Fresenius J Anal Chem (2000) 368 : 702–707
© Springer-Verlag 2000
O R I G I N A L PA P E R
Michael Krachler · Hendrik Emons
Extraction of antimony and arsenic from fresh and freeze-dried plant samples as determined by HG-AAS
Received: 15 June 2000 / Revised: 14 August 2000 / Accepted: 17 August 2000
Abstract Six extraction media (acetic acid, EDTA, tetrabutylammonium hydroxide, NaOH, MeOH/H2O, acetonitrile/H2O) were tested for their ability to extract antimony (Sb) and arsenic (As) from freeze-dried poplar leaves, pine shoots and spruce shoots, as well as from a peat matrix. Additionally, the extraction efficiency of Sb and As in fresh and freeze-dried elder leaves and poplar leaves was compared. Total concentrations of Sb and As of aliquots (~220 mg) of the freeze-dried samples were analysed by flow injection hydride generation atomic absorption spectrometry (FI-HG-AAS) after open vessel digestion with adequate mixtures of nitric, sulfuric, hydrochloric, and perchloric acid. Three reference materials GBW 07602 Bush Branches and Leaves, GBW 07604 Poplar Leaves, and SRM 1575 Pine Needles were analysed with every batch of samples to ensure the accuracy and precision of the applied analytical procedures. The use of hydrofluoric acid in the digestion mixture leads to distinctly lower As values (down to 40%) than actual concentrations in the investigated plant materials. Extraction efficiencies were generally low and lower for Sb than for As. Solutions of 0.66 mol L–1 NaOH liberated highest amounts of Sb with ~10% for poplar leaves, and ~19% each for pine shoots and spruce shoots. Distinctly higher concentrations of As in NaOH extracts of poplar leaves (22%), pine shoots (32%), and spruce shoots (36%) were quantified. Extraction experiments resulted in yields of 7–9% from fresh elder and poplar leaves, respectively, and 8–13% for freeze-dried samples for Sb. The corresponding values for As were 10–35% for the fresh material and 7–37% for the freeze-dried samples.
Introduction The toxicity of antimony (Sb) and arsenic (As) compounds is strongly dependent on their elemental species. There-
M. Krachler · H. Emons () Research Centre Juelich, Institute of Applied Physical Chemistry, 52425 Juelich, Germany e-mail:
[email protected]
fore, reliable speciation procedures are required for a meaningful risk assessment of Sb and As in different specimens. According to the latest IUPAC guidelines issued in 2000 [1] a “chemical species” is defined as a specific form of an element defined as to molecular, complex, or nuclear structure, or oxidation state. However, the intention of this study is to elucidate the “fractionation” of Sb and As in different plant matrices which is one important prerequisite for the development of successful speciation procedures. The term “fractionation” refers to the process of separation of an analyte or a group of analytes from a certain matrix according to physical (e.g. size, solubility) or chemical (e.g. bonding, reactivity) properties [1]. Thus, the present work focuses rather on the extraction of As and Sb from a homogeneous physical phase, i.e. finely-ground powder of plants, than on the identification of various As and Sb species (for example Sb(III), Sb(V),...) present in the native plant material. To this end, fractionation of As and Sb studying the extraction behaviour of both elements, is one important prerequisite for setting up trustworthy speciation procedures for metalloids in solid matter. The final goal is the achievement of a sufficiently high extraction yield, i.e. as much species of an element as possible should be extractable with adequate solvents from the specimens under investigation. If only a few percent of the total elemental concentration are extractable from a matrix, the application of speciation procedures becomes useless because most of the information remains within the sample itself. Another problem encountered with speciation is the preservation of species information during the extraction process. The species pattern of a particular sample might be severely altered by the pH, redox potential, and temperature of the extractant. Moreover, sample collection and storage might influence the species pattern. Frequently, the chemical and physical behaviour of As and Sb, neighbours of a main group in the periodic table of elements, are said to be quite similar [2–4]. Therefore, for example, extraction procedures developed and proved to work adequately for As, should produce comparable results for Sb. However, from our own experience, we cannot sup-
703 Table 1 Selection of extraction procedures for antimony from different matrices
Matrix
Extraction procedure
Reference
Contaminated soil
Single and sequential extraction procedures utilising various extraction media, horizontal shaking at room temperature Extraction with deionised water for 24 h Extraction with methanol-water (1+1) or with 0.2 mol L–1 acetic acid Extraction with water, methanol-water (1+1) or 0.1 mol L–1 KOH 0.3–0.5 g APM + 20 mL high-purity water, magnetic stirring for 2 days at room temperature 0.3 g APM + 6 mL extraction solvent (high-purity water, EDTA, phosphate), horizontal shaking for 4 h at room temperature
[6]
Contaminated soil Contaminated soil Sewage sludge, landfill samples Airborne particulate matter (APM) Airborne particulate matter (APM)
port this commonly accepted hypothesis as regards the similar behaviour of As and Sb. For example, results of the determination of Sb in various plant materials containing different amounts of silicates indicate that up to approximately 40% of Sb are associated with silicates [5]. Consequently, the use of appropriate amounts of hydrofluoric acid in the digestion mixtures is mandatory to achieve correct data for Sb [5]. On the contrary, mineralisation of identical samples without any hydrofluoric acid yields accurate results for As, but distinctly lower values than expected for Sb. In that light also clearly different extraction yields for Sb and As using the same solvents might be observed. So far only a few studies on the speciation of Sb in environmentally relevant samples have been reported in the literature [6–13]. These authors involved extraction procedures (Table 1) to liberate Sb species from the matrix (soil, sewage, and sewage sludge), other studies [2, 10, 11] reported only on water samples of different origin that have been analysed directly by various analytical methods. Extraction yields ranging from 8.6% to 26.9% for Sb from airborne particulate matter have been reported very recently by Zheng et al. [12, 13]. However, Lintschinger et al. [6] extracted less than 2% (0.5% in another study [7]) of Sb from heavily Sb-contaminated soil with water, and only a few percent by using EDTA. Much higher amounts (up to 23%) were extracted using alkaline conditions (0.1 M KOH) and even distinctly higher (51 to 92%) amounts of Sb were leached from soil with 175 mM C2H2O4 and (NH4)2C2O4 [6]. Similarly, only less than 0.5% of Sb was extractable from contaminated soil with a mixture of MeOH/water or 0.2 mol L–1 acetic acid [8]. Sb-extraction efficiencies of 86% and 21% from sewage and sewage sludge, respectively, with 0.1 M KOH have been reported [9]. Unfortunately, no data at all on results of extraction experiments of Sb from plant materials have been promulgated so far. In continuation of our research on the fate of Sb in biota [5, 14–16], this study was undertaken to elucidate two important issues, first, to compare the extraction efficiency for As to that of Sb using various solvents, and secondly, to investigate the extraction yields for both elements from fresh and freeze-dried plant materials.
[7] [8] [9] [12] [13]
Experimental Instrumentation Samples and extracts were mineralised in vessels made from glassy carbon (20 mL, Perkin Elmer, Norwalk, CT, USA) in an aluminium heating block (Gebrüder Liebisch, Bielefeld, Germany). A flow injection system (FIAS 400, Perkin Elmer), equipped with an autosampler (AS 90, Perkin Elmer) was coupled to an atomic absorption spectrometer (AAS 4100, Perkin Elmer) for the quantification of As and Sb in the digests. Reagents and standards For the preparation of all solutions MilliQ water (Millipore, Milford, MA, USA) was used. Acids for mineralisation were sulfuric acid (96%, suprapur®, Merck, Darmstadt, Germany), hydrofluoric acid (40%, suprapur®, Merck), perchloric acid (70%, suprapur®, Merck), and nitric acid (65%, analytical grade, Merck). For the flow injection system the carrier solution was prepared from hydrochloric acid (32%, analytical grade, Merck). NaBH4 solutions were prepared daily by dissolving appropriate amounts of powdered NaBH4 (analytical-reagent grade, Riedel-de Haën) in 0.04% (w/v) NaOH (30%, suprapur®, Merck). Sb(V) was reduced to Sb(III) and As(V) was converted to As(III) with aqueous solutions containing 30% (w/v) KI (suprapur®, Merck) and 5% (w/v) ascorbic acid (analytical grade, Merck). Calibration solutions (0.1 to 2 µg L–1) for As and Sb were prepared daily by diluting aliquots of stock standard solutions containing 1000 mg SbCl3 L–1 in 5 mol L–1 HCl (Merck) or 1000 mg As2O5 in water (prepared from Titrisol® ampoules, Merck) to the appropriate concentrations with 10% HCl. Extraction solutions were prepared from acetic acid (suprapur®, Merck), sodium hydroxide (suprapur®, Merck), EDTA (analytical grade, Sigma, Steinheim, Germany), methanol (analytical grade, Merck), acetonitrile (LiChrosolv®, gradient grade, Merck), and tetrabutylammoniumhydroxide (20% in water, analytical grade, Merck-Schuchardt, Hohenbrunn, Germany). Procedures Extraction Aliquots of approximately 1 g of powdered, freeze-dried samples, weighed to 1 mg, were poured into 15 mL graduated polyethylene tubes (Falcon, Becton Dickinson, Lincoln Park, NJ, USA). Subsequently, 9 mL of extraction medium were added into the tubes. Sample weights of 5 g and volumes of 30 mL of extractant (considering the lower concentration of biological material in the fresh samples) in 50 mL polyethylene tubes were employed for the ex-
704 traction of As and Sb from fresh samples. The sealed tubes were placed on a rotary mixer (Mixer 10, Greiner Labortechnik, Germany) and the soluble fraction of the powders was extracted for 24 h at room temperature. After centrifugation (Medifuge, Heraeus, Christ, Osterode, Germany) of the mixtures at 2000 g for 20 min, 4 mL of the supernatant were pipetted into the digestion vessels and subsequently mineralised.
Mineralisation For the determination of the total concentrations of As and Sb, aliquots (~220 mg) of powdered samples were mineralised with nitric, sulfuric, perchloric, and hydrofluoric acids as described in detail previously [5]. After digestion was completed, digests were quantitatively transferred into the 15-mL graduated polyethylene tubes and filled to 10 mL with high-purity water. Similarly, 4 mL of the supernatant of the centrifuged extracts were mineralised with 1 mL nitric acid, 0.5 mL sulfuric acid, and 0.3 mL perchloric acid. Extracts containing MeOH or acetonitrile were heated in a water bath at 90 °C for 1 h to remove most of the organic solvents before the digestion procedure was applied. The completely clear, homogeneous digests were transferred to the 15mL graduated polyethylene tubes and adjusted to 10 mL with highpurity water.
Determination of antimony and arsenic Concentrations of Sb in the digests were quantified according to our previously reported optimised procedure [5]. Briefly, Sb(V) in 4 mL of digest was spontaneously reduced to Sb(III) with 1 mL of the reduction solution containing KI and ascorbic acid [5, 14]. The identical procedure (reduction step, dilution, calibration curves) was applied to the determination of As in the digests. Detailed instrumental parameters of the FI-HG-AAS system for the determination of As and Sb are summarised in Table 2.
Table 2 Operating conditions for the hydride generation atomic absorption spectrometer for the determination of antimony and arsenic Hydride generation: NaBH4 solution concentration NaBH4 solution flow rate HCl solution concentration HCl solution flow rate Carrier gas flow rate Sample loop
0.3% (w/v), stabilised with 0.04% (w/v) NaOH 5 mL min–1 10% (w/v) 9 mL min–1 Argon, 50 mL min–1 500 µL
Atomic absorption spectrometer: Antimony: Wavelength Slit Lamp current (HCL)a Quartz tube atomizer temperature
217.6 nm 0.2 nm 20 mA 770 °C
Arsenic: Wavelength Slit Lamp current (EDL)b Quartz tube atomiser temperature
193.7 nm 2 nm 380 mA 900 °C
a HCL b EDL
hollow cathode lamp electrodeless discharge lamp
Samples Extraction experiments were carried out with freeze-dried, finelyground, homogeneous bulk materials representing poplar leaves, pine and spruce shoots, as well as a peat matrix. Aliquots of these bulk materials were also mineralised for the determination of total As and Sb in these samples. Additionally, aliquots of fresh specimens (elder leaves, poplar leaves) from the Environmental Specimen Bank (ESB) at the Research Centre Juelich were extracted with appropriate solvents. Fresh samples were also freeze-dried and subsequently extracted with the same extraction media as used for the fresh samples. All investigated samples had a particle size of 200 µm or lower and were collected and processed according to standard operating procedures of the ESB Juelich. Quality control Adequate reference materials (GBW 07602 Bush Branches and Leaves, GBW 07604 Poplar Leaves, both from the Institute of Geophysical and Geochemical Exploration, Langfang, The People’s Republic of China, and SRM 1575 Pine Needles, NIST, Gaithersburg, MD, USA) with certified Sb and As concentrations were analysed with every batch of samples to ensure the accuracy and precision of the applied analytical procedures. All reference materials were used as bottled. Results were corrected for the humidity content in the reference materials as determined on aliquots of each material by a moisture analyser (LP16, Mettler-Toledo, Greifensee, Switzerland).
Results and discussion Initially, we aimed at analysing the extracts directly for As and Sb by HG-AAS after centrifugation of the extraction mixtures considering the supernatant for analysis. However, when using HG-AAS for the determination of As and Sb for that purpose, two major problems hamper that approach. First, as signals for HG-AAS are well known to be strongly matrix-dependent, only matrix-matched calibration curves can be applied for the quantification of the two trace elements in these extracts. Therefore, each extractant would require a separate calibration. Secondly, the analysis of alkaline extraction solutions by HG-AAS would cause severe problems, because acidic solutions are needed for optimal production of stibines and arsines. In that context, the applied digestion of the extracts combines several advantages: (1) Any extraction medium (also alkaline solutions) that can be digested and evaporated at temperatures of ~300 °C can be used, (2) extraction samples become greatly comparable and uniform because the entire extraction solution is evaporated during the open vessel digestion procedure and (3) thus, no matrix-matched calibration curves have to be used for the quantification of As and Sb. Moreover, the employed digestion procedure revealed not at all to negatively influence blank levels or to cause worse analytical reproducibility or losses of analytes [5, 14]. For the determination of As the same experimental conditions (concentrations of NaBH4, HCl, and reduction solution) as applied to the determination of Sb were used except for the temperature of the quartz tube atomiser (900 °C) and the slit width (2 nm). These almost identical experimental conditions revealed to provide even better perfor-
705 Table 3 Certified and experimental concentrations (ng g–1) for As and Sb in three reference materials as determined by HG-AAS after open vessel acid digestion (means and standard deviations)
a indicative
value
Antimony
Arsenic
found
certified
found
certified
GBW 07602 Bush branches and leaves (n = 17)
68 ± 13
78 ± 15
930 ± 40
950 ± 80
GBW 07604 Poplar Leaves digestion with HF (n = 14) digestion without HF (n = 11)
42 ± 4 37 ± 3
45 ± 5
230 ± 30 360 ± 10
370 ± 60
NIST 1575 Pine Needles (n = 23)
159 ± 18
(200)a
210 ± 20
210 ± 40
Table 4 Total and extractable concentrations (ng g–1) of antimony in freeze-dried plant samples as determined by HG-AAS
Total concentration Extraction medium (n = 3) Acetic acid, 0.2 mol L–1 EDTA, 1.25 mmol L–1, pH 4.7 Sodium hydroxide, 0.66 mol L–1 MeOH/H2O, 9+1 (v/v) Acetonitrile/H2O, 1+1 (v/v) Tetrabutylammonium hydroxide, 10 mmol L–1 a determined
Poplar leaves 1
Poplar leaves 2
Pine shoots
Spruce shoots
Peat
86
59
61
62
267
±5
8.0 ± 0.3 8.1 ± 0.5 8.2 ± 0.8 1.7 ± 0.7 5.3 ± 0.2 5.0 ± 0.3
±2
5.2 ± 0.3 5.4 ± 0.4 6.6 ± 0.3 1.4 ± 0.4 4.3 ± 0.6 5.3 ± 1.0
±4
4.3 ± 0.5 4.4 ± 0.8 11.5 ± 0.8 1.5 ± 0.5 2.6 ± 0.2 3.3 ± 1.3
±4
3.2 ± 0.5 3.9 ± 0.2 11.4 ± 0.8 1.3 ± 0.2 3.5 ± 0.2 1.3 ± 0.5
± 21a
4.4 ± 7.6 ± 13.5 ± 1.3 ± 0.8 ± 7.3 ±
0.1 0.7 0.8 0.2 0.2 0.2
by instrumental neutron activation analysis (INAA)
mance for As than for the determination of Sb. Because concentrations of As in the investigated samples were generally higher than corresponding concentrations of Sb, experimental conditions for As were not further optimised.
Quality control The three reference materials GBW 07602 Bush Branches and Leaves, GBW 07604 Poplar Leaves, and SRM 1575 Pine Needles were analysed with every batch of samples to ensure the accuracy and precision of the applied analytical procedures. Experimental concentrations for Sb were always in good agreement with the certified or indicative values, similarly to the concentrations established previously [5]. For As, good results could be obtained for Bush Branches and Leaves and Pine Needles, but concentrations found for Poplar Leaves were about 40% lower than the certified target value (Table 3). However, when the use of hydrofluoric acid was avoided during the mineralisation of Poplar Leaves, accurate and precise results were achieved. A closer look to that problem revealed that most probably the formation of volatile compounds of arsenic and fluorine is the reason for the diminished concentrations of As found in the digests treated with HF. Consequently, all total As concentrations of the investigated specimens were examined in digests with and without HF to elucidate the influence of the matrix on the losses of As during mineralisation.
Freeze-dried plant materials Extraction yields for total antimony Extraction experiments were carried out with freeze-dried poplar leaves, pine shoots and spruce shoots, as well as with a peat material. The total Sb and As concentrations in the peat matrix have been analysed by instrumental neutron activation analysis (Actlabs, Canada) because the applied digestion procedure did not produce adequate solutions for this difficult-to-digest matrix. Table 4 summarises the total Sb concentrations of those specimens, as well as the concentrations extractable from these matrices using different solvents. Generally, independent of the investigated matrix, highest extraction yields were achieved using 0.66 mol L–1 sodium hydroxide followed by 1.25 mmol L–1 EDTA and 0.2 mol L–1 acetic acid. However, also under the most favourable conditions only 9.5% to 11.2% of the total Sb could be extracted from poplar leaves, whereas about 19% of the total Sb could be leached from both pine shoots and spruce shoots with NaOH (Table 4). Only 5% could be liberated from the peat matrix under these alkaline conditions. EDTA and acetic acid yielded comparable extraction yields somewhat lower than those obtained with NaOH. Interestingly, both solvents leached about 10% of the total Sb from poplar leaves – similarly to NaOH – but only about 7% and 6% from spruce shoots and pine shoots, respectively, could be extracted. The use of 10 mmol L–1 tetrabutylammonium hydroxide (pH 11.0) did not improve extraction yields (Table 4). As the investigated plant materials do not predominately consist of lipophilic components,
706 Table 5 Total and extractable concentrations (ng g–1) of arsenic in freeze-dried plant samples as determined by HG-AAS
Total concentration Digestion with HF (n = 5) Digestion without HF (n = 5) Extraction medium (n = 3) Acetic acid, 0.2 mol L–1 EDTA, 1.25 mmol L–1, pH 4.7 Sodium hydroxide, 0.66 mol L–1 MeOH/H2O, 9+1 (v/v) Acetonitrile/H2O, 1+1 (v/v) Tetrabutylammonium hydroxide, 10 mmol L–1 a no
Poplar leaves 1
Poplar leaves 2
Pine shoots
Spruce shoots
Peat
125 ± 16 282 ± 8
104 ± 3 268 ± 5
398 ± 58 423 ± 9
84 ± 8 111 ± 3
6750 ± 1080a
34 ± 1 20 ± 1 61 ± 4 12 ± 1 30 ± 1 36 ± 2
132 ± 5 124 ± 1 136 ± 2 10 ± 1 40 ± 1 98 ± 11
22 ± 1 21 ± 1 40 ± 3 15 ± 1 20 ± 1 27 ± 3
33 ± 35 ± 59 ± 18 ± 31 ± 36 ±
2 3 3 2 3 1
878 ± 229 ± 409 ± 185 ± 90 ± 197 ±
7 6 14 11 1 7
digestion, determination by instrumental neutron activation analysis (INAA)
the use of MeOH/H2O- and acetonitrile/H2O-mixtures provided the lowest extraction efficiencies. Extraction yields for total arsenic As already mentioned in the quality control section, concentrations of arsenic in the investigated specimens might be lower than actual values, if HF is present in the digestion mixture. To demonstrate the impact of HF on the As values, results for As in poplar leaves, pine shoots and spruce shoots are presented in Table 5 from digests containing and lacking HF. The use of HF lead to distinctly higher relative standard deviations compared to results obtained after digestion without HF. Losses of As were quite matrix dependent with poplar leaves responding most sensitive. Only about 40% of As of the actual values in poplar leaves could be found when samples were mineralised with digestion mixtures containing HF, whereas 94% and 76% of the actual concentrations of As were detected in pine shoots and spruce shoots, respectively. Distinctly higher amounts of As – compared to Sb – could be leached from the samples investigated as summarised in Table 5. Again NaOH yielded the highest extraction efficiencies, i.e. 21% and 23% for poplar leaves
Fig. 1 Extraction efficiencies for total antimony and arsenic from freeze-dried plant matrices using 0.66 mol L–1 NaOH as extractant (mean, standard deviation, n = 3)
as well as 32% and 36% for pine shoots and spruce shoots, respectively. Similarly to Sb, only 5% of the total As could be leached from the peat sample with NaOH. The same percentage of As could be leached from spruce shoots with NaOH and acetic acid, whereas for Sb only one third could be extracted with the latter solvent. Additionally, two-fold higher concentrations of As were found in peat extracts of acetic acid compared to that of NaOH (Table 5).
Comparison of extraction efficiencies from fresh and freeze-dried samples The storage and handling of freeze-dried samples is easier compared to fresh samples. That is most probably the reason for the almost exclusively extraction of freeze-dried samples during most of the speciation studies. However, freeze-drying might change the equilibrium, might lead to transformations of species [17] and might cause loss of the analyte. Thus, the native status of the species pattern can be destroyed irreversibly. Moreover, extraction yields for freeze-dried samples might be lower because the organic structure of the samples is altered upon freeze-drying and may lead to the formation of agglomerates that hamper the extraction of the analyte [Falk and Emons, unpublished data]. For this comparison of extraction efficiencies from fresh and freeze-dried samples elder leaves and poplar leaves were extracted with acidic (acetic acid) and alkaline (NaOH) extraction media. Shoots were not investigated because they consist of different parts (needles, stem) whose availability for extraction might be distinctly different. Thus, shoots are not as homogeneous as leaves. Table 6 depicts the results of these comparisons for Sb and As, respectively. For Sb, extraction efficiencies were similar for fresh and freeze-dried leaves and varied from 8.1% to 13.5% for elder leaves and from 7.1% to 10.1% for poplar leaves (Table 6). Using HF in the digestion mixture, only 90% and 50% of the actual As concentrations in elder leaves and poplar leaves, respectively, could be detected. Similarly to the results in Tables 4 and 5, again ex-
707 Table 6 Total and extractable concentrations (ng g–1) of antimony and arsenic in fresh and freeze-dried elder leaves and poplar leaves as determined by HG-AAS
All concentrations are recalculated related to freeze-dried specimens. Water content of fresh samples: Elder leaves 69.4%, poplar leaves 66.0%
Elder leaves
Poplar leaves
Antimony Total concentration (n = 5) Extraction medium (n = 5) Acetic acid, 0.2 mol L–1 Sodium hydroxide, 0.66 mol L–1
646 ± 10 fresh 55 ± 2 60 ± 3
freeze-dried 52 ± 3 87 ± 7
113 ± 2 fresh 8.6 ± 0.8 8.0 ± 0.9
freeze-dried 11.4 ± 0.3 8.9 ± 0.5
Arsenic Total concentration (n = 5) Extraction medium (n = 5) Acetic acid, 0.2 mol L–1 Sodium hydroxide, 0.66 mol L–1
172 ± 4 fresh 34 ± 1 25 ± 1
freeze-dried 41 ± 1 57 ± 3
223 ± 3 fresh 22 ± 2 79 ± 5
freeze-dried 15 ± 1 83 ± 5
traction efficiencies for As were higher compared to that of Sb, with 15% to 33% for elder leaves and 6.7% to 37% for poplar leaves (Table 6). No uniform tendency for higher or lower extraction yields using freeze-dried or fresh specimens could be established. Thus, freeze-drying does not reduce extraction efficiencies for total Sb and As, but, however, potential alteration of the species pattern due to the freeze-drying process remains to be further elucidated.
Conclusions For the determination of total Sb in plant materials the use of HF in the digestion mixture is mandatory to avoid diminished results due to capture of Sb within the silicates that have not been destroyed during mineralisation [5]. However, HF leads to the formation of volatile compounds between arsenic and fluorine during digestion resulting in diminished As values. Therefore, two separate digestion procedures (with and without HF) have to be applied for the determination of Sb and As when an open vessel digestion procedure is employed. Whether or not closed vessel digestion can overcome this problem has to be experimentally established. Extraction yields of the procedures tested in the present study are generally low and lower for Sb than for As. Therefore, future work will focus on the improvement of extraction efficiencies for Sb and As from plant materials by investigating different extraction procedures, i.e. microwave, ultrasonic or supercritical fluid extraction and by testing other solvents. The extraction of both elements might be different in case of plants grown on contaminated soils or sediments which remains to be further investigated. Acknowledgements This study was financially supported by the European Community through the Research Programme TMR, EU
contract No. FMBICT982889. The authors are thankful to F. Backhaus from the Environmental Specimen Bank Juelich for logistic support and help as regards the processing of the investigated samples. W. Shotyk, Geological Institute, University of Berne, Switzerland kindly provided the peat sample and the INAA results for As and Sb in that peat matrix.
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