Anal. Chem. 1998, 70, 4736-4742
Determination of (Ultra)trace Amounts of Arsenic(III) and Arsenic(V) in Water by Inductively Coupled Plasma Mass Spectrometry Coupled with Flow Injection On-Line Sorption Preconcentration and Separation in a Knotted Reactor Xiu-Ping Yan,* Robert Kerrich, and M. Jim Hendry
Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
A method has been developed for determination of (ultra)trace amounts of As(III) and As(V) in water by flow injection on-line sorption preconcentration and separation coupled with inductively coupled plasma mass spectrometry (ICPMS) using a knotted reactor (KR). The determination of As(III) was achieved by selective formation of the As(III)-pyrrolidine dithiocarbamate complex over a sample acidity range of 0.01-0.7 mol L-1 HNO3, its adsorption onto the inner walls of the KR made from 150cm-long, 0.5-mm-i.d. PTFE tubing, elution with 1 mol L-1 HNO3, and detection by ICPMS. Total inorganic arsenic was determined after prereduction of As(V) to As(III) in a 1% (m/v) L-cysteine-0.03 mol L-1 HNO3 media. The concentration of As(V) was calculated by difference (the total inorganic arsenic and As(III)). Owing to the groupspecific character of the chelating agent, and the use of an efficient rinsing step before elution, the interferences encountered in conventional ICPMS from common major matrix, alkali and alkaline earth metals, and chlorides were eliminated. The presence of organoarsenic species such as monomethylarsonate and dimethylarsinate in water samples had no effect on the results of As(III) and As(V). Thus, the method can be applied to the speciation analysis of inorganic arsenic at submicrogram per liter levels in aqueous solutions with high total content of dissolved solids and/or high content of chlorides. Using a preconcentration time of 60 s and a sample flow rate of 5 mL min-1, an enhancement factor of 22 was achieved in comparison with conventional ICPMS. The time required for a single determination was 200 s. The detection limits (3s) was evaluated to be 0.021 µg L-1 for As(III) and 0.029 µg L-1 for total inorganic arsenic. The precision for 14 replicate determinations of 1 µg L-1 As(III) was 2.8% (RSD) with drift correction and 3.9% (RSD) without drift correction. The concentrations of As(III) and As(V) in synthetic mixtures obtained by the present method were in good agreement with expected values. Results obtained by the proposed method for total arsenic in a river water reference material agreed well with certified and recently reevaluated values. The method was 4736 Analytical Chemistry, Vol. 70, No. 22, November 15, 1998
also applied to the speciation analysis of inorganic arsenic in porewaters. Arsenic is a ubiquitous trace element, and its environmental chemistry is complicated by the widely differing properties of generally occurring arsenic compounds.1 Many arsenic compounds are known to be toxic, and the exposure of humans and animals especially, and ecosystems in general, to arsenic remains of international concern.1 The toxicity of arsenic depends strongly on its chemical form.1 The inorganic compounds are far more toxic than their organic metabolites.1 Four of the more toxic arsenic compounds are arsenite (As(III)), arsenate (As(V)), monomethylarsonate (MMA) and dimethylarsinate (DMA), of which arsenite is the most toxic.1 In light of the differences in the toxicity and physiological behavour between the chemical forms, and in order to follow the pathways for interconversion in the environment, it is increasingly important to monitor the concentrations of the individual chemical species. Chromatographic techniques coupled with atomic spectrometry are widely used for arsenic speciation analysis in environmental samples.1-3 Recently, high-performance liquid chromatography on-line coupled with inductively coupled plasma mass spectrometry (ICPMS) has been shown to be promising for determination of various arsenic species in the environment.4-6 Methods based on direct chromatographic separation of the different species offer the advantages of minimizing chemical interferences and permitting simultaneous determination of various arsenic species including inorganic and organic compounds. The more important forms in natural waters are the inorganic arsenic species As(III) and As(V). As(III) and As(V) are likely to be found in water because of release of arsenic compounds from minerals. For the determination of these two major inorganic * To whom correspondence should be addressed: (tel) +1-306-966-8592; (fax) +1-306-966-8593; (e-mail)
[email protected]. (1) Harrison, R. M.; Rapsomanikis, S. Environmental Analysis Using Chromatography Interfaced with Atomic Spectroscopy; Wiley: Chichester, 1989. (2) Ebdon, L.; Hill, S.; Ward, R. Analyst 1986, 111, 1113-1138. (3) Ebdon, L.; Hill, S.; Ward, R. Analyst 1987, 112, 1-16. (4) Hill, S. J.; Bloxham, M. J.; Worsfold, J. Anal. At. Spectrom. 1993, 8, 499515. (5) Byrdy, F. A.; Caruso, J. A. Environ. Sci. Technol. 1994, 28, 528A-534A. (6) Sutton, K.; Sutton, R. M. C.; Caruso, J. A. J. Chromatogr., A 1997, 789, 85-126. 10.1021/ac980654e CCC: $15.00
© 1998 American Chemical Society Published on Web 10/17/1998
arsenic species in waters, highly sensitive and selective techniques are needed owing to the low concentrations of the species. ICPMS is a sensitive technique for the determination of the total concentration of many elements, but direct determination of (ultra)trace arsenic in high chloride matrixes is difficult by quadrupole ICPMS due to its limited sensitivity and 40Ar35Cl isobaric interference with the detection of monoisotope 75As. Thus, preconcentration and separation steps are required for the analysis of (ultra)trace arsenic in complex aqueous matrixes. To improve the sensitivity and selectivity, off-line procedures, such as ion exchange, solvent extraction, and coprecipitation, are generally used before detection. Such procedures operated in the batch mode are time-consuming, are labor-intensive, require large sample volumes, and suffer great risks of contamination and analyte loss. With on-line operation using flow injection (FI) techniques, the drawbacks of batchwise operation can be overcome to a great extent, while preconcentration can be further enhanced.7,8 Furthermore, FI on-line preconcentration offers the possibility for the differential determination of individual oxidation states of an element by selective preconcentration.9-12 Successful examples of such applications are the differential determination of As(III) and As(V)9 and Cr(III) and Cr(VI),10 selective determination of Se(IV)11 in natural waters by FI on-line C18 column preconcentration electrothermal atomic absorption spectrometry (ETAAS), and selective determination of Sb(III) in waters by FI on-line sorption preconcentration in a knotted reactor (KR) coupled with ETAAS.12 FI on-line preconcentration based on the sorption of metaldiethydithiocarbamate (DDC) on the inner walls of a KR made from PTFE tubing was originally proposed by Fang and coworkers13,14 for flame atomic absorption spectrometric (FAAS) determination of Cd in biological materials and Cu in water and rice samples. This method was extended to ETAAS determination of Pb in waters by Sperling et al.,15 as well as Sb(III) and Co in waters,12,16 and Pb, Tl, and Bi in biological and environmental samples by Yan and co-workers.17-19 Compared with FI on-line preconcentration systems using sorbent columns,9-11 those based on the sorption of metal-DDC,13-15 or -pyrrolidine dithiocarbamate (PDC)12,16-19 complexes on the inner walls of a KR permit the use of higher sample loading rates, thus achieving greater concentration efficiencies due to low hydrodynamic impedance in the KR, and allow the analysis to be performed at low cost, (7) Fang, Z.-L. Flow Injection Separation and Preconcentration; VCH: Weinheim, 1993. (8) Fang, Z.-L. Flow Injection Atomic Absorption Spectrometry; Wiley: chichester, 1995. (9) Sperling, M.; Yin, X.-F.; Welz, B. Spectrochim. Acta, Part B 1991, 46, 17891801. (10) Sperling, M.; Yin, X.-F.; Welz, B. Analyst 1992, 117, 629-635. (11) Yan, X.-P.; Sperling, M.; Welz, B., unpublished work; Bodenseewerk PerkinElmer GmbH, Germany, 1995. (12) Yan, X.-P.; Van Mol, W.; Adams, F. Analyst 1996, 121, 1061-1067. (13) Fang, Z.-L.; Xu, S.-K.; Dong, L.-P.; Li, W.-Q. Talanta 1994, 41, 2165-2172. (14) Chen, H.-W.; Xu, S.-K.; Fang, Z.-L. Anal. Chim. Acta 1994, 298, 167-173. (15) Sperling, M.; Yan, X.-P.; Welz, B. Spectrochim. Acta, Part B 1996, 51, 18911908. (16) Yan, X.-P.; Van Mol, W.; Adams, F. Lab. Rob. Autom. 1997, 9, 191-199. (17) Yan, X.-P.; Adams, F. J. Anal. At. Spectrom. 1997, 12, 459-464. (18) Ivanova, E.; Yan, X.-P.; Van Mol, W.; Adams, F. Analyst 1997, 122, 667671. (19) Ivanova, E.; Yan, X.-P.; Van Mol, W.; Adams, F. Anal. Chim. Acta 1997, 354, 7-13.
Table 1. Operational Conditions and Mass Spectrometer Settings of the ELAN 5000 ICPMS ICP radio frequency power argon outer gas flow rate argon auxiliary gas flow rate argon aerosol carrier gas flow rate sampler skimmer resolution
1000 W 12.0 L min-1 2.0 L min-1 1.0 L min-1
Mass Spectrometer Settings platinum tippled nickel, 1.1-mm orifice platinum tippled nickel, 0.89-mm orifice normal
Ion Lenses Settings photo stop (S2) -8.9 V Bessel box barrel (B) +9.9 V Einzel lenses land 3 (E1) +5.0 V Bessel box end lenses (P) -62.6 V Data Acquisition Parameters scanning mode peak hop transient number of sweeps 2 dwell time 95 ms numbers of readings 150
owing to the unlimited lifetime of the KR and its ease of construction with no need for packing materials.17 In the current study, an FI on-line KR sorption preconcentration and separation system was coupled to ICPMS for selective determination of As(III) in water samples. The selective determination of As(III) in the presence of As(V) and organoarsenic species was achieved by selective formation of the As(III)-PDC complex, its adsorption onto the inner walls of a KR made from PTFE tubing, elution with dilute nitric acid, and detection by ICPMS. The accuracy of the developed method was demonstrated by analyzing synthetic aqueous mixtures and a river water standard reference material SLRS-3. EXPERIMENTAL SECTION Reagents. All reagents used were of the highest available purity and at least of analytical-reagent grade. Doubly deionized water (DDIW; 18 MΩ cm-1), obtained from a Barnstead Nano Pure II system (Sybron-Barnstead, Boston, MA) was used throughout this work. The chelating agent solution was prepared by dissolving ammonium pyrrolidine dithiocarbamate (APDC; Sigma-Aldrich, Steinheim, Germany) in DDIW. Superpure nitric acid (BDH, Toranto, Ontario, Canada), further purified by sub-boiling distillation using quartz stills, was used to adjust sample pH. Standard solutions of As(III) and As(V) for calibration were prepared by stepwise dilution of 100 µg mL-1 stock solutions (CETAC Technologies Inc., Omaha, NE) just before use. Stock solutions of MMA and DMA were prepared by dissolving sodium methylarsonate (Carlo Erla, Milan, Italy) and sodium dimethylarsinate (Merck, Darmstadt, Germany) in DDIW, respectively. A 10% (m/v) L-cysteine solution was prepared as a prereductant by dissolving the solid TLC grade reagent L-cysteine (SigmaAldrich) in 0.03 mol L-1 HNO3. Apparatus. All ICPMS measurements were carried out using an ELAN 5000 ICPMS system fitted with a Meinhard nebulizer (Perkin-Elmer-Sciex, Rexdale, Ontario, Canada). The spray chamber was cooled to 18 °C using a refrigerated chiller model Analytical Chemistry, Vol. 70, No. 22, November 15, 1998
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Figure 1. FI manifold and operational sequences for the selective preconcentration and separation of As(III) on-line coupled with ICPMS: P1, P2, peristaltic pump; W, waste; KR, knotted reactor (150 cm long × 0.5 mm i.d. PTFE tubing); V, injector valve (the left side is the rator); broken lines, active lines; b, four-way connector.
HX-150 (Neslab Instruments, Portsmouth, NH). The ICPMS system was optimized daily and tuned to mass 75 for maximum sensitivity. The operating conditions are given in Table 1. Peak height of a transient signal was used for quantitation. The FI on-line preconcentration and separation were performed using a Perkin-Elmer model FIAS-400 FI system fitted with an AS-90 autosampler (Bodenseewerk Perkin-Elmer, U ¨ berlingen, Germany). The FIAS-400 consists of two peristaltic pumps and a standard rotary injector valve (four-port on the rotor and five ports on the stator). The rotation speed, actuation sequences of the two pumps, and actuation of the injector valve were programmed on and controlled automatically by the ELAN 5000 software. Approximately 65 cm of 0.35-mm-i.d. PTFE tubing connected the valve to the Meinhard nebulizer of the ICPMS instrument. The FI manifold used for the preconcentration and separation is shown in Figure 1. A laboratory-made KR from PTFE tubing (150 cm long × 0.5 mm i.d.), as described by Fang et al.,20 was used for collection of the analyte complex. The Tygon peristaltic pump tubings were employed to propel the sample and reagent. A small bore (0.35 mm i.d.) of PTFE tubing was used for all connections. These connections were kept as short as possible to minimize the dead volumes. Samples. Three porewater samples were collected from the bottom of three piezometers installed in a thick clay-till sequence (3.1-, 15.2-, and 61.0-m depth) located about 140 km south of Saskatoon, Saskatchewan, Canada. Detailed information on the site and its instrumentation is described elsewhere.21 Immediately after sampling, the porewaters were filtered through 0.45-µm (20) Fang, Z.-L.; Sperling, M.; Welz, B. J. Anal. At. Spectrom. 1991, 6, 301306. (21) Shaw, R. J.; Hendry, M. J. Can. Geotech. J., in press.
4738 Analytical Chemistry, Vol. 70, No. 22, November 15, 1998
Supor filters (Gelman Sciences), acidified to pH
