Improvement of Determination of Trace Amounts of

Food Anal. Methods (2014) 7:1016–1023
DOI 10.1007/s12161-013-9707-4
Improvement of Determination of Trace Amounts of Arsenic
and Selenium in Slim Coffee Products by HG-ICP-OES
Maja Welna & Anna Szymczycha-Madeja & Pawel Pohl
Received: 26 July 2013 / Accepted: 20 August 2013 / Published online: 31 August 2013
# The Author(s) 2013. This article is published with open access at Springerlink.com
Abstract A method for the determination of total inorganic
arsenic and selenium in slim instant coffees using hydride
generation inductively coupled plasma optical emission spectrometry (HG-ICP-OES) was proposed. Various sample preparation procedures, including the traditional total decomposition by the hot-plate or microwave heating in a HNO3/H2O2
mixture and alternative procedures based on the solubilisation
in aqua regia or tetramethyl ammonium hydroxide (TMAH)
and the dilution only with water or a low concentrated HNO3
solution were examined and compared. Corresponding As and
Se hydrides were generated in the reaction of an acidified
sample solution with the NaBH4 reductant in the presence of
antifoam A. A small sample preparation with aqua regia in an
ultrasonic bath followed by the pre-reduction with KI–ascorbic acid in the HCl medium for total As and the boiling with
HCl for total Se were found to be optimal. The external
calibration using standards treated and measured as the same as
samples were applied for the analysis. Limits of detection
(LODs) of 0.96 and 0.55 ng ml−1 were assessed for As and Se,
respectively. The precision (as the relative standard deviation
[RSD]) was within 1.6–7.1 %. The accuracy of the method
was confirmed by the recovery test and the analysis of a standard
reference material (non-fat milk powder, SRM 1459). The developed procedure was applied for the analysis of six commercial
instant slim coffee products available in the Polish market and it
was found that these products contain traces of As (0.114–
0.247 μg g−1) and Se (0.089–0.137 μg g−1).
Keywords Slim coffee . Sample preparation . Arsenic .
Selenium . hydride generation . ICP-OES
M. Welna (*) : A. Szymczycha-Madeja : P. Pohl
Chemistry Department, Analytical Chemistry Division,
Wroclaw University of Technology, Wybrzeże Wyspiańskiego 27,
50-370 Wroclaw, Poland
e-mail: maja.welna@pwr.wroc.pl
Introduction
The coffee brew prepared from ground roasted coffee beans or
soluble (instant) coffee belongs to a widely consumed nonalcoholic beverage worldwide. It is recognized as a rich source
of elements, including essential as well as toxic, that the coffee
plant may take up from a polluted soil, fertilizers or water and
air contaminations. A concise review concerning the elemental analysis of coffee has been presented very recently by Pohl
et al. (Pohl et al. 2013). Because of the ease of use and
cleanliness of the preparation, soluble/instant coffee enjoys
popularity (Vega-Carrillo et al. 2002; Oliveira et al. 2012) and
today, an interest in the consumption of specific soluble coffees as slim coffees, can be observed. They are mixtures of
instant coffee and various bioactive components, and therefore, the people, who wish to reduce their weight, prefer them.
So far, however, no information on the elemental composition
of such products has been reported.
Among various trace elements, Se is important and essential to the human health, while As is classified as one of the
most toxic. Therefore, the assessment of their total contents in
soluble coffee is of great importance, particularly because of
its increasing popularity and a habitual consumption of this
beverage. At present, the hydride generation (HG) technique
coupled with the atomic spectrometry detection is a common
method for the determination of traces of As and Se. The
reduction of hydride-forming elements (e.g., As, Bi, Sb, Se
and Sn) to volatile hydrides in the acid medium by sodium
tetrahydroborate (NaBH4) remarkably increases their detection performance as compared to conventional sample introduction by the pneumatic nebulisation (PN) (Pohl 2004).
Studies concerning Se and As in soluble coffee are rather
scarcely documented than those devoted to other elements
(Pohl et al. 2013). Accordingly, there have been only few
reports on the concentration of aforementioned elements
(Vega-Carrillo et al. 2002; Jose dos Santos and de Oliveira
Food Anal. Methods (2014) 7:1016–1023
2001; Ribeiro et al. 2003; Asfaw and Wibetoe 2005). In
general, the analysis of coffee on the content of As and Se
are performed using inductively coupled plasma optical emission spectrometry (ICP-OES) with PN (Ribeiro et al. 2003) or
HG (Jose dos Santos and de Oliveira 2001; Asfaw and
Wibetoe 2005) after the previous sample total decomposition
by means of the wet digestion in closed vessel microwave
(MW)-assisted (Jose dos Santos and de Oliveira 2001; Asfaw
and Wibetoe 2005) or open vessel (Ribeiro et al. 2003) systems in the presence of concentrated oxidative reagents, particularly mixtures of HNO3 +H2O2. The direct analysis of
solid coffee samples is also possible, carried out by the
INNA method (Vega-Carrillo et al. 2002).
The traditional approach to the sample preparation of coffee
prior to measurements of concentrations of various elements is
the most tedious and time-consuming step of the whole analysis; it commonly requires the use of hazardous reagents and
may lead to sample contamination or analyte losses. Alternative
methodologies, involving partial or no previous decompositions, certainly could avoid or minimize all inconveniences
related to sample digestion. Such procedures would be much
simpler, faster and cheaper and could simultaneously provide
accuracy and adequateness for routine analyses. Unfortunately,
alternatives to acid-based procedures of the coffee sample
preparation are rarely reported and utilised. To our best knowledge, there is no work on the evaluation of optimal conditions
being suitable for the determination of total As and Se in soluble
coffee products by the HG-ICP-OES hyphenated system with
as little sample treatment as possible. So far, an alkaline
solubilisation of instant coffee in tetramethyl ammonium hydroxide (TMAH) solution has been proposed by Ribeiro et al.
(Ribeiro et al. 2003) for ICP-OES multi-element measurements,
including Se. Asfaw and Wibetoe (Asfaw and Wibetoe 2005)
have tested the dissolution in a 0.36 mol l−1 HNO3 solution and
the solubilisation in aqua regia to determine hydride (Se) and
non-hydride forming elements by ICP-OES using a dual-mode
sample introduction system (MSIS) in various beverages (e.g.,
instant coffee). Alternative procedures using a slurry sampling
(SS) technique based on the dispersion of soluble coffee powders in a mixture of diluted HNO3 (1–2 %, v/v) and Triton X100 (1–10 %, v/v) solutions have also been proposed
(Anthemidis and Pliatsika 2005; Magalhaes et al. 1999), but
with a special attention to the determination of Al, Ca, Co, Cr,
Cu, Fe, Mg, Mn, Ni and Zn by graphite furnace atomic absorption spectrometry (GFAAS) or ICP-OES.
In this work we compare several different sample processes
(traditional and alternative), and propose a simple procedure,
i.e., with no previous complete digestion, which is accurate
and precise for the determination of total As and Se within a
low concentration range in soluble slim coffee products by
HG-ICP-OES. The HG reaction from As(III), As(V), Se(IV)
and Se(VI) was also optimised, taking into account its application in measurements of As and Se in real samples. The
1017
selected procedure was applied for the analysis of six different
instant slim coffees commercially available in Poland. To our
best knowledge, this marks the first report on the analysis of
such coffee products, aimed at the evaluation of traces of As
and Se with the undemanding sample treatment.
Experimental
Samples and Reagents
Six instant slim coffee products (numbered as SC1-SC6)
available in the Polish market were analysed. A standard
reference material used to test the accuracy was a non-fat milk
powder (SRM 1459) from National Institute of Standards and
Technology (NIST).
All chemicals were of analytical grade. Concentrated
HNO3 (Merck, Darmstadt, Germany), HCl (POCh, Gliwice,
Poland) and H2O2 (POCh) solutions and solid TMAH
(Sigma-Aldrich, Germany) were used for the sample preparation. Aqua regia was prepared by mixing 3:1 (v/v) concentrated HCl and HNO3 solutions. Stock standard solutions
(1,000 μg ml−1) of As(III), As(V), Se(IV) and Se(VI) were
obtained from their salts, i.e., Na3AsO3, NaHAsO4⋅7H2O
(Sigma-Aldrich), Na2SeO3 and Na2SeO4 (POCh). Working
standard solutions (up to 5.0 μg ml−1 for calibration curves
and 0.25 μg ml−1 for optimization investigations) were prepared by appropriate stepwise dilutions of concentrated solutions. A 1.0 % (m/v) reductant solution was made daily by
dissolving an appropriate amount of powder NaBH4 (SigmaAldrich) in 0.1 mol l−1 NaOH (POCh) and filtering (0.45 μm)
before being used. A 30 % aqueous emulsion of Anti foam A
(Sigma-Aldrich) was used as the anti-foaming agent during
the HG reaction. Solid KI, ascorbic acid and tiourea (POCh)
were used as pre-reducing agents. A 10 % (m/v) KI solution in
2.0 % (m/v) ascorbic acid and 10 % (m/v) tiourea were
prepared by dissolving reagents in water. Deionised water
(18.3 MΏ cm) from an EASYpure system (Barnstead,
Model D7033) was used in all experiments.
Sample Preparation Methods
Samples were prepared as described in the following
subsections.
Open Vessel Hot-Plate Heating Digestion (P1)
Coffee samples (0.5 g) were placed into 150-ml Pyrex beakers
with 12 ml of a concentrated HNO3 solution and left for the
pre-digestion overnight. Then, sample solutions were hotplate heated (85 °C) until all fumes of nitrogen oxides were
ceased. Next, 3 ml of a 30 % (v/v) H2O2 solution was added.
Resulting sample solutions were heated again to reduce their
1018
volumes to about 2 ml, and then quantitatively transferred into
25-ml volumetric flasks and made up to the volume with
deionised water.
Closed Vessel Microwave-Assisted Digestion (P2)
About 0.5 g of coffee samples was subjected to MW heating at
a maximum power of 600 W for 45 min using concentrated
reagents, HNO3 +H2O2 (6+1 ml). After cooling, residual solutions were quantitatively transferred into 25-ml volumetric
flasks and made up to the volume with deionised water.
Digests of samples from both procedures (P1,P2) were
clear and colourless solutions.
Solubilisation in Aqua Regia (P3)
About 0.5 g of coffee samples was weighed into 30-ml polypropylene (PP) centrifuge tubes; then 2 ml of aqua regia was
added and left to react. Next, resulting yellow-orange slurries
were sonicated in an ultrasonic bath for 15 min, made up to
25 ml with deionised water and finally centrifuged (10 min,
12,000 rpm). As a result, bright-yellow solutions were
obtained.
Solubilisation in TMAH (P4)
About 0.5 g of coffee samples was weighed into PP tubes, then
treated with 1.0 ml of a 25 % (m/v) TMAH aqueous solution
and left to react. Next, tubes with brown slurries were shaken
(80 °C, 30 min), cooled, diluted to 25 ml with deionised water
and finally centrifuged (10 min, 12,000 rpm). Resulting sample
solutions were bright-brown.
Dissolution in HNO3 (P5) or Water (P6)
About 1.0 g of coffee samples was weighed into PP tubes and
dissolved in 10 ml of a 2.0 % (v/v) HNO3 solution or water
and then centrifuged (10 min, 12,000 rpm) to remove any
solid particles. Dark-brown sample solutions were obtained.
All sample solutions were analysed in triplicate (n =3).
Coffee SC1 was selected and used for optimization studies.
With each set of sample solutions, blanks were prepared to
correct final results. To avoid differences between the composition of sample and standard solutions, working standard
solutions of As(III,V) and Se(IV,VI) were also processed
through all preparation procedures. The selected procedure
providing the most reliable results was applied to prepare
remaining slim coffees and analyse them for As and Se by
HG-ICP-OES against the external calibration.
Food Anal. Methods (2014) 7:1016–1023
Pre-treatment for Total As and Se Determination
by HG-ICP-OES
For total As and Se determination, any As(V) form present
was pre-reduced to As(III) with 1.0 % KI (in 0.2 % ascorbic
acid) and 3.0 mol l−1 HCl. Accordingly, an aliquot of 2.5 ml of
each sample solution was transferred to a 5-ml volumetric
flask; then 0.5 ml of 10 % KI in 2.0 % ascorbic acid and
1.25 ml of concentrated HCl were added and left to react for
about 30 min. After this time, the sample solution was diluted
to the volume with deionised water. For Se, any Se(VI) form
present was pre-reduced to Se(IV) by heating with 6.0 mol l−1
HCl. Accordingly, an aliquot of 1.5 ml of each sample solution was transferred to a 10-ml tube and poured with 1.5 ml of
concentrated HCl. The tube was stoppered and heated in a
water bath at 90 °C for 30 min. Total As (as As(III) in the first
sample aliquot) and total Se (as Se(IV) in the second sample
aliquot) were then measured by HG-ICP-OES.
Hydride Generation
As and Se hydrides were generated in a continuous flow
system with a gas–liquid phase separation system coupled to
the ICP-OES spectrometer. The system consisted of a modified cyclonic spray chamber, a parallel pneumatic nebulizer
(Burgener) and peristaltic pumps with delivery tubes (Welna
et al. 2011). In the manifold applied, an acidified sample
solution was mixed with the reductant solution in a Yjunction and then, the reaction mixture was introduced at the
bottom of the chamber to a special reaction cavity. Hydrides
and other gaseous co-products were swept by a carrier Ar
stream, introduced through the nebulizer gas inlet, and
transported into the plasma. The sample inlet of the nebulizer
was clogged. Post-reaction wastes were drained with the aid
of a peristaltic pump. For a better plasma stability, the level of
liquid in the chamber was controlled and kept at a constant
level. The same spray chamber and nebulizer were also used
to nebulise the sample solution.
Apparatus
Measurements of total As and Se was performed using a Jobin
Yvon (France) sequential ICP-OES instrument (JY 38S).
Working parameters for the HG reaction and the ICP-OES
detection are listed in Table 1. After the reaction mixture
passed into the chamber, 30 s were necessary to achieve
steady As and Se signals.
A Milestone (Italy) high-pressure MW digestion system
(MLS-1200 MEGA), equipped with a rotor MDR 300/10, was
used for the MW-assisted sample decomposition. An Elpin
(Poland) thermostatic water bath shaker (type 357) and a JP
Selecta (Spain) ultrasonic bath (UltrasonsH) were used for
experiments with aqua regia and TMAH-based sample
Food Anal. Methods (2014) 7:1016–1023
1019
Table 1 HG-ICP-OES operating parameters
Generator (MHz)
40.68
Rf power (W)
Injector i.d. (mm)
Observation zone
Ar flow rates (l min−1)
1000
2.5
12 mm above load coil
Plasma gas: 13.0
Sheath gas: 0.20
Carrier gas: 0.25
1.0
Solutions uptake (ml min−1)
(acidified sample, reductant)
Replicates
Wavelength (nm)
Hydride generation optimum condition
NaBH4 concentration (in 0.1 mol l−1 NaOH)/
% (m/v)
Sample acidity with HCl (mol l−1)
KI–ascorbic acid concentration, % (m/v)
3
As I 197.3
Se I 196.1
1.0
3.0 (As) and 6.0 (Se)
1.0–0.2
solubilisations. An MPW-350 centrifuge (MPW Med.
Instruments, Poland) was used to separate liquid phases from
solid particles.
Results and Discussion
Preliminary Studies
The effect of the oxidation state of As and Se for HG was
firstly studied. Typical HG reaction conditions were used for
this, i.e., NaBH4 and HCl concentrations were kept at 1.0 %
and 3.0 mol l−1, respectively. Single standard solutions of
As(III,V) and Se(IV,VI) were acidified with HCl, and corresponding hydrides were generated by merging them with the
NaBH4 solution. As could be expected, the Se(VI) form was
not reduced and no signal from this element was recorded.
Both As(III) and As(V) forms reacted with NaBH4, but with
different efficiency and response — the signal originated from
As(V) was about 70 % of that provided by As(III). In view of
this, the pre-reduction was necessary prior to the determination of total As and Se in real samples. In addition, it was
found that higher NaBH4 concentrations than 1.0 % made the
plasma unstable; therefore, 1.0 % NaBH4 was used in further
studies.
Next, conditions for the quantitative pre-reduction of
As(V) and Se(VI) into As(III) and Se(IV) were established.
Three reagents were tested: solutions of concentrated HCl,
1.0 % KI in 0.2 % ascorbic acid (KI) and 1.0 % tiourea (TU).
Medium of 3 mol l−1 HCl was used for pre-reduction with KI
and TU. As(III), As(V), Se(IV) and Se(VI) standard solutions
were subjected to the analysis and signals of elements were
measured. It was found that the KI–ascorbic acid mixture and
tiourea provide an effective reduction of As(V) at ambient
temperatures. Moreover, they enhance the As signal coming
from As(III) by about 38 % and 32 % for KI and TU,
respectively. The solution of HCl alone could not reduce
As(V) to its (III) oxidation state. In contrast, for Se, only the
boiling with HCl was established to be the most appropriate.
The quantitative reduction of Se(VI) was achieved by heating
with 6.0 mol l−1 HCl at 90 °C for 30 min. The use of KI–
ascorbic acid led to no signal recorded for Se, likely to the
reduction of Se(IV) and Se(VI) to the Se zero state (Uggerud
and Lund 1995). In the presence of tiourea, Se(VI) was found
to be partially reduced (~30 %) to Se(IV), while the Se signal
from Se(IV) was only 60 % of that obtained after the HCl
treatment.
Based on these results, 6.0 mol l−1 HCl (for Se) and both KI
and TU (for As) were selected for the investigation.
Next, the performance of the HG reaction was verified for
standards prepared and proceeded in the same way as real
samples (procedures P1–P6 and pre-reductions). Standard
solutions of Se(IV) in 6.0 mol l−1 HCl and of As(III) in
3.0 mol l−1 HCl were taken as references for Se and As,
respectively. Effects of the standard solution composition on
the efficiency of the As and Se hydride generation are detailed
in Table 2.
Independently of the sample preparation procedure used,
complete pre-reductions of As(V) and Se(VI) were obtained.
It was established that in the case of Se, the treatment with
TMAH (P4) results in a small increase (8–12 %) in its signal,
while the use of the dissolution in water only (P6) was found
to reduce the signal of this element by about 20 %. The use of
aqua regia (P3), the dissolution in diluted HNO3 (P5) or the
mineralization in different conditions (P1,P2) does not change
the signal of Se and provides sensitivities comparable to this
recorded for the reference Se standard. Considering these
results, it seems that various sample preparation methodologies for the total Se determination used here have a slight
effect on the efficiency of selenium HG. An opposite effect
was observed in the case of As. It was found that in the
presence of KI–ascrorbic acid, added to pre-reduce As(V),
the reduction of the As signal by about 15 % was observed for
conditions used in mineralization procedures (P1,P2). For the
remaining procedures (P3–P6), improvements of the As response were noticed (35–56 %), and this effect was more
pronounced (51–56 % increase) when the treatment with aqua
regia (P3) or TMAH (P4) was used. Taking into account the
enhancement of the As signal from As(III) after adding KI
(38 %) to the reference standard, it can be concluded that
diluted HNO3 (P5) and water (P6) do not influence the As
emission. Changes in the intensity of this element are likely
due to the KI addition. For procedures P3 and P4, it seems that
both aqua regia or TMAH used for sample preparation and KI
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Food Anal. Methods (2014) 7:1016–1023
Table 2 Ratio (R%) of intensities of elements recorded for standards prepared and proceed as real samples (procedures P1–P6 and pre-reductions) to
reference standardsa (optimal conditions for HG-ICP-OES measurements of As and Se)
Pre-reducing reagent
6.0 mol l−1 HCl
1.0 % KI–0.2 % ascorbic
acid (in 3.0 mol l−1 HCl)
Element
Se
As
1.0 % tiourea (TU)
(in 3.0 mol l−1 HCl)
Se(IV)
Se(VI)b
As(III)
As(V)b
As(III)
As(V)b
Procedure (P)
Digestion in HNO3/H2O2 (P1, P2)
97.9
105
81.8
86.4
65.3
60.8
Solubilisation in aqua regia (P3)
Solubilisation in TMAH (P4)
Dissolution in 2 % HNO3 (P5)
Dissolution in H2O (P6)
103
112
96.9
77.6
104
108
95.6
80.0
156
155
144
138
152
151
141
135
129
134
131
133
132
129
136
134
a
Reference (optimal) condition: Se(IV) in 6.0 mol l−1 HCl (Se) and As(III) in 3.0 mol l−1 HCl (As)
b
As As(III) or Se(IV) after pre-reductions
may lead to an improvement of the As signal. If TU was used
to pre-reduce As(V), the reduction of the As signal was also
noticed under mineralization conditions (P1,P2); however, the
effect was stronger (~35 %) than that obtained when KI was
employed. Similarly, for other procedures (P3–P6) the As
response was increased, but in this case the effect of the
reagent used for the sample preparation is rather negligible if
one takes into account the As signal enhancement coming
from addition of TU only (32 %) to the reference standard.
Therefore, the KI–ascorbic acid mixture was selected finally
as the pre-reducing reagent for the total As determination.
Noteworthy, these investigations confirmed the necessity of
the preparation of standard and sample solutions in the same
experimental conditions in order to obtain reliable results. It is
especially important in the case of comparison between external and standard addition calibrations when studying possible
matrix interferences.
Comparison of Sample Preparation Procedures
The comparison of six various preparation procedures of slim
coffee samples prior to the determination of As and Se by HGICP-OES was performed by evaluating the precision and the
accuracy of results achieved in addition to limits of detection
(LODs) of As and Se. The precision was expressed as the
relative standard deviation (RSD). LODs were determined as
concentrations corresponding to three times the standard deviation (3× SD) of ten executive measurements of respective
procedural blanks (3σ criterion). Spiking experiments and the
determination of recoveries of added As and Se were used to
check the accuracy. In these experiments, coffee SC1 was
spiked with known amounts of As and Se species (0.25 and
0.50 μg ml−1) and analysed following the same sample preparation procedures and the pre-reductions. Finally, recoveries
of As and Se for each sample preparation procedure followed
by the HG-ICP-OES measurements were assessed. These
results for P1–P4 procedures are listed in Table 3.
Traditional Procedures
As can be seen, results obtained using total wet digestion
procedures (P1,P2) demonstrate that both sample treatments
are quite suitable for the coffee sample preparation. The
precision of measurements and the accuracy obtained were
almost the same. RSD values varied between 1.4–6.1 % and
0.4–6.4 % for hot-plate (P1) and MW-assisted (P2) wet digestions, respectively. Recovery values were, independently on
the concentration and the speciation form of element, changed
from 95.7 % to 108.5 % (P1) and from 98.1 % to 107.1 %
(P2). Additionally, results for As(V) and Se(VI) prove the
recommended pre-reduction step for converting As and Se
into their lower oxidation states. The detectability of elements
was also comparable. Differences between As(III) or As(V)
and Se(IV) or Se(VI) were not marked; LOD values obtained
using the hot-plate digestion procedure (P1) were only slightly
higher (by 20–30 %).
Alternative Procedures
Among the procedures with no previous digestion (P3–P6),
the solubilisation in TMAH (P4) was established to be justified, but only to the As determination. As can be seen in
Table 3, quantitative recoveries were obtained for two considered As species (101.2–106.6 %), and RSDs (1.2–7.4 %) were
practically the same as those achieved for wet digestion procedures (P1,P2). LODs of both As species resulted from the
use of TMAH were however two times lower as compared to
those assessed when using traditional procedures (P1,P2).
Food Anal. Methods (2014) 7:1016–1023
1021
Table 3 The analytical characteristic of different coffee sample preparation procedures for the total As and Se determination by HG-ICP-OES
Procedure
Hot-plate heating
digestion (P1)
Element Addition
Parameter
0.25 μg ml−1 0.50 μg ml−1 0.25 μg ml−1 0.50 μg ml−1 0.25 μg ml−1 0.50 μg ml−1 0.25 μg ml−1 0.50 μg ml−1
As
As(III)
As(V)
Recovery (%)
RSD (%)
LOD (ng ml−1)
Recovery (%)
RSD (%)
LOD (ng ml−1)
Se
Se(IV)
Se(VI)
a
b
95.7
6.1
4.3
97.7
2.9
101.1
1.4
102.8
1.7
4.3a
Recovery (%) 106.1
RSD (%)
4.9
LOD (ng ml−1)
3.5
Recovery (%) 108.5
RSD (%)
4.1
LOD (ng ml−1)
3.5a
Microwave-assisted
digestion (P2)
107.1
6.4
3.4
103.2
1.3
102.2
5.2
98.5
0.4
3.4a
99.2
3.2
101.5
3.0
102.5
1.6
2.6
98.1
4.6
2.4a
Solubilisation in aqua
regia (P3)
103.1
3.5
0.97
103.5
1.6
95.7
4.2
103.0
4.9
0.96a
99.4
0.4
100.5
1.1
100.6
5.9
0.55
102.4
7.1
0.57a
Solubilisation in
TMAH (P4)
101.2
5.6
1.9
103.2
10.1
101.5
1.2
106.6
7.4
2.0a
104.1
2.4
NDb
108.8
2.1
NDb
As As(III) or Se(IV)
Not detected (0 % as recovery); solid residue after pre-reduction of Se(VI) into Se(IV) with 6.0 mol l−1 HCl
Unfortunately, this procedure was found ineffective for the Se
determination (no recovery of Se observed). It was established
that when the recommended pre-reduction step with HCl prior
to the Se determination by HG-ICP-OES in sample solutions
prepared was applied, a solid residue was formed in these
conditions that particles possibly could retain the volatile element species and/or tend to its decay. Although the sample
preparation procedure using TMAH for soluble coffee prior to
its elemental analysis (including the Se content) was previously
proposed in one work (Ribeiro et al. 2003), the conventional
PN was applied there for elements' measurements by ICP-OES.
Simple coffee samples dissolutions in diluted HNO3 (P5)
or water (P6) were unsuitable. It was evident mainly for Se,
which could not be determined (no recovery) for the same
reason as discussed previously when TMAH was used (P4)
(formation of solid residue after pre-reduction step with HCl).
In case of As, the application of a diluted HNO3 solution (P5)
led to much poorer recoveries (9.5–12.8 %) and the precision
(6.1–11 % as RSD), and up to two times higher detectability
(6.5 ng ml−1) as compared to that achieved from the digestion
procedures (P1,P2). Using water for the sample dissolutions
(P6), recoveries of As were high (80.3–90.1 %) but not
quantitative. RSDs (4.6–8.4 %) were within the range
obtained for wet digestions, while LODs (2.3 ng ml−1) were
better than those achieved when traditional procedures
(P1,P2) were used. Additionally, it must be noticed that samples treated with diluted acid or water only required definitely
the presence of Antifoam A in the reducing agent solution to
avoid an extensive foam formation during the HG reaction,
which affected the stability of ICP-OES.
The use of aqua regia (P3) was found to completely solubilise As and Se species, making them available for the HG
reaction. Similar results for Se were obtained by Asfaw and
Wibetoe (Asfaw and Wibetoe 2005). This treatment assures
the complete recovery of both As forms that were changed
from 100.6 % to 108.8 % (Se) and from 95.7 % to 103.5 %
(As). RSD values obtained in these conditions, i.e., 2.1–7.1 %
(Se) and 1.6–4.9 % (As), confirm the good precision of
measurements. Satisfactorily, LODs of As and Se evaluated
for this sample treatment were also the lowest. Accordingly,
they were from four (As) to six (Se) times lower than those
achieved for the MW-assisted wet digestion (P2) as well as 1–
2 orders on magnitude lower in comparison to those estimated
when applying PN to measure As and Se by ICP-OES (As:
88 ng ml −1 ; Se: 59 ng ml −1). Thus, it seems that the
solubilisation in aqua regia is adequate for the determination
of traces of As and Se in samples of soluble coffee. Sample
solutions obtained during the proceeding with aqua regia were
orange-yellow slurries, suggesting a partial decomposition of
the organic matter of coffee samples. No precipitation was
found when the pre-reduction step prior the total As and Se
determination by HG was carried out. In contrast, for the
remaining procedures (P4,P5,P6) resulting samples solutions
were brown, suggesting the simple coffee dissolution instead
of the partial decomposition, which can explain the later
difficulties in Se measurements.
1022
To conclude, the solubilisation in aqua regia offers a simple
and reliable alternative way to instant coffee analysis for the
total content of As and Se without the need for complete
digestion of sample matrix. Consequently, it was chosen for
further studies.
Food Anal. Methods (2014) 7:1016–1023
Table 4 The comparison of As and Se contents (mean±SD, n =3) in the
slim coffee SC1 and the NIST SRM 1549 obtained after the solubilisation
in aqua regia and the wet digestion of their samples and using the external
standard calibration
Procedure
Optimization and Verification of Alternative Procedure
An optimal S/L ratio (solid-to-liquid, m/v) for the sample
preparation with aqua regia (P3) was evaluated, assuring the
complete recovery of As and Se forms. The following sample
masses and final sample solutions (including 2 ml of aqua
regia added) were tested: 0.5/10, 0.5/25, 1/10 and 1/25 (g/ml).
Resulting sample slurries were sonicated for 15 min, made up
to the required volume with water and centrifuged before the
analysis. Concentrations of resulting sample solutions were
5 %, 2 %, 10 % and 4 % (m/v). The results were evaluated as
arsenic and selenium recoveries using the standard addition
method. It was observed that releasing of As and Se into
solutions depended on the S/L ratio and increased with the
dilution. The best results, i.e., quantitative recoveries, were
found for a sample solution concentration of 2 %. A lowering
in the recovery of As and Se for the higher concentration (4–
10 %) was suspected, because obtained sample solutions were
from non-clear (2–4 %) to even muddy (10 %) and required an
additional filtration before the analysis. Such an effect was not
observed for the 2 % solution.
In addition, spiking experiments on developed sample
preparation and pre-reduction procedures were carried out
and different concentrations of Se(IV,VI) and As(III,V) were
added (0.1–0.5 μg ml−1) as a single one or both of the As or Se
species considered. Recovery values obtained were from
97.8 % to 106.2 % (Se) and from 98.1 % to 106.5 % (As),
evidencing the absence of losses of As and Se or the contamination during all steps of the analysis.
Another experiment, made to verify the reliability of the
proposed methodology (P3), was based on the comparison of
concentrations of As and Se determined in instant coffee SC1
using this procedure with those determined in sample solutions resulted from the complete acidic digestion of respective
samples (P1,P2). Preliminary investigations showed that coffee chosen for the optimization (SC1) has Se below the LOD.
Therefore, in the case of Se, the NIST SRM 1549 with a
certified concentration of Se was analysed. As shown in
Table 4, results obtained with the partial sample decomposition (P3) correspond well with those obtained after the execution of total wet digestion procedures (in case of As) and
with the certified value, i.e., 102.4 % as recovery (in case of
Se). Moreover, the solubilisation in aqua regia was the only
procedure that was adequate for the Se determination in the
NIST SRM 1549. Additionally, the comparison of results
achieved using for the calibration the method of standard
additions and external standards (see Table 4) indicates that
P1
P2
P3
Concentration (μg g−1)
As
soluble slim coffee (SC1)
Se
SRM 1549a (non-fat milk)
0.232±0.013
0.221±0.006b
0.197±0.013
0.232±0.010b
0.242±0.008
0.248±0.021b
NDc
NDc
0.113±0.008
P1 hot-plate wet digestion, P2 microwave-assisted wet digestion, P3
solubilisation in aqua regia
a
Certified value 0.11±0.01 μg g−1
b
Using standard addition calibration
c
Not detected
no matrix effects were present when using the proposed
methodology.
Application
The proposed procedure (the solubilisation in aqua regia, P3)
was used to determine total As and Se concentrations in
various slim coffees (SC1–SC6) and in one common instant
coffee (C), for comparison. Results are presented in Table 5.
As can be seen, the precision typically varied between 6.0 %
and 9.0 % (as RSD). Much higher RSD values (11–13 %) were
obtained when extremely low concentrations of As and Se were
present in analysed coffees. It was established that concentrations of As ranged from undetectable to 0.247 μg g−1. In the
case of Se, it was from undetectable to 0.137 μg g−1. In two
samples, i.e., SC3 and SC6, both As and Se contents were
below their respective LOD values. Contents of As and Se
found in 100 % instant coffee (C) were close and up to two
Table 5 Total Se and As
concentrations in
analysed instant coffees
using the developed
methodology
a
Average values (n =3)
with relative standard of
deviation (RSD) in
brackets
b
Below LOD
Samples
Concentrationa (μg g−1)
Se
As
SC1
SC2
SC3
<LODb
0.089 (13)
<LODb
0.247 (8.5)
<LODb
<LODb
SC4
SC5
SC6
C
0.137 (9.0)
<LODb
<LODb
0.240 (6.6)
0.114 (11)
0.143 (7.6)
<LODb
0.282 (6.0)
Food Anal. Methods (2014) 7:1016–1023
times higher, respectively, than average values determined in
analysed slim coffee products. Ought to the lack of works
concerning the analysis of slim instant coffees for total As
and Se, concentrations of these elements were compared to
those reported for typical instant coffees. Our results were well
suited within concentrations ranges given by others authors,
As <0.200 μg g−1 (Vega-Carrillo et al. 2002; Jose dos Santos
and de Oliveira 2001) and Se <0.300 μg g−1 (Vega-Carrillo
et al. 2002; Jose dos Santos and de Oliveira 2001; Ribeiro et al.
2003; Asfaw and Wibetoe 2005).
Conclusions
The efficiency of As and Se HG was found to be strongly
dependent on the sample preparation procedure and reducing conditions. The degree of the sample decomposition
was critical and affected the H2Se formation. The developed and optimized analytical methodology based on the
partial decomposition of samples using their solubilisation
in aqua regia and ultrasonication of sample slurries formed
at room temperature, followed by the pre-reduction of
element species with KI–ascorbic acid (for total As) and
boiling with HCl (for total Se) and finally the detection of
both elements by HG-ICP-OES demonstrates the appropriate reproducibility, precision, accuracy, and sensitivity for
dependable determinations of traces of As and Se in soluble slim coffee products. The partial decomposition was
found to be especially essential in the case of Se, which
could be completely lost during the recommended prereduction step of Se(VI) with HCl prior to the HG reaction
if other sample preparation procedures were used, e.g., the
solubilisation in TMAH, the dissolution in diluted HNO3 or
water only. In the case of As, the treatment with aqua regia
followed by the pre-reduction of As(V) with KI–ascorbic
acid enabled to increase the signal of As by nearly 50 % as
compared to the result of an aqueous arsenic standard
acidified only with HCl (without any additives).
1023
The proposed procedure is safe, reduces the sample handling, minimizes the time and reagent consumption as well
eliminates losses of As and Se or the contamination of samples. Thus, it can be a vital alternative to traditional sample
treatment approaches based on the wet digestion with concentrated oxidative reagents.
The analysis of various soluble coffee products indicates
that some of them contain traces of As and Se, i.e., below
0.3 μg g−1.
Acknowledgments The work was financed by a statutory activity
subsidy from the Polish Ministry of Science and Higher Education for
the Faculty of Chemistry of Wrocław University of Technology.
Conflict of interest Maja Welna declares that she has no conflict of
interest. Anna Szymczycha-Madeja declares that she has no conflict of
interest. Pawel Pohl declares that he has no conflict of interest. This article
does not contain any studies with human or animal subjects.
Open Access This article is distributed under the terms of the Creative
Commons Attribution License which permits any use, distribution, and
reproduction in any medium, provided the original author(s) and the
source are credited.
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