Feasibility of Identification and Monitoring of Arsenic Species in Soil and Sediment Samples by Coupled Highperformance Liquid Chromatography — Inductively Coupled Plasma Mass Spectrometry† P. THOMAS*a , J. K. FINNIE‡b AND J. G. WILLIAMSc aInstitut Pasteur de L ille, Service Eaux Environnement, 1 rue Calmette, BP 245, F-59019 L ille Cedex, France bDepartment of Geology, Royal Holloway College, University of L ondon, Egham, Surrey, UK TW200EX cNERC ICP-MS Facility, Centre for Environmental T echnology, Imperial College, Silwood Park, Ascot, Berkshire, UK SL 5 7T E The determination of four arsenic species (AsIII, non-toxic. The diVerence in the toxicity of species is dramatic, dimethylarsinic acid, monomethylarsonic acid and AsV ) in soil with the inorganic forms having a toxicity comparable to that and sediments following a single microwave extraction of strychnine whereas the toxicity of the organic species is procedure was investigated using an on-line system involving similar to that of aspirin.2,3 high-performance liquid chromatography (HPLC) coupled Of interest to this study are the anthropogenic sources of with inductively coupled plasma mass spectrometry (ICP-MS).arsenic found in soils and sediments, such as arsenical pestic- Phosphoric acid was used in conjunction with an open focused ides, fertilisers, irrigation, dust from the burning of fossil fuels microwave system to extract arsenic compounds. This system and disposal of industrial wastes.Metal-containing waste was optimised with respect to acid concentration, microwave materials which may aVect groundwater pollution include solid power and time in order to obtain the maximum rate of wastes, dredged material and industrial by-products. More recovery whilst retaining the integrity of arsenic species. Using recent surveys4 undertaken in five EC countries indicate that an anion-exchange column and buVered phosphate solution about 89 000 contaminated industrial sites exist, which require with methanol added as the mobile phase, good separation and immediate treatment because they either present an environsensitivity were achieved.Under these conditions, recoveries mental health problem or cannot be re-used without being between 60 and 80% of the total As content were obtained, de-contaminated. With regard to loss of groundwater resources and the detection limits were in the range 1–2 mg kg-1 for all ascribable to contamination, it is important to address arsenic species.The HPLC–ICP-MS system was used for the species in order to assess the behaviour of these compounds determination of arsenic species in acid extracts of in-house in case of remediation or land usage. reference materials (soil and sediment samples from polluted A number of coupled techniques have been developed which areas). Only arsenate was found in soil but arsenite was the allow the speciation of arsenic in a variety of media.Highmain species found in a polluted river sediment. Three certified performance liquid chromatography (HPLC) coupled with reference materials were analysed to determine the inductively coupled plasma mass spectrometry (ICP-MS) proconcentrations of the four arsenic species, and the sum of the vides an ideal combination for the eYcient separation and arsenic concentration was compared with the certified total detection of arsenic species. Previously the separation has been arsenic value for each of the reference materials. This method achieved for water and marine biota samples5–8 using reversedallows the speciation of arsenic species in soil and sediment phase anion and cation pairing modes or ion-exchange colsamples to be determined at natural levels, and enables their umns.For soil extracts better results have been achieved with behaviour in the environment to be monitored. the anion-exchange mode.9,10 The main advantages of ICP-MS over other detection techniques is that it allows both on-line Keywords: Arsenic speciation; inductively coupled plasma mass real time analysis of the HPLC eluate with a high level of spectrometry; ion exchange; high-performance liquid sensitivity.ICP-MS is also an element specific detection chromatography; microwave extraction; soil; sediment method which allows the majority of interferences to be resolved from the analytical peaks. The main drawback of this Arsenic has a wide range of industrial uses and, as a conse- instrumentation is that arsenic has only one isotope (m/z 75), quence, anthropogenic emissions greatly exceed natural levels.which can suVer interference from the ArCl polyatomic ion The toxicity and mobility of arsenic in the environment are produced during extraction of the plasma through the interface. dependent on the chemical form or species in which it exists. However, this can be successfully attenuated by the addition It is well known that inorganic arsenic such as arsenite (AsIII) of organic modifiers.11–16 and arsenate (AsV) are the most toxic arsenic species.1 This technique requires that soil and sediment samples must Methylated arsenic such as monomethylarsonic acid (MMA), be in solution for analysis, and therefore the arsenic species dimethylarsinic acid (DMA) and trimethylarsine oxide must be extracted both quantitatively and in such a way that (TMAO) are less toxic and arsenic compounds such as arseno- the integrity is maintained.Approaches such as nitric acid or choline (AsC) and arsenobetaine (AsB) are considered to be perchloric acid digestion in conjunction with hydrofluoric acid are too aggressive and lead to the destruction of arsenic species. Acid leaches are an alternative, but the most commonly used † Presented at the 1997 European Winter Conference on Plasma of these, an aqua regia leach,17 is unsuitable for this technique Spectrochemistry, Gent, Belgium, January 12–17, 1997.owing to the presence of chlorine. A recent study to produce ‡ Present address: Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire, UK PE17 6LS. CRMs for speciation reported favourable results with the use Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1367–1372) 1367Table 1 ICP-MS operating conditions of phophoric acid combined with microwave heating for the extraction of arsenic from soils.18 That study used a low Forward power 1350 W concentration of acid (0.3 M) and the results were promising.Reflected power 1 W Nevertheless, the idea of certifying soils and sediment was Plasma gas 14 l min-1 abandoned because of several diYculties that arose during the Auxiliary gas 1.1 l min-1 Nebuliser gas 0.80 l min-1 first trial. In our study, we decided to investigate further the Nebuliser Glass concentric nebuliser, TR30A use of phosphoric acid as an extraction agent.Microwave (Glass Expansion, Camberwell, heating was chosen as it allows better temperature control Victoria, Australia) than heating blocks, which in turn leads to greater reproduc- Spray chamber Borosilicate glass Scott double pass, ibility of results. In addition, there is a reduced chance of water cooled (5 °C) arsenite oxidation with shorter heating times. Ion sampling— Sampling cone Nickel, 1.0 mm orifice This feasibility study involved work undertaken to produce Skimmer cone Nickel, 0.75 mm orifice an extraction procedure for arsenic in soils and sediments Sampling depth 13 mm from load coil using phosphoric acid which allows the integrity of the species T ime-resolved analysis peak-jumping parameters— to be maintained. The results are reported, together with those Time per slice 1.00 s from an anion-exchange HPLC–ICP-MS method.This method Points per peak 3 was found to be suitable for the qualitative and quantitative Detector mode Pulse counting Selected isotopes As at m/z 75, ClO at m/z 51 analysis of the acidic extracts produced. Composition of mobile phase A and B— A: (NH4)2HPO4–(NH4)H2PO4510 mmol l-1, pH 7.0, 3% EXPERIMENTAL MeOH B: (NH4)2HPO45100 mmol l-1, pH 8.5, 3% MeOH Standards and Reagents Arsenic standard solutions (1000 mg l-1) were prepared as follows: arsenite, 1.31 g of As2O3 (Aldrich, Milwaukee, WI, eVected by simply connecting a length of PEEK tubing USA) dissolved in 4 g l-1 of NaOH (Merck, Darmstadt, (0.17 mm id) from the exit of the column directly to the Germany); arsenate, 4.15 g of Na2HAsO4 7H2O (Aldrich) disnebulizer.The capillary tube was kept as short as possible in solved in distilled water; monomethylarsonic acid (MMA), order to minimise the dead volume. Prior to coupling the 3.91 g of CH3AsO(ONA)2 6H2O (Carlo Erba, Milan, Italy) HPLC system to the ICP-MS instrument, a peristaltic pump dissolved in distilled water; and dimethylarsinic acid (DMA), was used to aspirate an aqueous solution of arsenic as arsenite 2.90 g of (CH3)2AsO(ONa) 3.5H2O (Sigma, St.Louis, MO, (20 mg l-1) in the selected mobile phase; this was used to USA) dissolved in distilled water. optimise the ion lenses and nebuliser flow. For the hydride generation measurements, working standard For chromatographic separations, a gradient CM 41000 solutions were prepared by the appropriate dilution of a stock pump (LDC Analytical, Riviera Beach, FL, USA) was used standard solution of 1000 mg l-1 arsenic(III) chloride (Johnson with a 10 mm particle size (250×4.6 mm id) Hamilton PRP- Matthey, Karlsruhe, Germany) in 1% hydrochloric acid X100 anion-exchange column with a guard column fitted.A prepared from 32% general reagent acid (Merck). Rheodyne (Rheodyne, Cotati, CA, USA) Model 7125 injection For HPLC, mixtures of arsenic species were prepared daily valve with a 20 ml injection loop was used for sample introduc- in distilled water after appropriate dilution.Gradient elution tion. Gradient elution was carried out at a flow rate of was employed and the constituents of the two mobile phases 1.0 ml min-1. are given in Table 1. The chemicals used for the mobile phase An M301 microdigester (Prolabo, Fontenay-sous-Bois, were all of Fluka purum p.a quality (Sigma–Aldrich, Buchs, France) was used for the extraction procedure. Temperature Switzerland) and the water of HPLC grade was provided by measurements were made using a digital air thermometer in a Scharlau (FEROSA, La Jota, Barcelona, Spain).Methanol of borosilicate glass sheath (Prolabo Megal 500). It has a tem- HPLC grade (Carlo Erba) was also added to the mobile phase perature range of 0–500 °C with a precision of<3%. In-house as it has been shown to increase the signal sensitivity.7 The reference materials (soil and river sediment), after collection, resulting mobile phase was filtered through a 0.45 mm filter were freezed-dried, crushed and passed through a seive of mesh and degassed before use.Orthophosphoric acid (85%) (Merck) size 100 mm and bottled. A 0.3 g aliquot of sample was accu- was diluted to the appropriate concentration and used for the rately weighed into the digester flask and 50 ml of 1 M phos- microwave extraction procedure. phoric acid were added. An air condenser was fitted to the top of the flask and the latter was placed in the cavity of the Instrumentation and Sample Preparation microwave digestor, processed and allowed to cool.The contents were then filtered into a 100 ml calibrated flask and A PlasmaQuad PQ II+ ICP-MS instrument (VG Elemental, Winsford, Cheshire, UK) was used in the standard configur- diluted to volume with distilled water. From this 100 ml of solution, 5 ml were taken and diluted to 10 ml with distilled ation. The operating conditions are given in Table 1. The measurements were carried out using time resolved analysis water, ready for chromatographic analysis.Total arsenic determination was carried out using continu- (TRAVision). This is an option in the instrument software suite, designed specifically for the acquisition of multi-element ous flow hydride generation atomic fluorescence spectrometry (HG-AFS) (Excalibur Plus System, PSA, Orpington, Kent, time resolved signals. Data for arsenic (m/z 75) were collected using the peak jumping acquisition mode and stored in defin- UK).Samples were prepared using the dry ashing digestion procedure as follows: 1 g of sample with 2 ml of ashing aid able time slices and displayed as mass–intensity–time plots. Mass 51 (35Cl16O+) was also monitored as an indicator of (10% m/v ammonium nitrate, Fluka puriss. p.a. grade) at 450 °C for 2 h in a programmable muZe furnace. After ashing, potential interference at m/z 75 from 40Ar35Cl. Multi-element time resolved data were displayed in real time during the the residue was dissolved in 30 ml of 6 M HCl on a sand-bath and, when cool, the digested solution was filtered into a 50 ml chromatographic run.The acquired data were exported to additional software for more specialised processing,19 and a calibrated flask and diluted to volume with distilled water. An aqua regia leach was used to determine the proportion linear calibration curve was constructed for each As species with a top standard of 50 mg l-1. ofaqua regia-soluble arsenic in the sample; 0.5 g of freezedried sample was digested in 20 ml of aqua regia for 30 min at Coupling of the HPLC system to the ICP-MS system was 1368 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12a microwave power of 40 W. When cold, the digest solution in view of the possibility of other metals found in soil or sediment extracts precipitating in the column at high pH was transferred into a 100 ml volumetric flask and diluted to volume with distilled water. To determine the arsenic leached values.A pH of 8.5 is required to elute the arsenate in as short a retention time as possible; this was also found by Branch from the samples, HG-AFS was used. The conditions of this analysis were described in a previous paper.7 The same method et al.13 On increasing the pH of mobile phase A to 7, the resolution of DMA and MMA was improved and the retention of quantification was used to determine the ‘total’ arsenic extracted using phosphoric acid.The accuracy of this method time of arsenate was further reduced. It is thought that this is because there is less of a diVerence and so the column was assessed by using three reference materials (RMs), BCR 320, BCR 141 and IAEA soil-7. ‘conditions’ itself to mobile phase B more quickly. The starting mixture of the mobile phase was considered and the optimum conditions were found to be a 1+1 A–B mixture. With a higher proportion of mobile phase A, the RESULTS AND DISCUSSION retention time of arsenate was increased, and with a higher Optimisation of Anion-exchange Separation of the Analytes proportion of mobile phase B arsenite started to be retained and the peak to peak resolution between arsenite and DMA The methodologies employed by Demesmay et al.10 were used as a starting point for the study.The anion-exchange column was reduced. The chromatogram in Fig. 1 shows the separation of the four arsenic species using the selected gradient was used in conjunction with a phosphate based mobile phase for the separation of the arsenic species.Anion exchange allows programme. It was found that the concentration of acid in the extracts the use of a mobile phase with a higher buVer salt concentration than those used with an ion-pair reversed-phase column and had a significant eVect on the arsenate peak. Standards were produced for arsenite, MMA and arsenate with diVerent is thought to be less susceptible to matrix interferences such as from acids and other concomitant elements which may concentrations of phosphoric acid.Fig. 2 shows the eVect of increasing concentration of acid on the arsenate peak. These degrade the resolution and eYciency of the column. Phosphate has been used eVectively in a number of arsenic speciation results indicated that although there is an element of band broadening at 0.25 M H3PO4, it is not significant and these studies with either NH3 or Na being used as a counter ion.Sodium is unsuitable for use in conjunction with ICP-MS. The results were used when designing the extraction methods. As stated previously, arsenic has a single isotope at m/z 75 presence of alkali metals can greatly aVect the signal and lead to a build-up of salt deposits in the sampling cone. which can suVer from interference from the polyatomic ion 40Ar35Cl+ due to argon in the plasma and chlorine from the The exact mechanisms of ion-exchange chromatography are complex and not fully understood. However the resolution sample.A number of techniques have been employed to deal with this interference. Hydride generation has been used as a and the retention times of the analyte ions can be optimised through adjustments to the pH and ionic strength of the means of separating the Cl from the analyte in both total arsenic determinations14 and in arsenic speciation studies.15 mobile phase. With the literature pKa values,20 it is possible to predict how the species will behave at diVerent pH values; however, the total ionic strength of the mobile phase must also be considered and this is also related to the concentration. The behaviour of the ions was investigated through isocratic elution of mobile phases A and B at diVerent pH values.The pH ranges investigated were 6–7 for A and 7.95–9 for B. With the exception of arsenate, the species behave as predicted from their pKa values. Arsenite has an acidic proton and is not ionised until the pH is greater than 9, and it is therefore not retained by the column.DMA is fully protonated at pH 6 and is therefore also unaVected by changes in the pH. The first proton of MMA is removed at pH 6 and the second at pH 8; the retention time of MMA increased with increase in pH and indeed was very sensitive to changes in pH. Arsenate exhibited a reversal of the expected behaviour. For isocratic elution with mobile phase A at pH 6 and 6.5 arsenate was not eluted in an Fig. 1 Anion-exchange chromatogram of arsenic species in aqueous analysis time of 15 min. As the pH is increased it would be solution; 400 pg of each species injected. Peaks: 1, AsIII; 2, DMA; 3, MMA; 4, AsV. Response is normalised to the highest peak. expected that the retention time would increase; however, the opposite is observed and at pH 7.95 with mobile phase A, arsenate was eluted within 15 min; as the pH was further increased the retention time decreased. It would appear that there are interactions taking place other than those solely due to the ionic strength of the buVer.This behaviour can be used to reduce the total time of analysis and in doing so reduce band broadening eVects on the arsenate peak caused by long retention times. It was not found possible with isocratic elution to reduce the retention time of arsenate to around 10 min and optimise the conditions such that DMA and MMA were suYciently resolved for quantitative purposes.A gradient programme was developed using the results of the pH study on the individual species, and additional work was undertaken to examine the eVect of diVerent mixtures of the two mobile phases as starting points for the gradient. It was felt that it was desirable to keep Fig. 2 Magification of arsenate peak: eVect of acid concentration. 1, the programme as simple as possible by utilising a single ramp. 1.0 M H3PO4; 2, 0.50 M H3PO4; 3, 0.25 M H3PO4; 4, 1% nitric acid.Response is normalised to the highest peak. It was decided that a maximum pH of 8.5 should be imposed Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1369The alternative is to add molecular gases to the argon flow16 dilution was required. This would have brought the typical concentrations of arsenic species below the detection limit or organic solvents to the solution phase to act as suppressants. 11,12 The concentration of the organic solvents can achieved, so it was decided to lower the phosphoric acid concentration to 1 M, which meant that only a fourfold dilution be optimised to act as a Cl attenuator.Initially, 2% v/v methanol was added to the mobile phase to act as a dual of the neat extract was necessary. The eVect on the recovery of the heating time and power of purpose organic modifier to attenuate interferences and to increase the analyte signal. However, an interference was still the microwave oven was investigated by extracting the in-house reference at diVerent settings.The variation in the arsenic observed with the arsenate peak. The scan for ClO, m/z 51, showed a large peak at the corresponding retention time, extracted with respect to the total arsenic with the diVerent microwave programmes was negligible and so it was decided indicating that the problem was caused by chlorine. The methanol concentration was subsequently increased to 3%, to reduce the possibility of oxidation by keeping the power setting and time to the minimum.which attenuated the interference. The scan for m/z 51 did not return to the original background signal, but showed no peaks The stability of the arsenic species under the extraction conditions is an important factor, but the absence of RMs as had been previously observed. This had the eVect of increasing the retention time of arsenate, but compensation makes this very diYcult to assess. Experiments on simulated extracts were carried out.Solutions of 100 mg ml-1 of arsenite, was achieved by adjusting the starting mixture of the gradient to A–B 45+55. The final programme used was from 55% to DMA and arsenate were prepared; one 50 ml portion was put aside as a control and then 50 ml aliquots were taken and 100% B in 3 min, held at 100% B for 11 min and then ramped back down to 55% B in 2 min and allowed to equilibrate for measured at diVerent power and time settings. The results are summarised in Table 3.As can be seen, the two samples 10 min before injecting the next sample. subjected to 20% power for 20 min have a reasonably constant arsenite concentration that is similar to that of the control, Assessment of Results whereas samples treated at higher power or for a longer time showed a decrease in the amount of arsenite and an increase Currently no RMs are available for arsenic speciation studies. in the amount of arsenate greater than the margins of error. The same procedure was carried out, as detailed by Thomas Although inconclusive, these results suggest that with 20% and Sniatecki,7 of determining the total arsenic content in a power (40 W) for 20 min there is no significant oxidation of number of soil and sediment RMs by hydride generation.AsIII to AsV. This was confirmed by a limited spiking experi- These results were compared with the sum of total arsenic ment. Three portions of in house soil reference material were species found by phosphoric acid extraction and HPLC– taken, two of which were spiked with AsIII. All three were ICP-MS.This can only provide an indication of the level of extracted under the same conditions and analysed by HPLC– accuracy of the techniques; a true determination is only possible ICP-MS. The amount of AsIII recovered was less than the with appropriate RMs or by comparison with the results amount spiked; however, the amount of AsV remained the obtained using alternative methods.same within the margins of experimental error. Again, these The limit of detection for the system was determined with results are inconclusive and not extensive enough to allow 3% v/v methanol in the mobile phase and a 20 ml injection statistical analysis, but they do concur with the results of the loop. A blank solution of 0.25 M phosphoric acid was injected, stability experiments that there is no significant oxidation of the peak to peak amplitude was measured and this process AsIII to AsV under these operating conditions.was repeated five times. The mean was taken and the 3s The recoveries achieved with the phosphoric acid leach were response determined. The corresponding concentration for compared with those with the most commonly used leach, each species was calculated from the 50 mg l-1 standard run aqua regia. Five diVerent soil and sediment reference materials after the blanks. The precision of the instrumental technique were extracted with aqua regia and phosphoric acid.The total was determined by measuring a 20 mg l-1 standard solution amount of arsenic extracted was determined by HGFAS. The three times under the same conditions as for the limit of results are given in Table 4 along with the certified total arsenic detection. The RSD for each species was calculated. A summary content; the value for CRM 141 is an indicative value only. of these results is given in Table 2. As can be seen, the recoveries with the phosphoric acid leach are the same as or better than those with the aqua regia leach, Optimisation of the Extraction Procedure indicating that this extraction is suitable for a wide range of soil and sediment samples.To determine the optimum concentration of acid, the extraction A portion of each of the five phosphoric acid extracts was carried out with distilled water and then with increasing referred to above was analysed by HPLC–ICP-MS. Table 4 concentrations of phosphoric acid up to 3 M using the in-house gives the concentration of each species found and the sum of soil reference material.The total arsenic was determined by all the species analysed to allow comparison with the results HG-AFS. Initially 2 M phosphoric acid was chosen because with this concentration a higher percentage of total arsenic was extracted; however, this was shown to be unsuitable owing Table 3 Results from simulated extraction solutions (standard solutions of 100 mg l-1 of AsIII, AsV, MMA) to check the integrity of the to interferences as discussed earlier.To minimise the interspecies under various microwaves conditions ferences and bring the final acid content to 0.25 M an eightfold Microwave conditions AsIII (±6%) MMA (±3%) AsV(±4%) Table 2 Detection limits and precision with 3% v/v methanol in the Control 1 103–117 100–106 80–86 mobile phase using the HPLC–ICP-MS method with 20 ml injections 20%, 20 min 102–114 99–105 80–86 20%, 20 min 100–112 98–104 80–86 Species LOD* mg kg-1 RSD (%) 25%, 20 min 103–115 95–101 78–84 AsIII 1.3 8 Control 2 98–110 96–102 73–79 DMA 1.3 10 25%, 20 min 92–104 101–107 82–88 MMA 1.3 4 25%, 10 min 103–115 102–108 78–84 AsV 1.7 10 25%, 10 min 94–106 108–104 90–98 30%, 10 min 101–113 96–102 79–85 * Detection limits are based on three times the amplitude of the 30%, 10 min 104–118 109–115 85–93 baseline noise and are given as elemental As. 1370 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12Table 4 Results for each species found in some soil and sediment reference materials. Comparison between results obtained with phosphoric acid extract analysed by HPLC–ICP-MS, soluble aqua regia content and total As certified value. Results are expressed in mg kg-1 HPLC–ICP-MS HG-AFS H3PO4 Aqua regia Total As AsIII DMA MMA AsV Sum of extract extract certified Sample (±8%) (±10%) (±3%) (±10%) species (±5%) (±5%) values In-house soil nd* nd nd 10.5 10.5 11.1 11.6 13.7±0.3 In-house river 21.3 nd nd 4.0 25.3 41.1 37 41.2±1.4 sediment CRM 320 2.5 nd nd 39 41.5 62.8 59.8 76.7±3.4 CRM 141 nd nd nd 4.3 4.3 5.3 4.5 10.5 IAEA Soil-7 nd nd nd 8.0 8.0 11.6 9.0 13.4±0.9 * Below detection limit shown in Table 2.Fig. 5 Example of chromatogram of CRM 320 sediment. Two peaks Fig. 3 Example of chromatogram of in-house soil. Two peaks identidentified : 1, AsIII; 2, AsV. Total As content, 76.0 mg kg-1. Response ified: 1, AsIII; 2, As V .Total As content, 13.7 mg kg-1. Response is is normalised to the highest peak. normalised to the highest peak. industrial activity (i.e., a chlorine–alkali plant) and comes airdried, crushed, sieved and bottled.21 The results show that the main species is arsenate with a small amount of arsenite, indicating that this sediment comes from an aerobic situation. These results show that arsenic species found in river sediments seem to be very dependent on the sampling environment.CONCLUSIONS This method has the potential to form the basis of a routine procedure for the speciation of arsenic in soils and sediments, but issues of accuracy in the extraction process still need to be addressed. Further work is required on the behaviour of arsenic species during the sample pre-treatment to improve upon what Fig. 4 Example of chromatogram of in-house sediment. Two peaks is known about the changes to the element that take place identified : 1, AsIII; 2, AsV.Total As content, 41.2 mg kg-1. Response after sampling. is normalised to the highest peak. REFERENCES of the HG-AFS analyses of the same extracts. The concentration of arsenic species determined by HPLC–ICP-MS is 1 Leonard, A., in Metals and T heir Compounds in the Environment, lower than that determined by HG-AFS; this could be because ed. Merian, E., VCH, Weinheim, 1991, p. 751. the individual species are present at levels lower than the limits 2 Pershagen, G., in Environmental Carcinogens, Selected Methods of Analysis, ed.O’Neill, I. K., Schuller, P., and Fishbein, L., Oxford of detection. Three chromatograms (Figs. 3–5) are presented University Press, Oxford, 1985, vol. 8. to illustrate the main species in each diVerent sample. Fig. 3 is 3 Nriagru, J. O., Arsenic in the Environment. Part 1: Cycling and for in-house soil and shows a small arsenite peak; however, Characterisation, Wiley, New York, 1994. this was not quantifiable as it was below the limit of detection. 4 Fo� rstner, U., in Metal Speciation and Contamination of Soil, ed. Fig. 4 is a chromatogram of the in-house sediment RM, the Allen, H. E., Huang, C. P., Bailey, G. W., and Bowers, A. R., major species in this sample is arsenite, confirming that there CRC Press, Boca Raton, FL, USA, 1995. 5 Thomas, P., and Sniatecki, K., Fresenius’ J. Anal. Chem., 1995, is minimal oxidation, and there is also a small arsenate peak. 351, 410. This sediment was taken from a polluted river where the 6 Corr, J.J., and Larsen, E. H., J. Anal. At. Spectrom., 1996, 11, 1215. sediment is in a reducing environment. Although the sample 7 Thomas, P., and Sniatecki, K., J. Anal. At. Spectrom., 1995, 10, 615. had been freeze-dried, crushed and sieved, it was observed that 8 Larsen, E. H., PhD T hesis, National Food Agency of Denmark, AsIII seems unaVected by this kind of treatment. Fig. 5 is a 1993. chromatogram from CRM 320, a BCR river sediment. This 9 Beauchemin, D., Siu, K. W. M., McLaren, J. W., and Berman, S. S., J. Anal. At. Spectrom., 1989, 4, 285. CRM is typical of an aerobic situation in rivers with prolonged Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 137110 Demesmay, C., Olle, M., and Porthault, Fresenius’ J. Anal. Chem., 18 Amran, B., Lagarde, F., Leroy, M. J. F., Lamotte, A., 1994, 348, 205. Demesmay, C., Olle�, M., Albert, M., Rauret, G., and Lopez- 11 Larsen, E. H., and Studrup, S., J. Anal. At. Spectrom., 1994, 9, 1099. Sanchez, J. F., in Quality Assurance for Environmental Analysis, 12 Evans, E. H., and Ebdon, L., J.Anal. At. Spectrom., 1990, 5, 425. ed. Quevauviller, Ph., Maier, E. A., and Griepink, B., Elsevier, 13 Branch, S., Ebdon, L., Hill, S., and O’Neill, P., Anal. Proc., 1989, Amsterdam, 1995, vol. 17, p. 285. 26, 401. 19 Thomas, P., Koller, D., and Perriera, K., Analusis, 1997, 25, 19. 14 Sheppard, B. S., Caruso, J. A., Heitkemper, D. T., and Wolnik, 20 Kortum, G., Vogel, W., and Andrussov, K., Dissociation Constants K. A., Analyst, 1992, 117, 971. in Aqueous Solutions, Butterworths, London, 1961, p. 492. 15 Sheppard, B. S., Caruso, J. A., Heitkemper, D. T., and Perkins, L., 21 Griepink, B., and Muntau, H., EUR Report 11850, CEC, Brussels, J. Chromatogr. Sci., 1992, 30, 427. 1988, p. 4. 16 Hansen, S. H., Larsen, E. H., Pritzl, G., and Cornett, C., J. Anal. At. Spectrom., 1992, 7, 629. Paper 7/04149G 17 Lobinski, R., and Marczenko, Z., in Comprehensive Analytical Received June 13, 1997 Chemistry, ed. Weber, S. G., Elsevier, Amsterdam, 1996, vol. 30, p. 7. Accepted September 12, 1997 1372 Journal of Analytical Atomic Spectrometry, December 1997, Vol