First order speciation of As using flow injection hydride generation atomic absorption spectrometry with in-situ trapping of the arsine in a graphite furnace

A simple method is described to distinguish between As species that react with sodium tetrahydroborate (III) to form AsH₃ and the naturally occurring As species that are unreactive. Results for this rudimentary or “first order” speciation scheme are reported for biological tissue, aquatic plant material, urine and natural water samples. Biological tissue and aquatic plant samples were briefly solubilized in a mixture of 50% nitric acid, no sample preparation was required for the urine or natural water samples. Organoarsenic species which do not react with sodium borohydride under acidic conditions such as arsenobetaine, arsenocholine and tetramethylarsenic, are converted to As(V) by on-line photo-oxidation or microwave heating in a mixture of 0.5 M NaOH and 0.05 M K₂S₂O₈. The sample is subsequently acidified, reduced with sodium borohydride and the generated arsine is trapped in a heated graphite furnace prior to atomization. The superior detection limit (0.14 ng) of the trapping technique permits the dilution of most types of samples, minimizing or eliminating interference effects. Without photolysis or microwave heating a combined result for As(III), As(V), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) is obtained. Results are reported for the first order speciation of As in a suite of certified reference materials (CRMs) including National Research Council (NRC) biological tissues and natural water samples, Community Bureau of Reference (BCR) aquatic plant materials and the National Institute of Standards and Technology (NIST) SRM 267ON urine sample. The determination of a non-hydride forming As fraction in untreated urine and natural water certified reference materials (CRMs) has revealed a species of As previously undetected in NRC seawater CRMs.     

Stability studies of arsenic, selenium, antimony and tellurium species in water, urine, fish and soil extracts using HPLC/ICP-MS

The stability of arsenic, selenium, antimony and tellurium species in water and urine (NIST SRM 2670n) as well as in extracts of fish and soil certified reference materials (DORM-2 and NIST SRM 2710) has been investigated. Stability studies were carried out with As(III), As(V), arsenobetaine, monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), phenylarsonic acid (PAA), Se(IV), Se(VI), selenomethionine, Sb(III), Sb(V) and Te(VI). Speciation analysis was performed by on-line coupling of anion exchange high-performance liquid chromatography (HPLC) with inductively coupled plasma mass spectrometry (ICP-MS). Best storage of aqueous mixtures of the examined species was achieved at 3 degrees C whereas at -20 degrees C species transformation especially of selenomethionine and Sb(V) took place and a new selenium species appeared within a period of 30 days. Losses and species transformations during extraction processes were investigated. Extraction of the spiked fish material with methanol/water led to partial conversion of Sb(III), Sb(V) and selenomethionine to two new antimony and one new selenium species. The other arsenic, selenium and tellurium species were almost quantitatively extracted. For soil spiked with MMA, PAA, Se(IV) and Sb(III), recoveries after extraction with water and sulfuric acid (0.01 mol/L) were below 20%.     

New method of gold-film electrode preparation for anodic stripping voltammetric determination of arsenic (III and V) in seawater

A new method of efficient rotating gold-film glassy-carbon electrode preparation prior to the determination of As(III) and As(V) in seawater by anodic stripping voltammetry (ASV) is described. Factors affecting sensitivity and precision including pH, deposition time and potential, rotation and scan rate, and the nature of working electrode were investigated. Electroinactive As(V) was reduced to As(III) by gaseous SO(2) prior to ASV determination. For a deposition time of 4 min the determination limit was approximately 0.19 ppb. Precision of the proposed method was very good (RSD=2-0.6% at 1-5 ppb) and a relatively good accuracy determined by analysis of certified reference seawater (CASS-1) and seawater samples spiked with an arsenic standard solution, was also obtained.     

Application of sample pre-oxidation of arsenite in human urine prior to speciation via on-line photo-oxidation with membrane hydride generation and ICP-MS detection

A pre-oxidation procedure which converts arsenite [As(III)] into arsenate [As(v)] was investigated in urinary arsenic speciation prior to on-line photo-oxidation hydride generation with ICP-MS detection. This sample pre-oxidation method eliminates As(III) and As(v) preservation concerns and simplifies the chromatographic separation. Four oxidants, Cl2, MnO2, H2O2 and I3-, were investigated. Chlorine (ClO-aq) and MnO2 selectively converted As(III) into As(v) in pure water samples, but the conversion was inefficient in the complex urine matrix. Oxidation of As(III) by H2O2 was least affected by the urine matrix, but the removal of excess H2O2 at pH 10 proved difficult. The most appropriate oxidant for the selective conversion of As(III) into As(v) with minimal interference from the urine matrix is I3- at pH 7. Unlike H2O2, excess oxidant can be easily removed by the addition of S2O3(2-). The I3-(-)S2O3(2-) treatment on a fortified sample of reconstituted NIST SRM 2670 freeze dried urine indicated that arsenobetaine (AsB), dimethlyarsinic acid (DMA), monomethylarsonic acid (MMA) and As(v) were not chemically degraded with recoveries ranging from 95 to 102% for all arsenicals. Sample clean-up involved pH adjustment prior to C18 filtration in order to achieve efficient As(III) conversion and quantitative recoveries of AsB and DMA. The concentrations determined in NIST SRM 2670 freeze dried urine were AsB 17.2 +/- 0.5, DMA 56 +/- 4 and MMA 10.3 +/- 0.3 with a combined total of 83 +/- 5 micrograms L-1 (+/- 2 sigma).     

Low uncertainty determination of manganese and vanadium in environmental and biological reference materials by instrumental neutron activation analysis

The elements Mn and V were determined by INAA in about 5 mg and 100 mg aliquots of NIST SRM 1648 to elucidate discrepancies between our previous results for the 0.5 mg to 15 mg aliquots and the NIST certified and/or information values. Simultaneously, other NIST SRMs 1633a, 2704, and BCR CRMs 038, 101 and 143 were also analyzed. Special attention was given to evaluating and minimizing uncertainties of all steps of analysis. Our results compared very well with the respective certified and/or information values (if available) of all SRMs and CRMs studied, except for NIST SRM 1648. For this SRM we have found significantly lower results than the NIST values which suggests that the NIST values are positively biased by about 10%. A new value for V in BCR CRM 143 was also obtained.     

Hollow fiber liquid phase microextraction combined with electrothermal atomic absorption spectrometry for the speciation of arsenic (III) and arsenic (V) in fresh waters and human hair extracts

A new method of hollow fiber liquid phase microextraction (HF-LPME) using ammonium pyrrolidine dithiocarbamate (APDC) as extractant combined with electrothermal atomic absorption spectrometry (ETAAS) using Pd as permanent modifier has been described for the speciation of As(III) and As(V). In a pH range of 3.0-4.0, the complex of As(III)-APDC complex can be extracted using toluene as the extraction solvent leaving As(V) in the aqueous layer. The post extraction organic phase was directly injected into ETAAS for the determination of As(III). To determine total arsenic in the samples, first As(V) was reduced to As(III) by l-cysteine, and then a microextraction method was performed prior to the determination of total arsenic. As(V) assay was based on subtracting As(III) form the total arsenic. All parameters, such as pH of solution, type of organic solvent, the amount of APDC, stirring rate and extraction time, affecting the separation of As(III) from As(V) and the extraction efficiency of As(III) were investigated, and the optimized extraction conditions were established. Under optimized conditions, a detection limit of 0.12 ng mL⁻¹ with enrichment factor of 78 was achieved. The relative standard deviation (R.S.D.) of the method for five replicate determinations of 5 ng mL⁻¹ As(III) was 8%. The developed method was applied to the speciation of As(III) and As(V) in fresh water and human hair extracts, and the recoveries for the spiked samples are 86-109%. In order to validate the developed method, three certified reference materials such as GBW07601 human hair, BW3209 and BW3210 environmental water were analyzed, and the results obtained were in good agreement with the certified values provided.     

Determination of heavy metals at sub-ppm levels in seawater and dialysis solutions by FAAS after tetrakis(pyridine)-nickel(II)bis(thiocyanate) coprecipitation.

A coprecipitation method has been developed for the determination of Cr(III), Mn(II), Fe(III), Co(II), Cu(II), Cd(II) and Pb(II) ions in aqueous samples by flame atomic absorption spectrometry (FAAS) with the combination of pyridine, nickel(II) as a carrier element and potassium thiocyanate as an auxiliary complexing agent. The obtained coprecipitates were dissolved with nitric acid and measured by FAAS. The coprecipitation conditions, such as the effect of the pH, amounts of nickel, pyridine and potassium thiocyanate, sample volume, and the standing time of the precipitate formation were examined in detail. It was found that the metal ions studied were quantitatively coprecipitated with tetrakis(pyridine)-nickel(II)bis(thiocyanate) precipitate (TP-Ni-BT) in the pH range of 9.0 - 10.5. The reliability of the results was evaluated by recovery tests, using synthetic seawater solutions spiked with the analyte metal ions. The obtained recoveries ranged from 96 to 101% for all of the metal ions investigated. The proposed method was validated by analyses of two certified reference materials (NIST SRM 2711 Montana soil and HPS Certified Waste Water Trace Metals Lot #D532205). It was also successfully applied to seawater and dialysis solution samples. The detection limits (n = 25, 3s) were in the range of 0.01-2.44 microg l(-1) for the studied elements and the relative standard deviations were < or =6%, which indicated that this method could fully satisfy the requirements for analysis of such samples as seawater and dialysis solution having high salt contents.     

Arsenic speciation in natural water samples by coprecipitation-hydride generation atomic absorption spectrometry combination

A speciation procedure for As(III) and As(V) ions in environmental samples has been presented. As(V) was quantitatively recovered on aluminum hydroxide precipitate. After oxidation of As(III) by using dilute KMnO₄, the developed coprecipitation was applied to determination of total arsenic. Arsenic(III) was calculated as the difference between the total arsenic content and As(V) content. The determination of arsenic levels was performed by hydride generation atomic absorption spectrometry (HG-AAS). The analytical conditions for the quantitative recoveries of As(V) including pH, amount of aluminum as carrier element and sample volume, etc. on the presented coprecipitation system were investigated. The effects of some alkaline, earth alkaline, metal ions and also some anions were also examined. Preconcentration factor was calculated as 25. The detection limits (LOD) based on three times sigma of the blank (N: 21) for As(V) was 0.012 μg L⁻¹. The satisfactory results for the analysis of arsenic in NIST SRM 2711 Montana soil and LGC 6010 Hard drinking water certified reference materials for the validation of the method was obtained. The presented procedure was successfully applied to real samples including natural waters for arsenic speciation.     

Use of Escherichia coli immobilized on amberlite XAD-4 as a solid-phase extractor for metal preconcentration and determination by atomic absorption spectrometry.

A sensitive solid-phase extraction technique (SPE) for the enrichment of Fe(III), Co(II), Mn(II) and Cr(III) prior to atomic absorption spectrometric analysis is described. Escherichia coli immobilized on Amberlite XAD-4 was used as a solid-phase extractor. The effects of the pH, amount of solid-phase, eluent type and volume of the sample solution on the recovery of the metal ions were investigated. The effect of diverse ions was also investigated. The recoveries of Fe(III), Co(II), Mn(II) and Cr(III) under the optimum conditions were found to be 99 +/- 2, 99 +/- 3, 98 +/- 2, 98 +/- 3%, respectively, at the 95% confidence level. The detection limits of the metal ions were found as to be 2.4, 3.8, 1.3 and 1.7 ng ml(-1) for Fe(II), Co(II), Mn(II) and Cr(III) respectively, by applying a preconcentration factor of 25. The proposed enrichment method was applied to the determination of analytes in Atatürk Dam water samples, and alloy samples (RSD < 5%). The accuracy of the method was verified on certified alloy samples (NBS SRM 85b and NBS SRM 59a). The analytes were determined with a relative error lower than 5% in water and alloy samples.     

Development of an inexpensive and sensitive method for the determination of low quantity of arsenic species in water samples by CPE-FAAS

The simple and rapid preconcentration technique using cloud point extraction (CPE) was applied for the determination of As(V) and total inorganic arsenic (As(V) plus As(III)) in water samples by means of FAAS. As(V) has formed an ion-pairing complex with Pyronine B in the presence of cetyl pyridinium chloride (CPC) at pH 8.0 and extracted into the non-ionic surfactant Triton X-114, after centrifugation the surfactant-rich phase was separated and diluted with 1.0 mol L⁻¹ HNO₃ in methanol. The proposed method is very versatile and economic because it exclusively used conventional FAAS. After optimization of the CPE conditions, a preconcentration factor of 120, the detection and quantification limits of 1.67 and 5.06 μg L⁻¹ with a correlation coefficient of 0.9978 were obtained from the calibration curve constructed in the range of 5.0-2200 μg L⁻¹. The relative standard deviation, RSD as a measure of precision was less than 4.1% and the recoveries were in the range of 98.2-102.4%, 97.4-101.2% and 97.8-101.1% for As(V), As(III) and total As, respectively. The method was validated by the analysis of standard reference materials, TMDA-53.3 and NIST 1643e and applied to the determination of As(III) and As(V) in some real samples including natural drinking water and tap water samples with satisfactory results. The results obtained (34.70 ± 1.08 μg L⁻¹ and 60.25 ± 1.07 μg L⁻¹) were in good agreement with the certified values (34.20 ± 1.38 μg L⁻¹ and 60.45 ± 1.78 μg L⁻¹).     

Determination of arsenic, selenium, and antimony in cloud water by inductively coupled plasma mass spectrometry

The capability of inductively coupled plasma mass spectrometry in determining trace levels of As, Se, and Sb in cloud water was evaluated. Preliminary studies focused on identifying and eliminating potential interferences in the cloud water matrix, the choice of appropriate internal standards, and system optimization. The detection limits for As, Se, and Sb were 20, 100, 20 pg/mL using pneumatic nebulization, and 5, 25, 5 pg/mL, respectively, using ultrasonic nebulization with a precision of better than 5% RSD. The accuracy was demonstrated by the analysis of a NIST commercial reference material, SRM 1643d. In all cases, the results from ICP-MS analysis agreed within 4% of the certified values. Comparative analysis of cloud water samples obtained from a site downwind from large pollution sources (Whiteface Mountain, New York) and Changlagali Pakistan, a rural mountain peak, was carried out by hydride generation atomic absorption (HGAA) spectrometry. There was excellent agreement between the ICP-MS and HGAA results.     

Application of fast ultrasound water-bath assisted enzymatic hydrolysis--high performance liquid chromatography-inductively coupled plasma-mass spectrometry procedures for arsenic speciation in seafood materials

Enzymatic hydrolysis of seafood materials for isolating arsenic species (As(III), As(V), DMA and AsB) has been successfully performed by assisting the procedure with ultrasound energy (35 kHz) supplied by an ultrasound water-bath. The use of pepsin, as a proteolytic enzyme, under optimized operating conditions (pH 3.0, temperature 40 °C, enzyme to sample ratio of 0.3) led to an efficient assistance of the enzymatic process in a short period of time (from 4.0 to 30 min). The enzymatic extract was then subjected to a clean-up procedure based on ENVI-Carb™ solid phase extraction (SPE). An optimized anion exchange high performance liquid chromatography (HPLC) coupled to inductively coupled plasma-mass spectrometry (ICP-MS) permitted the fast separation (less than 15 min) of six different arsenic species (arsenite, As(III); arsenate, As(V); dimethylarsinic acid, DMA; and arsenobetaine, AsB; as well as monomethylarsonic acid, MMA; and arsenocholine, AsC) in a single run. Relative standard deviations (n = 11) of the over-all procedure were 7% for AsB and DMA, 11% for As(III) and 9% for MMA. HPLC–ICP-MS determinations were performed using aqueous calibrations covering arsenic concentrations of 0, 5, 10, 25, 100 and 200 μg L⁻¹ (expressed as arsenic) for As(III), As(V), MMA, DMA and AsC; and 0, 125, 250, 500, 750, 1000 and 2000 μg L⁻¹ (expressed as arsenic) for AsB. Germanium (5 μg L⁻¹) was used as an internal standard. Analytical recoveries from the anion exchange column varied from 96 to 105% (enzymatic digests spiked with low target concentrations), from 97 to 104% (enzymatic digests spiked with intermediate target concentrations), and from 98 to 103% (enzymatic digests spiked with high target concentrations). The developed method was successfully applied to two certified reference materials (CRMs), DORM-2 and BCR 627, which offer certified AsB and DMA contents, and also to different seafood samples (mollusks, white fish and cold water fish). Good agreement between certified and found AsB concentrations was achieved when analyzing both CRMs; and also, between certified and found DMA concentrations in BCR 627. In addition, the sum of the different arsenic species concentrations found in most of the analyzed samples was statistically similar to the assessed total arsenic concentrations after a total sample matrix decomposition treatment.     

Speciation analysis of inorganic arsenic by microchip capillary electrophoresis coupled with hydride generation atomic fluorescence spectrometry

A novel method for speciation analysis of inorganic arsenic was developed by on-line hyphenating microchip capillary electrophoresis (chip-CE) with hydride generation atomic fluorescence spectrometry (HG-AFS). Baseline separation of As(III) and As(V) was achieved within 54 s by the chip-CE in a 90 mm long channel at 2500 V using a mixture of 25 mmol l(-1) H3BO3 and 0.4 mmol l(-1) CTAB (pH 8.9) as electrolyte buffer. The precisions (RSD, n=5) ranged from 1.9 to 1.4% for migration time, 2.1 to 2.7% for peak area, and 1.8 to 2.3% for peak height for the two arsenic species at 3.0 mg l(-1) (as As) level. The detection limits (3sigma) for As(III) and As(V) based on peak height measurement were 76 and 112 microg l(-1) (as As), respectively. The recoveries of the spikes (1 mg l(-1) (as As) of As(III) and As(V)) in four locally collected water samples ranged from 93.7 to 106%.     

Determination of ultra trace arsenic species in water samples by hydride generation atomic absorption spectrometry after cloud point extraction

Cloud point extraction (CPE) methodology has successfully been employed for the preconcentration of ultra-trace arsenic species in aqueous samples prior to hydride generation atomic absorption spectrometry (HGAAS). As(III) has formed an ion-pairing complex with Pyronine B in presence of sodium dodecyl sulfate (SDS) at pH 10.0 and extracted into the non-ionic surfactant, polyethylene glycol tert-octylphenyl ether (Triton X-114). After phase separation, the surfactant-rich phase was diluted with 2 mL of 1 M HCl and 0.5 mL of 3.0% (w/v) Antifoam A. Under the optimized conditions, a preconcentration factor of 60 and a detection limit of 0.008 μg L⁻¹ with a correlation coefficient of 0.9918 was obtained with a calibration curve in the range of 0.03–4.00 μg L⁻¹. The proposed preconcentration procedure was successfully applied to the determination of As(III) ions in certified standard water samples (TMDA-53.3 and NIST 1643e, a low level fortified standard for trace elements) and some real samples including natural drinking water and tap water samples.     

Differential determination of arsenic(III) and total arsenic using flow injection on-line separation and preconcentration for graphite furnace atomic absorption spectrometry

Arsenic(III) can be quantitatively extracted using sodium diethyldithiocarbamate (NaDDTC) as the complexing agent and C18 reversed phase packing as the column material for solid phase extraction. Arsenic(V) must be reduced to its trivalent oxidation state prior to extraction. A mixture of sodium sulphite, hydrochloric acid, sodium thiosulphate and potassium iodide was found to be optimum for on-line reduction. When the sorbent extraction is carried out without and with the addition of the reduction mixture, arsenic(III) and total arsenic can be determined sequentially by graphite furnace atomic absorption spectrometry with detection limits (3 σ) of 0.32 ng for As(III) and 0.43 ng for total arsenic. A 7.6-fold enhancement in peak area compared to direct injection of 40 μl samples was obtained after 60 s preconcentration. Results obtained for sea water standard reference materials, using aqueous standards for calibration, agree well with certified values. A precision of 5.5% RSD was obtained for total arsenic in a sea water sample (1.65 As). Results obtained for synthetic mixtures of trivalent and pentavalent arsenic agreed well with expected values.     

Determination of polychlorinated dibenzo-p-dioxin and dibenzofuran congeners in air particulate and marine sediment standard reference materials (SRMs)

Due to the limited number of environmental matrix certified reference materials (CRMs) with assigned values for natural levels of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), an interlaboratory study was undertaken by the National Institute of Standards and Technology (NIST) and Environment Canada to establish reference concentration values for selected PCDD/Fs in two well-characterized NIST Standard Reference Materials (SRMs): SRM 1649a (Urban Dust) and SRM 1944 (New York/New Jersey Waterway Sediment). Results from 14 laboratories were used to provide reference values for the seventeen 2, 3, 7, 8-substituted PCDD/F congeners, the totals for individual tetra- through hepta-substituted PCDD/F homologues, and the total amount of tetra- through hepta-substituted PCDD/Fs. The mass fractions for the individual 2, 3, 7, 8-substituted congeners range from approximately 0.01 microg/kg to 7 microg/kg dry mass.     

Determination of polychlorinated dibenzo-p-dioxin and dibenzofuran congeners in air particulate and marine sediment standard reference materials (SRMs)

Due to the limited number of environmental matrix certified reference materials (CRMs) with assigned values for natural levels of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), an interlaboratory study was undertaken by the National Institute of Standards and Technology (NIST) and Environment Canada to establish reference concentration values for selected PCDD/Fs in two well-characterized NIST Standard Reference Materials (SRMs): SRM 1649a (Urban Dust) and SRM 1944 (New York/New Jersey Waterway Sediment). Results from 14 laboratories were used to provide reference values for the seventeen 2, 3, 7, 8-substituted PCDD/F congeners, the totals for individual tetra- through hepta-substituted PCDD/F homologues, and the total amount of tetra-through hepta-substituted PCDD/Fs. The mass fractions for the individual 2, 3, 7, 8-substituted congeners range from approximately 0.01 μg/kg to 7 μg/kg dry mass.     

Simultaneous speciation of inorganic arsenic and antimony in water samples by hydride generation-double channel atomic fluorescence spectrometry with on-line solid-phase extraction using single-walled carbon nanotubes micro-column

A new method was developed for the simultaneous speciation of inorganic arsenic and antimony in water by on-line solid-phase extraction coupled with hydride generation-double channel atomic fluorescence spectrometry (HG-DC-AFS). The speciation scheme involved the on-line formation and retention of the ammonium pyrrolidine dithiocarbamate complexes of As(III) and Sb(III) on a single-walled carbon nanotubes packed micro-column, followed by on-line elution and simultaneous detection of As(III) and Sb(III) by HG-DC-AFS; the total As and total Sb were determined by the same protocol after As(V) and Sb(V) were reduced by thiourea, with As(V) and Sb(V) concentrations obtained by subtraction. Various experimental parameters affecting the on-line solid-phase extraction and determination of the analytes species have been investigated in detail. With 180 s preconcentration time, the enrichment factors were found to be 25.4 for As(III) and 24.6 for Sb(III), with the limits of detection (LODs) of 3.8 ng L^− 1 for As(III) and 2.1 ng L^− 1 for Sb(III). The precisions (RSD) for five replicate measurements of 0.5 μg L⁻¹ of As(III) and 0.2 μg L⁻¹ of Sb(III) were 4.2 and 4.8%, respectively. The developed method was validated by the analysis of standard reference materials (NIST SRM 1640a), and was applied to the speciation of inorganic As and Sb in natural water samples.     

Speciation of Cr(III) and Cr(VI) in environmental samples by using coprecipitation with praseodymium(III) hydroxide and determination by flame atomic absorption spectrometry

A speciation procedure has been established for the flame atomic absorption spectrometric determination of Cr(III) and Cr(VI) based on coprecipitation of Cr(III) by using praseodymium(III) hydroxide (Pr(OH)₃) precipitate. In the presented system, Cr(III) was quantitatively (>95%) recovered at the pH range of 10.0?C12.0 on Pr(III) hydroxide, while the recoveries of Cr(VI) were below 10%. The method was applied to the determination of the total chromium after reduction of Cr(VI) to Cr(III) by using hydroxylamine hydrochloride. The concentration of Cr(VI) is calculated by difference of total chromium and Cr(III) levels. The analytical parameters including pH of the aqueous medium, amount of Pr(III), centrifugation speed, sample volume were optimized. The influences of matrix ions were also investigated. The method was validated by the analysis of TMDA 70 fortified lake water certified reference material. The method was applied to the speciation of chromium in water samples.     

Determination of arsenic, selenium, and antimony in cloud water by inductively coupled plasma mass spectrometry

The capability of inductively coupled plasma mass spectrometry in determining trace levels of As, Se, and Sb in cloud water was evaluated. Preliminary studies focused on identifying and eliminating potential interferences in the cloud water matrix, the choice of appropriate internal standards, and system optimization. The detection limits for As, Se, and Sb were 20, 100, 20 pg/mL using pneumatic nebulization, and 5, 25, 5 pg/mL, respectively, using ultrasonic nebulization with a precision of better than 5% RSD. The accuracy was demonstrated by the analysis of a NIST commercial reference material, SRM 1643d. In all cases, the results from ICP-MS analysis agreed within 4% of the certified values. Comparative analysis of cloud water samples obtained from a site downwind from large pollution sources (Whiteface Mountain, New York) and Changlagali Pakistan, a rural mountain peak, was carried out by hydride generation atomic absorption (HGAA) spectrometry. There was excellent agreement between the ICP-MS and HGAA results. Received: 31 July 1997 / Revised: 28 October 1997 / Accepted: 31 October 1997