Anal. Chem. 1998, 70, 3649-3655
Determination of Anionic, Neutral, and Cationic Species of Arsenic by Ion Chromatography with ICPMS Detection in Environmental Samples Ju 1 rgen Mattusch* and Rainer Wennrich
Department of Analytical Chemistry, UFZsCenter for Environmental Research Leipzig/Halle Ltd., Permoserstrasse 15, D-04318 Leipzig, Germany
Ion chromatographic methods developed to separate either cationic, neutral, and anionic arsenic species or soluble and suspended arsenic species were successfully used in DORM-2 standard reference material and in water samples of environmental interest. The most effective separation of the analytes within 10 min was achieved with a nitric acid gradient elution using a strong anionexchange stationary phase with additional capacity for hydrophobic interactions (IonPac AS7). The elementalspecific detection mode allows the sensitive determination of the arsenic species in the submicrogram per liter range. The calibration results were compared with those obtained by an alkaline water-methanol mixed eluent combined with a weak anion-exchange column (IonPac AS4A-SC). Differences in sensitivities were eclipsed by the low level of the baseline and the noise when using nitric acid. The gradient method was used to determine arsenic species in highly ferrous/ferric-contaminated leachates of lignite spoil. The companion elements underwent parallel screening to explain the interactions of arsenic species with the major elements. Compounds and forms of arsenic in aquatic, terrestrial, and biological systems have been the target of increasing attention in recent years. Being able to determine different species and compounds of arsenic is important if their transport mechanism in the environment, toxicological risks, and interrelationships are to be specified. Due to their varying toxicity, highly efficient separation techniques have been combined with sensitive detection devices to investigate the presence of species and their metabolic reactions in biological and environmental samples.1-3 Irgolic et al.5 studied the accumulation and transformation of inorganic arsenic species in mushrooms using anion-exchange and reversed-phase (RP) columns for chromatographic separation. Microcolumns with * Corresponding author. E-mail:
[email protected]. Fax: +49 341 235 2625. (1) Le, X.-C.; Cullen, W. R.; Reimer, K. J. Talanta 1994, 41, 495-502. (2) Ding, H.; Wang, J.; Dorsey, J. G.; Caruso, J. A. J. Chromatogr. A 1995, 694, 425-431. (3) Hwang, C.-j.; Jiang, S.-J. Anal. Chim. Acta 1994, 289, 205-213. (4) Tera¨sahde, P.; Pantsar-Kallio, M.; Manninen, P. K. G. J. Chromatogr. A 1996, 750, 83-88. (5) Ku ¨ hnelt, D.; Go¨ssler, W.; Irgolic, K. J. Appl. Organomet. Chem. 1997, 11, 289-296. S0003-2700(98)00257-1 CCC: $15.00 Published on Web 07/15/1998
© 1998 American Chemical Society
equivalent characteristics were also used to separate metal species by Gjerde et al.6 The application of micro-HPLC in the RP ionpairing mode was studied to eliminate the interference of naturally occurring organic arsenic compounds during the determination of phenylarsonic compounds.7 To determine anionic, cationic, and neutral arsenic species, different columns were combined in series.4 For the sensitive and selective determination of various arsenic species in various sewage samples of environmental interest, two ion chromatographic methods were developed, allowing the detection of both suspended and soluble arsenic species as well as neutral and cationic forms of this element during one chromatographic run. Water samples studied from effluents penetrating the tailings of a tin ore settling plant were found to contain high levels of total arsenic (2 mg L-1), iron (10 mg L-1), and manganese (10 mg L-1). As soon as the effluent is exposed to the air, the redox conditions rapidly change from reducing to oxidizing, as indicated by the precipitation of ferric oxyhydroxides and related compounds.8 EXPERIMENTAL SECTION Apparatus. A DX 100 ion chromatograph (Dionex, Sunnyvale, CA) was connected to an ELAN 5000 inductively coupled plasma mass spectrometer (Perkin-Elmer/Sciex) as an element-specific detector. The two devices were interfaced by a cross-flow nebulizer. The eluent was pumped isocratically at a flow rate of 1.5 mL min-1. The samples were introduced by a sampling loop of the pneumatic injection valve of 25 or 200 µL, depending on the existing analyte concentration. The parameter set of the ICPMS was concluded as shown in Table 1. The gradient elutions of the differently charged arsenic species were performed by the LC 250 binary pump (Perkin Elmer). The injection valve of the DX 100 was also used for sample injection. Reagents. Bidistilled water (18 MΩ cm-1) was used to prepare the analyte stock solutions and the eluent solutions. All solutions were prepared from analytical grade chemicals if available. Titrisol arsenic standard with a concentration of 1000 mg of As L-1 was used as arsenate (As(V)) stock solution. Arsenite (As(III)) stock solution was prepared by dissolving the (6) Gjerde, D. T.; Wiederin, D. R.; Smith, F. G.; Mattson, B. M. J. Chromatogr. A 1993, 640, 73-78. (7) Pergantis, S. A.; Heithmar, E. M.; Hinners, T. A. Analyst 1997, 122, 10631068. (8) Vink, B. W. Chem. Geol. 1996, 130, 21-30.
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998 3649
Table 1. Parameter Set for ICPMS Detection basic parameter
value
eluent flow rate rf power Ar auxiliary gas flow rate Ar plasma gas flow rate Ar nebulizer gas flow rate dwell time data acquisition masses (m/z)
min-1
1 mL 1050 W 0.85 L min-1 15 L min-1 0.90 L min-1 1000 ms graphic mode, Microcal origin As, 75; Fe, 54; P, 31
Table 2. Gradient Program step
time (min)
solvent A (%)
solvent B (%)
comments
0 1 2 3 4 5
0 0-1 1-2 2-12 12-13 13-20
100 100 0 0 100 100
0 0 100 100 0 0
injection pH(calcd) 3.3 linear gradient pH(calcd) 1.3 linear gradient reconditioning
a
Solvents: A, 0.5 mM HNO3; B, 50 mM HNO3.
corresponding amount of arsenic trioxide (Fluka) in oxygen-free water to obtain a concentration of 1000 mg of As L-1. Dimethylarsinic acid trihydrate (DMA, for synthesis; Merck) was dissolved in water to prepare the stock solution (1000 mg of As L-1). Monomethylarsonic acid (MMA), arsenocholine (AsC), arsenobetain (AsB), trimethylarsine oxide (TMAO), and tetramethylarsonium bromide (TMA) stock solutions (each 1000 mg of As L-1) were kindly provided by the group headed by K. J. Irgolic (Graz, Austria). All stock solutions were stored in the dark (at 4 °C), and the final standard solutions were prepared daily. Chromatographic Procedures. To determine inorganic arsenic species of suspended and soluble forms, a weakly anionexchange column (IonPac AS4A-SC, 4 mm) was used in combination with the guard column IonPac AG4A-SC (Dionex), together with an eluent containing 5 mM sodium carbonate, 40 mM sodium hydroxide, and 4% (v/v) methanol (p.a., all supplied by Merck). To separate anionic, neutral, and cationic arsenic species, nitric acid gradient elution was combined with the IonPac AS7 highcapacity anion-exchange column (Dionex). During the chromatographic run, the nitric acid concentration was changed from 0.5 to 50 mM (pH 3.3 to 1.3). The optimized gradient scheme tested for the sufficient separation of the inorganic and organic arsenic species is summarized in Table 2. Sample Preparation. To avoid precipitation and oxidation reactions during the sampling period prior to injection, the effluent water samples were taken with a deaerated 10-mL syringe (HSW, Tuttlingen, Germany) combined with a hydrophilic cellulose acetate 0.45-µm syringe filter (Sartorius, Go¨ttingen, Germany) and a stopcock system (Braun Melsungen, Melsungen, Germany). To avoid further contamination, the cooled samples (6 °C) were injected without any pretreatment into the chromatographic system. RESULTS AND DISCUSSION Isocratic Separation in Alkaline Eluent. Inorganic arsenic species can be separated on an anion-exchange stationary phase 3650 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
Figure 1. IC gradient chromatogram of an arsenic species standard and bromide (internal standard) solution. Peak identification: 1, As(III); 2, DMA; 3, As(V); 4, AsB; 5, AsC; 6, Br- (100 µg L-1 of each). Chromatography: 200-µL injection volume, IonPac AS7, flow rate 1.0 mL min-1. ICPMS detection: m/z 75, m/z 79, dwell time 1000 ms.
depending on their charge. To minimize analysis time and achieve sufficient resolution for the soluble and suspended parts of the arsenic species, sodium hydroxide and methanol were added to the conventional mobile phase in ion chromatography. Bearing in mind the analysis of real water samples, a concentration of sodium hydroxide of 40 mM and a methanol level of 4% (v/v) seem to be sufficient for separation as mentioned above. The methanol content of the eluent is also limited by influencing the rf plasma properties of the ICPMS device. With this eluent composition, the separation of As(III) and As(V) is achieved within 5 min. The calibration for these two arsenic species was performed over 3 orders of magnitude up to 2 mg of As L-1 for each component, with a linear correlation coefficient of r2 > 0.999. There was no difference in the sensitivity of the calibration plot (area values) as the oxidation state of arsenic was changed. The analysis of effluent samples penetrating tin ore tailings located near Altenberg (Saxony) shows high concentrations of As(III) and lower concentrations of As(V) accompanied by “suspended arsenate” (s-As(V)). The dependence of the retention time and peak area on the pH value of the sample was examined in order to identify the nature of the additional peak. The subsequent acidification of the effluent sample from its natural pH of 6.5 to 2 by hydrochloric acid led to a shift in the peak maximum to shorter retention times and to an enhanced As(V) signal. As the decrease of the peak area of the s-As(V) closely correlated with the rising peak area of soluble As(V), it can be assumed that this broad peak was formed by arsenate adsorbed on colloidal ferric oxyhydroxides with a wide range in terms of charge and size. The complete identification of the suspended material has not yet been achieved as no
Figure 3. Calibration graphs of As(III) and As(V) using IonPac AS7 and AS4A-SC columns. Chromatography: 200-µL injection volume, optimized conditions. ICPMS detection: see Table 1. Figure 2. Dependence of retention time on nitrate concentration for selected arsenic species. Chromatography: nitric acid gradient elution with IonPac AS7, 200-µL injection volume, 100 µg L-1 of each arsenic species.
additional elements (Fe, Mn, Al, Ca) with the same retention behavior could be found during chromatographic analysis. Moreover, these chromatographic signals were not detected in artificial solutions reflecting the conditions of the real effluent, either. With respect to the s-As(V), both the peak area and the peak position varied sometimes, depending on the sampling site and time. This suggests that the transport of arsenic along the river occurs not only via soluble ionic species but also in the suspended matter of various sizes and charges. The formation and existence of these colloids may also depend on organic additives such as surfactants, which play a significant role in the flotation process. Further investigations will thus focus on identifying the nature of s-As(V) by combining ion chromatography (IC) with highresolution ICPMS (HR-ICPMS). Gradient Separation in Acidic Eluent. In order to investigate organic arsenic species, too, a chromatographic method was developed based on gradient elution by nitric acid and using the column IonPac AS7 (Figure 1). The gradient program shown in Table 2 was worked out in order to minimize analysis times and ensure high resolution. Arsenite, dimethylarsinic acid, arsenate, arsenobetaine, and arsenocholine were well separated with a run time of 10 min. Under these conditions, TMA and TMAO coeluted with arsenocholine. The retention behavior of MMA (pKa ) 3.6) showed some abnormalities, reflecting the existence of two protonated forms at the beginning of the chromatographic separation. The order of retention of the analytes differed from that published by Caroli9 for separation in alkaline solution when
using this column. This behavior can presumably be attributed to the shift of the protonation equilibrium of the analytes as well as their resulting capability for ion-exchange and hydrophobic interactions with the stationary phase. To identify the effect of the pH of the mobile phase and the concentration of nitrate ions on separation efficiency, the nitrate concentration was varied in the range 0-20 mM. The eluent composition was changed such that sodium nitrate was added with increasing concentrations of subsequent runs to both stock solutions A and B of the eluent. The dependence of retention time on nitrate concentration is illustrated by different slopes for the four selected arsenic species (Figure 2). This is attributed to the dissimilar separation mechanism on the high-capacity column IonPac AS7. The addition of small amounts of sodium nitrate quickly led to the highest acceleration rate for arsenate, followed by DMA, AsC, and AsB. This behavior reflects a dual mechanism responsible for the separation of the arsenic species. The retardation of As(V) and DMA is mainly attributed to a high degree of ion-exchange interactions with the stationary phase at the beginning of separation. Judging by the nitric acid gradient applied, the pH is high enough to deprotonate both species to single-charged anions. By increasing the nitric acid concentration to 50 mM, arsenate with a pKa value of 2.2-2.310,11 will pass a short distance of the column in its charged form and continue uncharged. DMA (pKa ) 6.210,12) penetrates the column only in (9) Caroli, S. Element Speciation in Bioinorganic Chemistry; Chemical Analysis Series 135; John Wiley & Sons, Inc.: New York, 1996; pp 445-460. (10) Gailer, J.; Irgolic, K. J. Appl. Organomet. Chem. 1994, 8, 129-140. (11) Dean, J. A. Lange’s Handbook of Chemistry, 13th ed.; McGraw-Hill: New York, 1985. (12) Hansen, S. H.; Larsen, E. H.; Pritzl, G.; Cornett, C. J. Anal. At. Spectrosc. 1992, 7, 629-634.
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
3651
Table 4. Calibration Characteristics for IC-ICPMS with the IonPac AS7 Columna
As species
abbr
arsenite phenyl arsonate dimethyl arsinate arsenate arsenobetaine arsenocholine
As(III) PhAs DMA As(V) AsB AsC
a
peak slope LOD LOD tR width (counts‚ (µg of (µg of (s) (s) L‚µg-1) As L-1) compd L-1) 159 265 357 445 599 714
57 69 70 71 69 68
1627 2140 2281 1764 2192 1880
0.37 0.34 0.32 0.42 0.33 0.38
0.61 0.91 0.58 0.78 0.79 0.84
Noise ) 7 counts/s; LOD ) 3(noise)(peak width/2)/slope.
Figure 4. Dependence of signal intensity on methanol content in the eluent solution. Conditions: continuous pneumatic nebulization of the alkaline (5 mM Na2CO3, 40 mM NaOH) and acidic (10 mM HNO3) eluent mixed with different percentages of MeOH; eluents were dotted with 250 µg of As L-1. Table 3. Calibration Characteristics for IC-ICPMS with the AS 4A-SC Columna As species arsenite arsenate a
abbr
tR (s)
peak width (s)
slope (counts‚ L‚µg-1)
LOD (µg of As L-1)
LOD (µg of compd L-1)
As(III) As(V)
88 267
33 42
4710 5443
0.26 0.29
0.43 0.54
Noise ) 25 counts/s; LOD ) 3(noise)(peak width/2)/slope.
its protonated form. Therefore, the ion-exchange interactions are lower than those in the case of arsenate but stronger in comparison to those of the protonated As(III), indicating the retention order. The separation of DMA from As(III) can be also effected by its hydrophobic interaction with the polymeric stationary phase. With respect to its protonation equilibrium, AsB exists as a zwitterion above pKa ) 2.212 which ought to be able to interact in both ion-exchange and reverse-phase modes. In contrast to the previous compound, AsC is mainly retained by hydrophobic interaction because it is a quaternary arsine throughout the entire pH range used for separation. The differences of the chromatographic pattern with alkaline9 and acidic mobile phase seem to be the result not only of the charge of the species but also of its ability to form ion pairs with the eluent anion, e.g., carbonate. The proposed method has the advantage that arsenite determination is not affected by either the arsenobetaine peak tailing or the shortening of the analysis time. Comparison with the separation order given by Manninen et al.,4 who used anionic and cationic columns in series with an alkaline eluent, reveals differ3652 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
Figure 5. IC-ICPMS of arsenic species in an aqueous extract of DORM-2 reference material. Chromatography: gradient elution, 25µL injection volume; for others see Experimental Section. ICPMS detection: dwell time 600 ms; for other see Experimental Section. Peak quantification: As(III), 0.08; DMA, 0.28; As(V), 0.48; AsB, 16.5; and AsC, 0.08 mg kg-1.
ences in the case of AsB and DMA because of the different ionic states depending on the pH of the eluent. Calibration and Comparison of Both Methods. To estimate the sensitivity of the proposed methods, calibrations were performed in the concentration range of 0-1000 µg of As L-1 for each component. A comparison of the calibration plots of arsenite and arsenate for isocratic ion chromatography in alkaline media and gradient technique with an acidic eluent is presented in Figure 3. Under identical operations modes for element-specific ICPMS detection (rf power, nebulizer conditions), the sensitivity of both arsenate and arsenite in the alkaline water-methanol eluent is about 3 times higher than that in the acidic one. This effect is probably due to the methanol content facilitating more efficient nebulization and the transportation of the eluent aerosol into the ICP or a more energetic plasma16 with a higher ionization efficiency. The influence of methanol on the signal intensity was therefore tested by the continuous pneumatic nebulization of
Figure 6. Multichannel IC-ICPMS analysis of seepage water of a tin ore tailings. Chromatography: IonPac AS7, for gradient elution see Experimental Section, 200-µL injection, 0.45-µm filtered. ICPMS detection: m/z 54 (Fe), 31 (P), 75 (As); for others see Table 1. Peak quantification: As(III), 1.17; and As(V), 0.074 mg L-1.
solutions containing arsenate and the major eluent components shown in Figure 4. The MeOH content was varied between 0 and 20% (v/v) in a 10 mM nitric acid and 5 mM carbonate/40 mM sodium hydroxide solution respectively containing 250 µg of As L-1. With regard to maximum sensitivity, a concentration range of 2-4% (v/v) MeOH was found to be optimal for both the acidic and the alkaline solutions, which tallies with the results of Larsen and Stu¨rup.16 Higher MeOH levels lead to a decrease in sensitivity of As determination, caused by the deterioration of the (13) Goossens, J.; Vanhaecke, F.; Moens, L.; Dams, R. Anal. Chim. Acta 1993, 280, 137-143. (14) Corr, J. J. J. Anal. At. Spectrosc. 1997, 12, 537-546. (15) Wennrich, R.; Mattusch, J.; Morgenstern, P.; Dzeheryan, T. G.; Shkinev, V. M.; Spivakov, B. Y. Fresenius J. Anal. Chem. 1997, 359, 161-166. (16) Larsen, E. H.; Stu ¨ rup, S. J. Anal. At. Spectrosc. 1994, 9, 1099-1105.
plasma properties for arsenic ionization.13 However, the application of the organic modifier MeOH is allowed only in the case of the column IonPac AS4A-SC, which is compatible with organic solvents. The calibration characteristics for both methods are summarized in Tables 3 and 4. As is to be expected for arsenicspecific detection, the LODs are very similar for each compound with respect to their arsenic elemental concentration. The small differences in the sensitivity response could be attributed to variations between species in their physicochemical properties, resulting in slightly different degrees of As ionization. They range from 0.3 to 0.4 µg of As L-1, with the mean relative standard deviation being less than (6.5% (n ) 5) for separation in acid solution (column IonPac AS7) and 0.3 and 0.32 µg of As L-1 for arsenite and arsenate, respectively, with the IonPac AS4A-SC Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
3653
Figure 7. IC-ICPMS analysis of seepage water of tin ore tailings. Chromatography: IonPac AS4A-SC, alkaline eluent, 200-µL injection. ICPMS detection: m/z 75, dwell time 1000 ms; for others see Experimental Section. Peak quantification: As(III), 1.13; As(V), 0.093; and s-As, 0.375 mg L-1.
column. Despite the difference in sensitivity by a factor of 2-3, fairly similar LODs were obtained with both methods due to a low baseline level and a very small baseline noise of 7 counts/s in nitric acid, overcompensating the sensitivity enhancement in the mixed methanol-alkaline eluent. Calibration plots based on four concentrations of each compound investigated show high linearity (r2 > 0.9996) over 3 orders of magnitude. If a sufficient reconditioning step is applied after each run, the reproducibility and repeatability of both separation and quantification are very good, with the exception of those for MMA. In comparison to the high loading of the ICPMS equipment by the alkaline solution, both the nitric acid eluent and its flow rate closely coincide in terms of the exclusion of interference by contamination and impurities. Application. The proposed gradient method with nitric acid as eluent was applied to the determination of inorganic and organic arsenic species in DORM-2scertified NRCC reference material for trace metals. Comparison of the chromatograms of the aqueous DORM-2 extract with the arsenic standard solution (Figure 5) shows the existence of the following species and their concentrations: As(III) (0.08 mg kg-1), DMA (0.28), As(V) (0.48), AsB (16.5), and AsC (0.08). These findings closely correspond to those of other authors14 and the certified concentration of total arsenic of 18.0 ( 1.1 mg kg-1. This method was also used on other environmentally relevant samples. Seepage water samples from tin ore tailings (Altenberg, Germany)15 were analyzed with respect to arsenic species as part of a toxicological assessment. Figure 6 shows a gradient ion chromatogram of such a sample, in which 75As, 31P, and 54Fe were simultaneously analyzed during one run. The main arsenic component leaving the tailings was found to be arsenite, ac3654 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
Figure 8. IC-ICPMS analysis of a leachate of a lignite spoil (Ro¨ tha, Germany). Chromatography: gradient elution, IonPac AS7, 200-µL injection volume, 0.45-µm filtered. ICPMS detection: m/z 75, 54, dwell time 1000 ms; for others, see Experimental Section.
companied by a smaller amount of arsenate formed by air oxidation. Very small, noisy peaks were found at 222 and 354 s, while a high, noisy baseline was detected behind the arsenate. These traces of unknown arsenic species have not yet been identified. Phosphate was also analyzed simultaneously at a retardation time of 455 s using this method. The peak (m/z ) 31) at 628 s is also an unknown component, probably coming from the flotation additive styrenephosphonic acid. As discussed above, the presence of ferrous ions (10 mg L-1) and their redox and precipitation reactions are a major factor within the seepage samples analyzed and were, therefore, detected at m/z ) 54. Interference at this mass by 40Ar14N was negligible for qualitative considerations. As can be seen from Figure 6, iron appears simultaneously with arsenate and traces of phosphate, possibly indicating a conjugate formation of arsenate (phosphate) and iron species. This could be the reason for the enhanced level of the baseline m/z ) 75 behind the arsenate peak. The chromatogram in Figure 7 shows the IC separation under alkaline conditions of the same sample. In addition to the soluble arsenic forms arsenite and arsenate, suspended matter containing arsenate is to be observed in the broad, fuzzy peak behind the soluble arsenate. These particles are small enough to penetrate the IC column and 0.45-µm filter and may be, therefore, responsible for the transportation of toxic arsenic-containing material along the river. Arsenic species were also determined in leachates of lignite spoil (Ro¨tha, Germany) using the gradient method. The highly polluted water leachate contains 9.6 ( 0.3 g of Fe L-1 and about 5 mg of As L-1. The ion chromatogram in Figure 8 shows wellseparated peaks of species for both masses detected (75As, 54Fe). The arsenic peaks correspond with arsenite (2.83 mg of As L-1) and arsenate (2.28 mg of As L-1). Two well-defined peaks were also obtained when detecting the m/z ) 54. The collection of
Table 5. Ferric and Ferrous Ion Concentrations and Their Ratios in the Collected Chromatographic Fractions
fraction
time range (s)
F1 F2 F3 F4
0-130 130-270 270-450 450-720
concn (mg L-1) Fe(II)a Fe(III)b 0.8 165 0.1 3.9
25.5 654 0.65 70.4
Fe(II)/Fe(III) 0.03 0.25 0.15 0.05
a Photometric determination by complex formation with 2,2′-dipyridyl. b Difference of the total Fe concentration determined photometrically after reduction with ascorbic acid and the Fe(II) concentration.
four eluent fractions during the chromatographic run and photometric analysis of the ferric and ferrous concentrations shows the existence of Fe(III) species accompanied by smaller amounts of ferrous ions in both peaks. The concentrations of the iron species in each fraction and their ratios are summarized in Table 5. CONCLUSION The proposed ion chromatographic methods using elementalspecific detection (ICPMS) enables the determination of either inorganic arsenic species including suspended matter containing arsenic or both inorganic and organic arsenic species. Modifying the alkaline eluent in combination with the IonPac AS4A-SC column by methanol improved the sensitivity of ICPMS detection and shortened the chromatogram for suspended arsenic materials.
The components accompanying the 75As+ peak are currently being identified by means of high-resolution ICPMS. The gradient elution of differently charged arsenic species in acidic eluent enables the separation of arsenic species to be finetuned. Current work is focusing on the complete separation of the neutral and cationic arsenic species with only one column and gradient elution modified by pH, ionic strength, and ion-pair reagents. One particularly interesting application of the gradient method was demonstrated by the chromatographic analysis of highly polluted leachates of lignite spoil containing ferric and ferrous ions in the grams per liter range. The gradient method also allows the direct determination of arsenic species in 0.1 M phosphoric acid and acidic plant extracts. The advantages of the nitric acid eluent also include the low background, low noise, and negligible amounts of contaminants. In the future, we will also pay more attention to the simultaneous determination of species of different elements within one chromatographic run. As can be seen from the chromatogram of 54Fe and 75As, extremely sharp peaks were obtained for iron and arsenic species with very different concentrations in the sample. However, explaining the retention behavior and the composition of iron species generating these peaks requires further investigations.
Received for review March 6, 1998. Accepted June 4, 1998. AC9802574
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
3655
