it to
load the ice sample. Via valve V2o, the sublimation vessel
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
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Fig. 3. Flow scheme of the tube cracker-GC-IRMS line with a device to introduce CO2 /N2 O pulses into the GC carrier gas (red box), the
tube cracker (green box), followed by a Nafion dryer, cryofocus capillary, GC, 2nd Nafion dryer and finally the inlet system with the open
split leading to the ion source of the IRMS and the reference port to inject CO2 std/on off peaks and to admit a continuous CO background
(yellow box).
Fig. 4. Photo showing the closed sample tube.
can be connected to the turbo pump, passing water trap 1
(with VFV4 closed). Onto the right feedthrough, an inlet
capillary is mounted which allows the continuous introduction of reference air (see section on air reference inlet). Via
valve V3, the sublimation vessel is connected to the external
water trap and the consecutive traps.
2.2.3
External water trap
Although the internal water trap already removes 99 % of the
water vapour, an additional, external water trap was needed
to achieve the requirements for extreme low pH2 O in the
final CO2 sample. A compact water trap to minimize the total surface area of the system was aimed at. Since in our
application the trapped CO2 and with it any H2 O traces are
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ultimately transferred into a miniature glass capillary with a
volume of only 15 µl, a H2 O amount of just 0.1 µg is sufficient to form a liquid phase. In case the H2 O flux exceeds
0.1 µg, liquid H2 O forms within the tube after the transfer
from the CO2 trap to the glass capillary and warming to ambient temperature. The presence of a liquid phase within the
tube allows CO2 to exchange oxygen with H2 O. This shifts
the δ 18 O values to more depleted values of up to 5 ‰ depending on the δ 18 O value of the ice sample. Therefore,
temperatures as cold as −150 ◦ C in the external water trap
are needed to reach the required H2 O vapour pressure. Since
classical cooling systems, e.g. dry ice/pentane slush (Leckrone and Hayes, 1997) are not readily suitable for this temperature range and closed-cycle He coolers are too bulky, we
constructed a trap to match our requirements. The trap is
made of silitec coated stainless steel tubing (Restec, USA)
with 1/400 o.d., 0.53 cm i.d., 20 cm length to reduce adsorption of CO2 on the cold surface of the trap. This tubing
rests within an aluminium block, which is cooled with LN2
droplets and cold nitrogen gas; for details see Schmitt (2006)
and Bock et al. (2010). A second reason for the usage of a
LN2 cooled water trap is the general exclusion of solvents
within our laboratory as traces of organic contaminants are
reported to interfere with the IRMS measurement due to isobaric interferences (Francey et al., 1999; Leuenberger et al.,
2003; Eyer, 2004). The temperature of the trap is automatically controlled via a thermocouple and a PID controller with
an output current driving the “LN2 pump” similar to the air
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
cooling system of the sublimation vessel. Since the trap is
operated only 25 ◦ C above the corresponding CO2 saturation
pressure during sublimation conditions (pair = 0.1 mbar with
pCO2 ∼ 2.8 × 10−5 mbar), cold spots in the trap are of special concern. To detect potential cold spots, we tested the
trap by filling it with CO2 at 0.001 mbar and held the trap at
−150 ◦ C for 10 min to observe a pressure drop due to condensation or adsorption. Within the range of precision of the
pressure measurement no loss of CO2 was observed, thus, we
are confident that the trap shows no cold spots.
2.2.4
CO2 trap and glass capillary
From the dried air stream provided by the external water
trap, the CO2 trap removes CO2 , N2 O and organic impurities from the bulk air (O2 , N2 , Ar), which is adsorbed in the
consecutive air trap. The CO2 trap is a 8 cm long, U-shaped,
1/800 stainless steel tubing which can be immersed in a dewar filled with LN2 during the trapping procedure (Fig. 3
shows the CO2 trap in immersed position). After sample collection, the CO2 trap can be rapidly heated to transfer the
gases into the glass capillary. For this, a heating jacket is
wrapped around the CO2 trap to allow heating the trap from
−196 ◦ C to +100 ◦ C within 50 s. The tubing and valves enclosed between V4–V5x and V6 (red area in Fig. 1) are permanently heated to 100 ◦ C to minimise CO2 adsorption during the transfer of the CO2 to the glass tube.
The tip of an ordinary Pasteur glass pipet is used to collect
and store the extracted CO2 . Prior to usage the glass capillaries are cleaned in an ultrasonic bath with diluted HCl and
thoroughly rinsed with deionised water to eliminate organic
and inorganic contaminants from the glass surface. The capillary’s tip is flame sealed and the diameter of the open end
(3 mm o.d.) is adjusted and rounded with a hand torch to
fit into the 1/800 o.d. Cajon-Ultratorr adapter with which the
glass tube is connected to the CO2 trap via V5. To pump
off the atmospheric air from the glass tube and tubing, valve
V5x can be connected to the fore-vacuum or after checking
the pressure at PV5 to the high vacuum (VFV4 open, VFV3
closed).
2.2.5
Air reference inlet
The prerequisite of accurate measurements for isotope analysis is the so called principle of “Identical Treatment” (IT)
coined by Werner and Brand (2001). To perfectly fulfil
this requirement, one would need artificial ice with air inclusions of known composition. As such a reference material is not available the second best referencing strategy
is to continuously admit an air reference during the sublimation of gas free ice (“blank ice”). This treatment mimics the air release from the sample during the sublimation as closely as possible. Except for the actual gas release from the ice surface and possible impurities, all subsequent steps are then identical for reference and sample.
Atmos. Meas. Tech., 4, 1445–1461, 2011
For referencing we use two pressurised air cylinders with
known δ 13 C values and known atmospheric mixing ratios
for CO2 and N2 O. These two air cylinders contain current
atmospheric air, with CO2 mixing ratios being reduced to
obtain CO2 mixing ratios covering the minimum and maximum values during the last 800 ka years. Cylinder CA06195
(“Boulder 1”) has a CO2 mixing ratio of 182.09 ± 0.04 ppmv,
δ 13 C of −7.92 ‰ ± 0.003 ‰ with respect to VPDB-CO2 (the
international reference material Vienna Peedee belemnite)
and δ 18 O−4.756 ‰ ± 0.007 ‰ with respect to VPDB-CO2 .
Cylinder CA06818 (“Boulder 2”) has a CO2 mixing ratio of
296.80 ± 0.02 ppmv, a N2 O mixing ratio of 263.4 ± 3.7 ppbv,
δ 13 C of −8.421 ‰ ± 0.003 ‰ with respect to VPDB-CO2
and a δ 18 O − 4.800 ‰ ± 0.014 ‰ with respect to VPDBCO2 . The two cylinders and the values given above were
obtained by the Stable Isotope Lab (SIL) at the Institute for
Arctic and Alpine Research (INSTAAR), University of Colorado, in cooperation with the Climate Monitoring and Diagnostics Laboratory (CMDL) of the National Oceanic and
Atmospheric Administration (NOAA).
Since the gas release during sublimation is a continuous
but slow process, a reference inlet was needed to allow a constant flow of air into vacuum conditions without fractionation
and, thus, to mimic the gradual air release during the ice sublimation process. Our reference gas inlet system consists of
three components (Fig. 1): (1) the pressurised air cylinders
with pressure regulators representing the reference gas supply, (2) an inlet capillary through which a defined amount of
air enters the extraction vessel, (3) a vent capillary to keep
the system well flushed. The pressure of the inlet capillary
can be adjusted from 100 to 350 kPa with a pressure regulator
(0–100 psi, Air Gas, USA), to span the gas release rates during the sublimation of samples (0.02 to 0.06 ml STP min−1 ).
The inlet capillary has an i.d. of 0.05 mm and provides a viscous flow regime at this pressure range. The outlet of this
capillary is some mm above the ice cube surface to achieve
similar flow and mixing conditions of gas and water vapour.
The flow rate of typically 0.04 ml STP min−1 through this
inlet capillary is far too low to flush the reference system
efficiently, a prerequisite for stable measurements. Otherwise long lasting drift phenomena for δ 13 C and CO2 and bad
reproducibility would result. Therefore, the reference system is equipped with a purge valve (“S4”) directing a higher
flow of approx. 1 ml STP min−1 at 200 kPa to a vent capillary (fused silica, 0.05 mm i.d., 15 cm) with which the system can be continuously flushed during operation. The inlet
system is connected to the vacuum system via S5, which is
closed when reference gas is admitted to the sublimation vessel. For ice core samples S5 has to be opened to evacuate the
inlet system, while S4 and S3 are closed. With this reference
inlet, air is admitted during the sublimation of blank ice to
achieve identical treatment of sample and reference. Consequently, however, only the same throughput like for the
real ice samples is achieved this way, i.e. at best two samples a day. When the sublimation device is not used the air
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
reference inlet is flushed with helium via valve S1c while S4
and S6 are closed.
2.3
2.3.1
2.2.6
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Tube cracker GC system
General remarks
Volumetric determination of mixing ratios
Although the main purpose of the system lies in the precise
determination of δ 13 C, it is crucial to measure the mixing
ratio of CO2 and N2 O on the same ice sample with high precision, because: (1) a significant deviation in the CO2 mixing
ratio from neighbouring samples or from different extraction
techniques is a valuable tool to detect contamination or loss
processes during the whole analysis. (2) For a quantitative
interpretation of global atmospheric δ 13 C changes with models, it is imperative to have the data of both the isotopic and
the mixing ratio at the same time interval. Although highly
resolved time series on CO2 mixing ratio are available for
the Holocene (Etheridge et al., 1996; Monnin et al., 2001;
Siegenthaler et al., 2005a; MacFarling Meure et al., 2006),
temporal resolution is yet poor during MIS3 and older periods and dating uncertainties as well as a mismatch in the gas
age–ice age difference of different cores weaken the precision necessary to disentangle global carbon fluxes. (3) Diffusion and fractionation processes in the transformation zone
between bubble and clathrate ice and below call for extraction techniques which allow 100 % extraction efficiencies to
provide unfractionated CO2 concentrations and to validate
the measurements on such ice using the classical dry extraction techniques (Lüthi et al., 2010).
With the mass spectrometer providing the amount of CO2
via the peak area, the amount of the corresponding air is
volumetrically determined from the collected gases in the
air trap. For this we use a glass tube (1/400 o.d., 1/800 i.d.)
filled with 5 Å molecular sieve in pellets (diameter 5 mm,
length 20 mm). Before usage and for weekly regeneration,
water is removed by heating the molecular sieve to 140 ◦ C
for a few hours. During the sublimation, the air trap is immersed into LN2 and acts as a vacuum pump. At −196 ◦ C
the equilibrium air pressure above the loaded molecular sieve
(2 ml STP air) is <0.0020 mbar (“PM ” pressure transducer,
10−4 − 1 Torr max). It is therefore the molecular sieve
that drives the pressure gradient from the sublimation vessel through the external water trap and CO2 trap to the air
trap, thus acting as a cold finger. The quantitative release
of the adsorbed gases is accomplished by heating the trap to
+100 ◦ C. The desorbed air components are expanded into a
2-l expansion volume which is thermally insulated to get a
stable temperature reading. After pressure stabilizes within
the system, the final pressure and temperature of the expansion volume are read out.
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Tube cracker applications are used in many fields of isotope
analysis for many decades. Usually, a glass tube containing the extracted gas sample acts as the interface coupling
a separate sample preparation step to the isotopic measurement at the dual inlet IRMS. Tube outer diameter dimensions normally used with the latter techniques are 1/400 or
even 3/800 and the gas is expanded into the bellow or cold
finger of the MS after breaking the tube and before admitting it to the changeover valve. This direct route to the IRMS
is not feasible for CO2 samples from our ice core samples
for three reasons. (1) Only small amounts of CO2 are available since the amount of ice is limited and/or a high temporal
resolution and replicates are preferred to get a robust record.
(2) Precise δ 13 C values on atmospheric CO2 samples containing N2 O either involve a mathematic correction for isobaric interference, or the separation of both gases via chromatography (Ferretti et al., 2000). (3) The most important
reason for a gas chromatographic separation step prior to the
IRMS measurement is organic contaminants which can interfere in the MS measurement: traces of drilling fluid, which
often accompany deep ice core samples, were occasionally
observed on the Taylor Dome ice core during the measurements of the data set for Smith et al. (1999) and on Dome C
ice by Eyer (2004). Interferences with the solvent ethanol
were reported by Francey et al. (1999) and Elsig (2009) reported contamination from an unknown organic released during the mechanic extraction process. Since some organic
components, like the drilling fluid, behave physicochemically similar to CO2 during the extraction process, minute
traces of this substance can simultaneously reach the ion
source if not separated before. We observed such problems
during earlier measurements of drill fluid contaminated ice
core samples, which manifested themselves by excessively
high m/z 45 and 46 traces yielding highly enriched δ 13 C ratios (>1000 ‰). Note, all measured ice cores in this study
used a drill liquid consisting of two components: The densifier HCFC-141b, 1,1-dichloro-1-fluoroethane, and Exxsol
D30, a mixture of hydrocarbons (Augustin et al., 2007). It
is likely that HCFC-141b causes the interference on m/z 45,
as this mass is one of the main fragments during the ionisation of this molecule (http://webbook.nist.gov). Additionally, HCFC-141b has a lower boiling point (32 ◦ C) compared
to Exxsol D30 (100 ◦ C–150 ◦ C), thus, traces of the densifier
vapour are likely to pass the cold region (−150 ◦ C) of the external water trap. Indeed, at −150 ◦ C the saturation vapour
pressure of HCFC-141b is still 1.5 × 10−5 mbar (extrapolation of data from http://webbook.nist.gov), thus comparable
to the partial pressure of CO2 with 2.8 × 10−5 mbar calculated for the conditions within the external water trap. Consequently, HCFC-141b is able to pass the cold region of the
external water trap.
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Our cryofocus and GC separation avoids these contamination issues. Although a severe contamination can easily
be detected by unusual, or physically or climatologically unrealistic δ 13 C values and excluded from the data set, minor
contaminations are more difficult to detect and require many
replicates or a comparison of different cores. In conclusion,
gas chromatographic separation of CO2 from N2 O and organic impurities is indispensable. From this follows the special design of our CF–IRMS application consisting of an
injection port for a CO2 /N2 O mixture, a flow-through tube
cracker, a Nafion dryer, a cryofocus, a gas chromatograph
and a 2nd Nafion dryer (Fig. 2). Our system is linked to a
MAT 253 via the open split of a conventional Precon GPbox system (Thermo Electron, Bremen). The components
with their special features are described in detail in the following sections.
2.3.2
CO2 /N2 O injection device
To fulfil the requirement of identical treatment of sample
and reference within the cracker-GC-IRMS system (Fig. 3),
a device was constructed allowing CO2 /N2 O pulses to be
injected into the cracker with a 10 µl loop attached via the
“SP1” six-port valve (Valco, USA). The CO2 and N2 O concentration can be adjusted to appropriate values (∼4 % CO2
in He, or ∼8 nmol absolute per 10 µl filling) by dilution of
CO2 and N2 O with He in a mixing chamber. The CO2 used
for these pulses is identical to our monitoring gas and has
a δ 13 C of −4.48 ‰ with respect to VPDB and a δ 18 O of
−19.89 ‰ with respect to VPDB. With two pressure regulators (Porter 8286-SMVS-30, USA) the flow rate is set to
∼0.1 ml min−1 for the CO2 /N2 O mixture and 15 ml min−1
for helium. This mixing device allows a convenient adjustment of the signal height of the CO2 peaks without the
need to switch between different loop sizes. To introduce a
CO2 /N2 O pulse, the SP1 valve is switched to the “inject” position and the GC flow (0.85 ml min−1 ) flushes the CO2 /N2 O
mixture from the 10 µl loop via the “6P2” valve to the cracker
device (Fig. 3).
2.3.3
Cracker
Instead of the CO2 /N2 O pulses described above, a sealed
sample tube containing the trapped CO2 -N2 O mixture from
an ice core sample can be introduced via the cracker device.
The schematic layout of the cracker system can be seen in
Fig. 3. The sealed tubes with the extracted CO2 and N2 O
are mounted into a flow-through tube cracker. After flushing the cracker with helium, the tube is manually broken,
its content is transferred out of the cracker passing a Nafion
dryer and trapped on a cryofocus capillary. With an actuator
the capillary is lifted out of the LN2 to release the trapped
gases, which are then directed to a GC column, where CO2
is separated from N2 O and organic impurities before entering a continuous-flow isotope ratio mass spectrometer via an
Atmos. Meas. Tech., 4, 1445–1461, 2011
open split. A similar line for δ 13 C determination at a precision of <0.05 ‰ was published earlier (Ferretti et al., 2000;
Ribas-Carbo et al., 2002). The main difference is that a discrete amount of CO2 (∼20 nmol) enters our system via the
special tube cracker device which allows us to achieve the
same precision from only 2 ml STP compared to a total sample consumption of up to 45 ml from a large flask sample
reservoir (Ferretti et al., 2000). To reduce the internal volume of the cracker to a minimum, the tube cracker is made of
a 1/16–1/800 o.d. Swagelok reducing union, a 4.5 cm 1/800 o.d.
stainless steel tubing, which is the flexible part and houses
the glass tube, and a 1/8–1/1600 o.d. Valco column end fitting. This fitting is equipped with a 2-µm stainless steel frit
to prevent glass particles from the cracker entering the down
stream part and potentially clogging the valve. The total internal volume of the cracker is only 160 µl, and the sample
is flushed with a He flow rate of only 0.85 ml min−1 to the
cryofocus capillary.
2.3.4
Humidifier
High precision δ 13 C measurements require low and constant
water levels within the ion source since HCO+
2 production
causes apparent sample enrichment (Leckrone and Hayes,
1998; Meier-Augenstein, 1999; Rice et al., 2001). Therefore,
water vapour is generally kept at low levels within the carrier
gas stream of CF-IRMS applications. Contrary to this common notion, a special humidifier device saturating the carrier
gas with H2 O had to be inserted prior to the tube cracker device and consecutively the H2 O is removed again from the
carrier via a Nafion dryer after passing the cracker. The reason for this unusual procedure was that although CO2 pulses
admitted to the cracker via the loop resulted in reproducible
δ 13 C values, similar measurements of CO2 prepared in glass
tubes revealed a serious fractionation with poor precision
(>1 ‰). We found the cause of this problem to be a strong
adsorption and fractionation occurring on the fresh glass surfaces after breaking the tubes in the cracker. To prove this,
empty tubes were evacuated and sealed off at high vacuum.
These tubes were then inserted into the cracker and a first
measurement was conducted where a pulse of ∼8 nmol CO2
from the loop was flushed to the cracker and flowing around
the intact glass tube. A second CO2 pulse was then admitted
to the cracker with the empty tube broken prior to arrival of
the gas at the crushed glass particles. Whereas no effect was
visible in case of the intact tube, a strong fractionation occurred with the crushed tube. A considerable reduction of the
peak area of up to 15 % was noticed, depending on the breaking conditions with more or less glass particles produced
during the cracking. Moreover, for the isotopes, the δ 13 C
and δ 18 O ratios were shifted by ∼+1 ‰ and ∼+2 ‰, respectively. From this we deduced, that a fractionating adsorption
process occurs on the fresh glass surface. Although the tube
cracker technique is well established for many decades (Des
Marais and Hayes, 1976; Caldwell et al., 1983), this method
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
is generally applied for dual inlet-Multiport measurements
with CO2 amounts a factor of 100 larger than in our case.
Although the amount of glass particles can be considerably reduced by scoring the tube, this is not an easy task and
not reliable enough due to the tiny dimensions of the capillary (1 mm o.d.). Consequently, the other possibility was
to inhibit the CO2 adsorption. This is done by providing a
strong adsorbent in excess: water vapour. The helium flow of
the GC carrier gas is bubbled through deionised water within
a 1/400 o.d. glass tube similar to the humidifier used by Leckrone and Hayes (1997). The excess of H2 O compared to CO2
(molar ratio H2 O/CO2 ∼20 within the cracker) then prevents
CO2 from adsorbing at the glass surfaces, accordingly, the
fractionation phenomenon disappeared after installation of
the humidifier. After the cracker an extra long Nafion dryer is
needed again to remove the high load of water vapour before
the He stream enters the cryofocus capillary. We use a Nafion
membrane (0.0300 o.d. and 50 cm length, Ansyco, Germany)
housed in a 1/800 o.d. glass tube and a counter current He
stream of 5 ml min−1 to dry the GC flow.
2.3.5
Cryofocus, GC, IRMS injection
As cryofocussing capillary we use a loop of deactivated fused
silica (0.32 mm o.d., 20 cm length) which can be immersed
in LN2 by a pneumatic actuator. The sharp sample pulse
from the cryofocus is directed to the GC, where separation of CO2 from N2 O and organic drilling contaminants is
achieved at 110 ◦ C using a GS-Carbonplot column (length
30 m, o.d. 0.32 mm, Agilent). Note that at this temperature
the drilling fluid components are strongly retained on the GC
column, thus, during the IRMS measurement we do not observe a separate peak from these compounds. These compounds are only removed at the high temperatures used in
the stand-by mode at 200 ◦ C. A sample chromatogram shows
that CO2 is sufficiently separated from N2 O (Fig. 6a). An
even better separation could be achieved using a lower GC
temperature, however, this also results in a stronger separation of the CO2 isotopologues, thus, increasing the time shift
between m/z 44 and m/z 45 (Meier-Augenstein et al., 1996).
Although this time shift can be accounted for by the software, we observed a stronger dependence of the isotopic ratio on the amount of the sample (usually referred to as linearity effects or amount dependency), a common phenomenon
with GC-IRMS applications (Hall et al., 1999; Schmitt et al.,
2003). Therefore, we choose a GC temperature of 110 ◦ C,
which results in a sufficient separation between CO2 and
N2 O of 14 s (see Fig. 6a) with still a moderate time shift of
typically <0.1 s between the m/z 44 and m/z 45 beams. Nevertheless, we usually observe a small amount dependency of
the measured δ 13 C values, which has to be corrected for (see
section δ 13 C correction).
Finally, the CO2 and N2 O peaks pass the 2nd Nafion dryer
and are transferred to a modified GP-Interface (Thermo Electron, Bremen, Germany) to be admitted to the ion source of
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1453
the MAT 253 via the open split and inlet capillary with a flow
rate of ∼0.3 ml min−1 STP.
Via the reference port and reference capillary (Fig. 3) we
continuously introduce a small amount of carbon monoxide
(∼200 mV for m/z = 28 on the major cup with the standard resistor of 3 × 108 ) to reduce adsorption-desorption effects
of CO2 at the surfaces of the ion source (Elsig and Leuenberger, 2010).
3
3.1
Analysis procedure
Sample preparation and sublimation
Prior to the sublimation of an ice sample, each ice cube
has to be prepared in the following way to provide a clean
surface. With a band saw a cube is cut to the dimensions
3.5 cm × 3.5 cm with 4.5 cm length. To fit this cube into the
sublimation vessel with an internal diameter of 3.3 cm, the
edges are rounded and trimmed with a stainless steel knife to
a cylinder of ∼3.2 cm diameter and a weight of ∼30 g. The
ice is inserted in the precooled sublimation vessel which is
mounted to the flange via a copper gasket. First the atmospheric air is pumped out of the vessel until a constant value
is reached corresponding to the vapour pressure of the ice.
Since CO2 adsorption on the surfaces is a critical issue, two
hours at vacuum to decontaminate the system is necessary.
As pointed out by others, CO2 desorption from surfaces is
most effective at high H2 O pressures (Zumbrunn et al., 1982;
Güllük et al., 1998). With the cooling system, a temperature
of ∼ −34 ◦ C is adjusted within the vessel leading to a pH2 O
of ∼0.25 mbar (pressure transducer “PS ” in Fig. 1). If the
system is leak free, a final pressure of <2 × 10−7 mbar is
achieved at the high vacuum side with gas free blank ice and
∼5 × 10−7 for bubble or clathrate ice due to slow sublimation of the ice, which releases some air. After two hours at
vacuum, a few millimetres of ice have been removed and the
surfaces of the vessel and the traps are sufficiently clean. The
sublimation is started by increasing the current of the halogen bulbs and simultaneously decreasing the temperature of
the cooling gas stream to −115 ◦ C and adjusting its flow rate
such that a pH2 O of ∼0.25 mbar is obtained. Sampling is
then started by cooling down the CO2 trap and the air trap
and opening V3. The steady gas stream liberated from the
ice is now flowing towards the air trap. Collection of gas is
stopped after 50 min by closing V3, V4 and V6 and cooling
down the glass tube. After V5 is opened, the CO2 trap is released from the LN2 Dewar and warmed up for 60 s and CO2
is transferred from the trap to the glass tube. Finally, the glass
tube is flame sealed and stored until the GC-IRMS measurement. Parallel to the last steps, the air trap is removed from
LN2 as well and warmed to 100 ◦ C. The air quantitatively
desorbs from the molecular sieve and the constant pressure
is read out at “PM ” (Fig. 1) to calculate the CO2 and N2 O
mixing ratios.
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Note, that only ∼85 % of the ice sample is sublimated during the 50 min duration leaving around 5 g of the ice sample
in the vessel. A complete sublimation of the ice sample is
not intended since ice impurities (mineral dust and organics)
may gradually accumulate at the ice surface and could enhance chemical reactions when highly concentrated at the final stage. Additionally, the sublimation rate of the remaining
ice piece is getting extremely slow and highly variable leading to scattered results. Regardless of this reasoning, leaving some ice unsublimated does not compromise our 100 %
extraction efficiency, as the remaining ice stays entirely intact. This is different for conventional mechanical extractions, where initially the entire ice sample is crushed but the
gas content is released only partially giving rise to fractionation of the measured mixing ratios for partially clathrated ice
(Lüthi et al., 2010; Schaefer et al., 2011).
3.2
Cracker-GC-IRMS measurement scheme
The high precision attainable using dual-inlet IRMS is based
on the direct comparison of sample and reference of similar
signal amplitude. Gas is admitted to the ion source by identically crimped capillaries. In contrast, for continuous flow
applications, this identical treatment of sample and reference
is accessible only to a certain degree, e.g. by using an internal reference inlet (Meier-Augenstein, 1997). The favoured
strategy is to bracket a sample peak with two references and
try to adjust their signal heights as precisely as possible (Rice
et al., 2001). For practical reasons, a precise adjustment is
not completely possible, therefore the data has to be corrected using empirical functions for the amount dependency
in δ 13 C (see Sect. 4.2). Our strategy outlined here is based
on two different sets of “referencing peaks”. Air standards
admitted to the sublimation vessel during the sublimation of
blank ice offer by far the closest analogue to gas samples
extracted from ice. Due to large effort to produce them, it
is unfeasible to use this type of reference for all correction
purposes. But they are the basis on which the samples are
referred to in terms of absolute value of δ 13 C and the mixing
ratios of CO2 and N2 O. In contrast, CO2 /N2 O injections automatically admitted to the cracker system with subsequent
cryofocusing and GC separation face the same treatment as
sample tubes and feature their identical peak shape. With
these CO2 /N2 O injections the system’s δ 13 C drift with time
and the dependence of δ 13 C on the peak area (“linearity”) are
corrected for (see linearity section in Fig. 5). As pointed out
above, identical measurement conditions are of paramount
importance to obtain a high precision δ 13 C record from the
samples. Therefore, we extract the air from several ice core
samples and store the CO2 in glass tubes until they are all
measured in a single run. A typical measurement session
comprises up to 20 h, starting with a “linearity section” consisting of pairs of an “equilibration peak” (EQ) followed by
a “linearity peak”. The size of the “linearity peak” is varied to produce three different peak heights (L3, L2 and L1),
Atmos. Meas. Tech., 4, 1445–1461, 2011
Fig. 5. Chromatogram showing the first 400 min of a typical measurement session comprising up to 1500 min. Within the first 10 min
CO2 pulses are introduced by the reference port of GP box (so
called std on/off peaks). The “linearity section” runs from 15 min
to 260 min, where pulses of a CO2 /N2 O mixture are introduced
into the tube cracker GC system. The “linearity peaks” are produced in three size classes, L3 in blue, L2 in green, L1 in magenta;
each “linearity peak” is preceded by an “equilibrium peak”, EQ
in black. This scheme of EQ-L3, EQ-L2, EQ-L1 (= one block) is
then repeated five times. In the following “sample section”, starting at 260 min, the first eight measured sample tubes are shown (in
red), with the sample 1 and sample 7 being empty tubes. Identical measurement conditions for each of these peaks are provided by
an equilibration peak (EQ in black) preceding each of them. After
all sample tubes have been processed, a second “linearity section”,
consisting of four blocks, followed by a series of std on/off closes
the measurement session (not shown).
which can later be used to correct for amount effects. The
injection of the CO2 -N2 O mixture to the carrier gas stream
is carried out by switching the valve “6P1”, which injects
∼8 nmol CO2 into the He stream. To produce linearity peaks
with different peak sizes, the loop is filled and flushed out
one, two or three times. A filling-injection cycle lasts for
3 s. Due to the cryofocus the peak shape for different sizes is
identical for tube samples, which is a prerequisite for applying the identical treatment principle. The aim of preceding
each “linearity peak” or sample peak with an equilibration
peak (EQ) is to provide identical conditions by equilibrating both the cracker-GC system and the ion source prior to
all samples. This alternating EQ-L3, EQ-L2, EQ-L1 scheme
is repeated five times until the actual sample tubes (SA) are
measured (sample section in Fig. 5). Again, an equilibration
peak precedes each sample tube, leading to an alternating
EQ-SA scheme. As the process is fully automated except
for the tube cracker itself, only the opening/closing of the
cracker and breaking the tube needs personal attendance.
To be loaded with a new sample tube, the cracker is opened
and the glass shards of the last tube are removed. During
that time the GC carrier flow (0.85 ml min−1 ) bypasses the
cracker via the “6P2” valve (Fig. 1), whereas the second
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
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He flow (cracker flow: 5 ml min−1 ) sweeps the atmospheric
air out to the vent after the cracker is closed. Flushing the
cracker lasts for 60 s, the “6P2” valve is then switched back
and the GC carrier flows through the cracker again. Before
the sample tube is processed further, the preceding equilibration CO2 peak is prepared by switching the “6P1” valve three
times, injecting in total 24 nmol CO2 from the CO2 /N2 O
mixture. The injected mixture flows through the cracker to
mimic the sample and then towards the cryofocus capillary
immersed in LN2. After quantitative trapping (90 s) the capillary is lifted out of the LN2 and the sample directed to the
GC where CO2 is separated from N2 O. Via the open split, the
CO2 and N2 O peaks enter the ion source of the IRMS. After completion of the EQ peak, the sample tube is processed
through bending the cracker, which breaks the scored tube
into two pieces releasing the stored gases into the GC carrier.
All following steps are identical to the preparation of the EQ
peak. Following this alternating EQ-SA scheme, all sample
tubes are measured. A final “linearity section” completes the
IRMS run to check whether the linearity of the system has
changed during the measurement of the samples (not shown
in Fig. 5).
4
4.1
Data processing and correction
Peak integration
To allow for a flexible and transparent peak integration, we
use a self developed peak integration routine, similar to the
software used by Bock et al. (2010). While the start of the
CO2 peak is defined with a slope threshold criterion, the end
of the CO2 integral is determined using the first derivative of
the m/z 44 beam (derivative decreases with the advent of the
N2 O peak).
A special feature of our integration procedure is shown
in Fig. 6b, which shows the zoomed background region of
the CO2 peak. About 90 s before the CO2 peak starts, the
background drops by ca. 3 mV in case of m/z 44 beam. This
step is due to the immersion of the cryofocus in LN2, which
collects not only the current CO2 /N2 O from the cracker for
90 s, but also a small fraction from the tail of the previous
peak. To account for this collected background, we calculate
the “background area” for each beam and subtract this area
from the proper peak area. Although this “background area”
is small (∼0.3 Vs compared to a m/z 44 area 120 Vs for a
typical sample peak) and almost constant, correcting for it
generally reduces the amount dependency of the δ 13 C values,
as this constant background area would otherwise add to a
variable peak area. Note that it is advantageous that the CO2
of the CO2 -N2 O mixture has a similar δ 13 C value to the δ 13 C
of ice core samples, in our case −4.48 ‰ compared to the
sample range of typically −6 ‰ to −7 ‰, thus, minimizing
the influence of a peak-to-peak contamination.
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Fig. 6. (a) Sample chromatogram showing the separation of CO2
from N2 O for an ice core sample. The N2 O peak is clearly visible
in the measured m/z 45/44 beam ratio as a negative bump since the
average molecular mass of N2 O is lower than that of CO2 . However, as the typical atmospheric abundance of N2 O is only 1/1000
of CO2 , the N2 O peak is not visible in the intensity plot below.
(b) Zoom into the region prior to the CO2 peak start to illustrate
the background in front of the peak. The background is determined
around 85 s before the peak start.
To calculate the peak area for N2 O, only the beam m/z 44
is used, since we do not calculate isotope ratios. As the N2 O
peak sits on the shoulder of the ca. 1000 fold larger CO2
peak, the background correction is a critical step in determining the N2 O peak area. From the start of the N2 O peak
backwards we take 10 s from the CO2 tail and 5 s after the
end of the N2 O peak. An exponential fit is calculated covering those background points before and after the N2 O peak.
Using this fit, the N2 O peak is separated from the CO2 peak
and its area is integrated.
4.2
Referencing strategies and δ 13 C corrections
Motivated by the identical treatment principle, we apply the
following hierarchic referencing and correction strategy analogue to Behrens et al. (2008) to account for drift and fractionation processes throughout the entire system.
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1. CO2 pulses of our monitoring gas are introduced by
the reference port of the GP box at the beginning and
at the end of a GC-IRMS measurement run (Fig. 5).
These on/off pulses are used to monitor a drift in the
isotope ratios or the beam intensity of the IRMS during the measurement run. These pulses cannot be used
for any drift correction as they are admitted only at the
start and end of the measurement, but they are helpful
for troubleshooting and excluding errors in the IRMS
source.
2. CO2 /N2 O mixtures injected to the cracker system are
used twofold. First, to correct for the temporal δ 13 C
drift caused by instability of the mass spectrometer itself, changing water levels, but also due to equilibration/saturation processes of the GC column occurring
during the measurement time of >15 h (“drift correction”). Secondly, to provide the empirical relationship
of the m/z 44 peak area to the δ 13 C value (“linearity
correction”), with which the amount dependency of the
δ 13 C values is corrected for.
3. Air reference samples processed with the sublimation
system are the final reference basis for δ 13 C and the
mixing ratios of CO2 and N2 O. Currently, we apply a
“1-point calibration” using the results from the Boulder 2 cylinder, which is isotopically sufficiently close
to our ice core samples. Within each GC-IRMS measurement run consisting of approximately 30 ice core
samples, four to five of these air samples are randomly
measured.
Therefore, the following data processing and correction
scheme is applied on the raw δ 13 C results of the peak integration routine. In a first step, the mean of the air reference
samples is set to the assigned values of the cylinder. The next
step is to correct for a δ 13 C drift during the measurement run.
Using all EQ peaks during the whole run, a cubic spline interpolation is performed and applied to all measured peaks
types (EQ, L1-L3, and SA). Typically, the drift is on the order of 0.04 ‰ h−1 . After this trend correction, the area-δ 13 C
relationship from three different peak sizes (L1, L2, L3) is
calculated and all measured peaks are corrected for “linearity” effects (Hall et al., 1999; Schmitt et al., 2003). The typical slope of the area-δ 13 C function is 0.002 ‰ Vs−1 . For ice
core samples, ranging between 120 Vs for interglacial CO2
values to 80 Vs for glacial values, this translates to a correction of maximum 0.08 ‰. After these corrections, the mean
value of the air reference peaks is adjusted again to the assigned value of the cylinder, thus, referencing all peaks of
the measurement run with respect to Boulder 2. The referencing and correction strategy for δ 13 C presently relies only
on a 1-point calibration. Additionally, the δ 13 C values of the
reference cylinders are 1–2 ‰ more negative than the typical
ice core samples we measure. These two shortcomings onto
Atmos. Meas. Tech., 4, 1445–1461, 2011
the measurement accuracy can be improved in the future using two air standards with δ 13 C values that bracket the δ 13 C
values of the ice core samples.
5
5.1
Procedure verification and comparison
Measurement reproducibility
In the following, we discuss the measurement precision of
the system, starting with the CO2 /N2 O pulses admitted to
the cracker-GC-IRMS, followed by the air reference samples processed with the sublimation system and finally the
ice core samples. The results are summarised in Table 1.
As can be seen for the CO2 /N2 O injections, the precision for
δ 13 C depends on the peak size, with 1 σ standard deviation of
the small “L1” peaks of 0.05 ‰ compared to 0.03 ‰ for the
three times larger “L3” peaks. While for dual-inlet IRMS the
measurement precision can be close to the theoretical shot
noise limit (Merritt et al., 1995), for continuous flow-IRMS
the precision is limited rather by the more complex sample
preparation and the GC system. The precision of our air reference gases Boulder 1 and 2 processed with the sublimation
system is 0.05 ‰ and 0.04 ‰, thus, comparable to that of the
CO2 /N2 O pulses. Therefore, the additional error from the
gas processing in the sublimation system is small. During
the last two years, several samples from different ice cores at
different depth intervals were measured in replicate. These
measurement precisions are compared in Table 1 as well.
Overall, the precision ranges between 0.04 ‰ and 0.08 ‰,
which is slightly lower than the precision for the reference
gases. This is not surprising as additional effects can contribute to the variance of ice core samples. Notably chemical
reactions of impurities within the ice core sample or during
the sublimation, physical fractionation processes during the
bubble close-off from the firn air and also processes during
the drilling and storage of the ice core can alter the δ 13 C composition. It is beyond the scope of this paper to discuss all
these effects, nor does the current data basis yet allow firm
conclusions, but there is indication that ice from the Talos
Dome ice core is more reliable than from other cores. One
hint for its “good ice quality” might be that its inorganic impurity content during the glacial is much lower compared to
the other Antarctic ice cores (Delmonte et al., 2010), reducing the likelihood for chemical reactions and in-situ production of minute amounts of CO2 . This feature of the coastal
Talos Dome ice core can be observed as well for new N2 O
mixing ratio measurements recently obtained (Schilt et al.,
2010), which showed no clear signs of in situ N2 O production in contrast to other ice cores, like the inland cores
EDML or EDC. However, besides these observations, there
is no proven causal link between Talos Dome’s low impurity
concentration, missing N2 O insitu production and its slightly
better reproducibility of CO2 and δ 13 C measurements. More
work is necessary and underway on these issues. As expected
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Table 1. The table shows the average reproducibility (1-σ standard deviation) for measurements of calibrated air samples (Boulder 1 and 2
cylinders) and ice core samples for the parameters δ 13 C, and the mixing ratios of CO2 and N2 O. Ice measurements were done on three
different Antarctic ice cores, two EPICA drill sites (European Project for Ice Coring in Antarctica), EPICA-Dome C, or EDC, (at the
Dome C drill site) and EPICA-DML (drill site at Dronning Maud Land), or EDML and the Talos Dome ice core, TALDICE. From these
different cores, different depth ranges were analysed providing a range of physical and chemical ice properties. The column “n” denotes
the number of replicates per sample. Column “m” denotes the total number of samples, where replicates have been measured. Note that for
CO2 /N2 O pulses no CO2 and N2 O mixing ratios can be derived using our volumetric method.
Type of measurement
CO2
peak size
(nmol)
CO2 /N2 O pulses (L1)
CO2 /N2 O pulses (L2)
CO2 /N2 O pulses (L3)
“Boulder 1” CA06195
“Boulder 2” CA06818
bubble ice EDC (100 m–600 m)
clathrate ice EDC (>1200 m)
mixing zone Talos (600 m–1200 m)
clathrate ice Talos (>1200 m)
clathrate ice EDML (>1200 m)
8
16
24
16
26
16–24
16–24
16–20
18–24
18–24
when using a quantitative extraction technique, ice samples
from the difficult bubble-clathrate mixing zone can be measured for δ 13 C and the CO2 mixing ratio as reproducible as
samples from the bubble ice zone or fully clathrated samples
(Fig. 1).
The precision for the mixing ratios for the air references
and the ice cores are shown in Table 1. For CO2 we obtain
an average 1 σ precision of ca. 1 ppmv for the air references
and 2–3 ppmv for the different ice cores. This precision is
comparable to the precision of mechanical extraction techniques and other studies using sublimation (Etheridge et al.,
1996; Güllük et al., 1998; Siegenthaler et al., 2005b; Ahn et
al., 2009; Lourantou et al., 2010a; Lüthi et al., 2010). For
N2 O we obtain a precision for reference air sample between
1.6 ppbv and 6.9 ppbv, and between 8.2 ppbv and 18.1 ppbv
for the different ice cores. As pointed out above, most glacial
sections of Antarctic and Greenland ice cores show problems
with in situ N2 O production at certain age intervals. Therefore, the high scatter observed for EDML is at least in part
due to the internal variability of the ice rather than an analytic
problem. For the Talos Dome ice core we obtain a precision
of 8–9 ppbv, which is a little less precise compared to the
range of 5–6 ppbv obtained by others (Sowers et al., 2003;
MacFarling Meure et al., 2006; Schilt et al., 2010).
If required, the precision for the mixing ratios can be improved further. As the primary focus of this new technique
lies in high-precision δ 13 C measurements, the devices and
measurement procedure was optimised for δ 13 C, with the
mixing ratios being 2nd priority. The main source of uncertainty in the mixing ratio results from flow changes in the
cracker-GC-IRMS system, which translates into changes in
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n
m
δ 13 C
1σ
(‰)
CO2
1σ
(ppmv)
N2 O
1σ
(ppmv)
4
4
3
3
3
3
3
50
50
50
3
12
28
27
4
15
8
0.05
0.04
0.03
0.050
0.037
0.061
0.082
0.036
0.048
0.053
–
–
–
0.9
1.3
1.9
2.6
1.5
2.6
1.8
–
–
–
1.6
6.9
8.6
9.9
8.9
8.2
18.1
peak size. This behaviour is visible in Fig. 5, where the amplitude of the “EQ” and “L3” peaks slightly increases between 200 min and 300 min. Although this does not affect
the δ 13 C values, it directly translates into the precision of the
mixing ratios.
5.2
Comparison with previous ice core results
While determining the precision of a new method is relatively easy, it is more troublesome to quantify the accuracy.
This is especially true for ice core analytics since properties
like extraction behaviour of the ice sample can substantially
differ from the reference material, typically a cylinder with
pressurised air. The general practise for well mixed atmospheric gases is to compare results obtained with different
methods from different ice cores. If the results agree within
their errors there is confidence that the results represent the
correct past atmospheric composition. Figure 7 provides a
comparison of our results with measurements of previous
studies using different extraction principles and measurement techniques. For δ 13 C we compare our sublimation data
with results using a dry extraction (Elsig et al., 2009). The
mean of all 65 differences is 0.014 ‰ with a 1 σ standard deviation of 0.095 ‰, thus, no significant differences between
the two methods for bubble ice within their error limits exist
(Fig. 7a). For the CO2 mixing ratio, a depth interval of the
EDC core with bubble ice was selected for this comparison
as only here the efficiency of the mechanical extraction is
sufficiently high (typically around 80 %). Additionally, this
interval covers a wide range of CO2 concentrations between
185 ppmv and 265 ppmv (2001). Here, the mean difference
Atmos. Meas. Tech., 4, 1445–1461, 2011
1458
J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
Fig. 7. (a) Comparison of δ 13 C results obtained with the sublimation technique with results using a mechanical extraction device on
the same ice core (Elsig et al., 2009). Shown are the differences between the sublimation and the results of the mechanical extraction
device measured on the EDC ice core over the depth interval from
110 m to 580 m (bubble ice). Differences were calculated where
both methods measured neighbouring ice samples, which have air
occluded which can be regarded identical for this purpose (mean
age differences < 25 years), when taking the averaging effect of the
bubble enclosure process into account. (b) Comparison of CO2
concentration measurements obtained with our sublimation system
with published data from the same ice core using a mechanical extraction device (Monnin et al., 2001). Within the covered depth
range the air is exclusively trapped in bubbles (“bubble ice”). The
measured depth interval covers the time interval of the last deglaciation, thus, covers CO2 concentrations between 185 and 265 ppmv,
i.e. almost the full range of the glacial/interglacial CO2 variability. (c) Comparison of N2 O concentration measurements obtained
with our sublimation system with published data from the same ice
core using a melt extraction device (Schilt et al., 2010). At this
depth range, the bubbles have transformed into clathrate hydrates
(“clathrate ice”). The selected time interval (65–90 ka) includes
rapid N2 O variations between 210 and 265 ppmv, thus, covers almost the full natural variability during the last 800 ka.
between the methods is 2.7 ppmv with a 1 σ of 3.3 ppmv. The
same holds for N2 O, where we compared our data with recent
measurements on Talos Dome ice. Here, the mean difference
is only 0.9 ppbv, with a 1 σ of 10.9 ppbv.
A second approach to assess the accuracy of gas reconstructions from ice cores is to provide a temporal overlap
between atmospheric measurements, firn air measurements
and air trapped in ice. This approach requires a special setting, like an ice core drill site with a very high accumulation
rate where air from recent decades is enclosed. These conditions were fulfilled in studies by Etheridge et al. (1996),
Francey et al. (1999), and MacFarling Meure (2006), using
Atmos. Meas. Tech., 4, 1445–1461, 2011
ice from the Law Dome ice cores, which overlap with firn air
measurements and direct atmospheric samples from the Cape
Grim Air Archive. A temporal overlap between archived air,
firn air and air extracted from ice was accomplished for the
year 1978, where all three archives provided data. However,
due to age spread within the firn column of about 12.5 years
(Francey et al., 1999), the ultimate precision of the temporal
match is set by the age spread of the firn air. These studies
convincingly showed that their CO2 and δ 13 C measurements
on these ice cores match their firn air measurements, and the
archived air samples within the combined measurement uncertainty. In a recent publication (Elsig et al., 2009) we could
demonstrate that our data is in agreement with the ice core
results by Francey et al. (1999), which verifies our results in
terms of accuracy. We acknowledge, that due to the large age
spread of the EDC core (about 170 years for Holocene conditions; Spahni et al., 2003) the comparison with the decadally
resolved Law Dome record cores is challenging given the
observed δ 13 C trend during this interval in the Law Dome
record. A tighter connection of our δ 13 C ice core measurement with the Law Dome-air archive link could be established by remeasuring Law Dome ice with our system.
6
Conclusions
We have presented an analysis system capable of high precision measurements of δ 13 C on CO2 on ice core samples.
When both the ice quality, such as samples from Talos Dome,
and the measurement conditions are optimal then we obtain for δ 13 C measurements a 1 σ precision for neighbouring samples of usually 0.05 ‰. Given the typical long term
variations of atmospheric δ 13 C during the late Pleistocene
of 0.5 ‰, this precision allows for analyses with a signal to
noise ratio of 10, thus, sufficient to provide key constraints
for biogeochemical models. Our system uses for the first
time a sublimation technique, allowing a quantitative extraction for gases trapped in ice core samples. Additionally, the
mixing ratios of CO2 and N2 O can be derived with a precision of 2 ppmv and 8 ppbv, respectively, similar to conventional extraction systems. The various cold traps of the analysis system are equipped with cooling systems using liquid nitrogen regulated by controllers, allowing a highly automated
system free of organic solvents.
The performance of the system was tested on a variety of
Antarctic ice cores and different ice properties ranging from
bubble ice, ice from the bubble/clathrate transition zone and
fully clathrated ice proving the reliability and very high precision of our new method. With the 100 % extraction efficiency of our sublimation method we have now for the first
time an extraction at hand that ensures unfractionated, highprecision δ 13 CO2 analyses also in partially or fully clathrated
ice. This opens the window to the δ 13 CO2 archive in central Antarctic ice from depths greater than about 700 m,
where clathrate formation begins. For instance in the EPICA
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J. Schmitt et al.: A sublimation technique for high-precision measurements of δ 13 CO2 and mixing ratios
Dome C ice core these depths correspond to the time interval
from 25 000 to 800 000 yr BP, i.e. the major part of the entire ice core record. The same applies for CO2 concentration
measurements on clathrated ice, where our new sublimation
technique can act as a reference method for the mechanical
extraction techniques.
Acknowledgements. We thank Hinrich Schäfer and the second
anonymous reviewer for their very constructive and detailed
comments which helped to improve the paper. This work is a
contribution to the “European Project for Ice Coring in Antarctica” (EPICA), a joint European Science Foundation/European
Commission (EC) scientific program, funded by the EC under the
Environment and Climate Program and by national contributions
from Belgium, Denmark, France, Germany, Italy, The Netherlands, Norway, Sweden, Switzerland, and UK. This is an EPICA
publication. Funding by the German Ministry of Education and
Research (BMBF) through the German climate research program
DEKLIM (project RESPIC) is acknowledged. The assistance of
Melanie Behrens is gratefully acknowledged for running the MS
and the organization of the lab at the Alfred Wegener Institute for
Polar and Marine Research, Bremerhaven, Germany. We also thank
for the technical advice from Klaus-Uwe Richter.
Edited by: A. Hofzumahaus
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