Selective determination of airborne hexavalent chromium using inductively coupled plasma mass spectrometry

Talanta 57 (2002) 1143– 1153 www.elsevier.com/locate/talanta

Selective determination of airborne hexavalent chromium using inductively coupled plasma mass spectrometry Yarong Li, Narayan K. Pradhan, Roy Foley, Gary K.C. Low * Analytical and En6ironmental Chemistry Section, New South Wales En6ironment Protection Authority, PO Box 29, Lidcombe, NSW 1825, Australia Received 12 September 2001; received in revised form 8 April 2002; accepted 12 April 2002

Abstract A new method for determining ultra-trace levels of hexavalent chromium in ambient air has been developed. The method involves a 24-h sampling of air into potassium hydroxide solution, followed by silica gel column separation of chromium (VI), then preconcentration by complexation and solvent extraction. The chromium (VI) complex was dissolved in nitric acid. The resultant chromium ions were determined by inductively coupled plasma mass spectrometry (ICP–MS) using a dynamic reaction cell (DRC) with ammonia as the reactive gas to reduce polyatomic interferences. The interconversion of chromium in potassium hydroxide solution and air sample matrix were investigated under ambient conditions. It was found that there was no conversion of chromium (VI) into chromium (III) species. However, it was observed that some chromium (III) species were converted into chromium (VI) species. For a KOH solution containing 100 mg l − 1 of chromium (III) species, the rate of conversion was found to be 3% after 24 h exposure, 8% after 48 h, 10% after 72 h and no further conversion was observed thereafter. However, in a solution containing air sample matrix, 9.3% of chromium (III) converted to chromium (VI) within 6 h, and during the course of a 11-day exposure period, 13% (range 8 – 17%) of chromium (III) converted to chromium (VI). The method detection limit (MDL) for chromium (VI) in potassium hydroxide solution (0.025 M) was found to be 2 ×10 − 2 mg l − 1. This is equivalent to 0.2 ng m − 3 (for 23 m3 air sampled into 200 ml of KOH solution over a 24-h period). The recovery of spiked chromium (VI) from solutions containing air sample matrix was 95 9 9% (n =8). Matrix related interferences were estimated to be less than 10% based on recovery studies. The concentration of airborne chromium (VI) in Sydney residential areas was found to be less than 0.2 ng m − 3, however, in industrial areas the concentrations ranged from 0.2 to 1.3 ng m − 3 using this analytical procedure. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hexavalent chromium; Air; Complexation; Separation; Preconcentration; Interconversion; Inductively coupled plasma mass spectrometry; Dynamic reaction cell; Polyatomic interferences

1. Introduction * Corresponding author. Fax: + 61-2-964-62755 E-mail address: [email protected] (G.K.C. Low).

Chromium is a naturally occurring metal in the environment. It may exist in several oxidation

0039-9140/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 1 9 6 - 0

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states ranging from chromium(II) to chromium(VI). However, the forms of chromium that commonly exist in the environment occur in the III and VI oxidation states. The toxicity of a chromium species is related to its bioavailability, which is in turn linked to its chemical form. For example, hexavalent chromium can exist as chromium trioxide, dichromic acid and dichromate salts, which are all considered to be carcinogenic [1– 3]. In contrast, chromium(III) compounds are essential to humans at low levels where they are involved in the metabolism of glucose, lipid and protein [1]. Based on a constant lifetime exposure level of 1 mg m − 3, the USEPA and WHO have reported the lifetime cancer risk for inhalation exposure to chromium (VI) is in the range of 1.1×10 − 2 – 1.3 × 10 − 1 [2–4]. At the risk level of ten in 1 million cancers, these risk estimates equate to respective concentrations of 7× 10 − 5 and 9 ×10 − 4 mg m − 3 of chromium(VI) in air. Typically, for monitoring chromium(VI) species in air, approximately 24 m3 of air is passed through 200 ml of KOH trapping solution over a 24 h period. Under these conditions, the lifetime cancer risk levels reported by the USEPA and WHO are equivalent to concentrations ranging from 8× 10 − 3 to 0.1 mg l − 1 of chromium(VI) species in the solution. Consequently, for an analytical procedure to be applicable, it must be capable of detecting chromium(VI) species at these ultra-trace concentrations. There are several methods available for chromium speciation in environmental samples [5– 24]. However, the relatively high detection limits make these methods unsuitable for the determination of ultra-trace levels of airborne chromium(VI). The sampling and analytical methods for detecting hexavalent chromium under ambient conditions are a challenge due to the very low concentrations expected [2,3], interferences and the potential for interconversion of the chromium species. There are several factors that control the interconversion of chromium(III) and chromium(VI) species. These include the presence of oxidizing and reducing agents, the electrochemical potential of the oxidation and reduction reactions, UV light, and acid-base reactions [25,26]. The formal reduction potential (Eh) of Cr(VI)/Cr(III) in solution

changes from − 0.04 (pH 13) to 0.52 (pH 7.4) and then to 1.07 (pH 2) [26]. Chromium(III) is thermodynamically stable under low Eh and low pH, while high Eh and high pH favor the existence of chromium(VI). Hence, alkaline conditions are used to stabilize chromium(VI). Studies have also shown that the oxidation of chromium(III) to chromium(VI) is highly dependent on its chemical forms: Cr2O3 and aged Cr(OH)3 are resistant to oxidation whilst Cr3 + and freshly precipitated Cr(OH)3 are relatively easy to oxidize [25,27]. In this study, a DRC–ICP –MS method [28,29] using ammonia as the reactive gas, was developed for the determination of ultra trace levels of chromium(VI) in air. The effects of interferences and interconversion of chromium species in KOH solution were elucidated, and the method was found to be applicable to monitoring cancer risk levels of chromium(VI) in air.

2. Experimental

2.1. Inducti6ely coupled plasma mass spectrometry (ICP –MS) A Perkin –Elmer SCIEX Elan® 6100DRC™ ICP –MS (Concord, Ont., Canada) equipped with a dynamic reaction cell (DRC) was used in this study. The sample delivery system consisted of Perkin–Elmer AS93 plus auto-sampler, peristaltic pump and a Glass Expansion (GE) ‘Tracey’ Cyclonic spray chamber with a conical type AR30-1F1 standard concentric glass nebulizer. The instrument was fitted with platinum sampler and skimmer cones and optimized daily. Indium was used as an internal standard to compensate for matrix interferences and ammonia was used as the reaction gas in the DRC –ICP –MS to remove the polyatomic interferences. The operating conditions for the ICP –MS set-up are given in Table 1.

2.2. Reagents, solutions and materials Millipore (Bedford, MA, USA) Milli-Q™ 18 MV water was used throughout this study. Potassium hydroxide (AR grade, BDH, UK) solution (0.25 M) was purified by eluting through an anion

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exchange column and diluted 10-fold for use as the impinger solution. Nitric acid (ARISTAR grade, BDH, UK) was purified by distillation and chloroform (ARISTAR grade, BDH, UK) was purified by extraction with 10% (v/v) nitric acid. Ammonium pyrrolidine dithiocarbamate (APDC) (AR grade, Sigma, Switzerland) solutions (5% w/v) were prepared daily. Hexavalent chromium stock standard solution (1000 mg l − 1) was prepared from potassium dichromate (AR grade, BDH, UK) in 2% (v/v) nitric acid. Trivalent chromium stock standard solution (1000 mg l − 1) was prepared from chromium(III) potassium sulfate dodecahydrate (AR grade, BDH, UK). Chromium(III) and chromium(VI) intermediate and working standards were prepared daily. The Indium internal standard solution (100 mg l − 1) was prepared from 1000 mg l − 1 stock (Alpha Resources Inc., USA). The environmental standard reference material, NIST 1648 Urban Particulate Matter (US Department of Commerce National Institute of Table 1 DRC–ICP–MS operating conditions Plasma Plasma flow rate (l min−1) Auxiliary gas flow rate (l min−1) Radio frequency power Sample cone Skimmer cone Measurement parameters Analysis mode Analysis isotopes Nebulizer gas flow rate (l min−1) Nebulizer aspiration rate (ml min−1) Sweeps per reading Dwell time (ms) Replicates Total analysis time (s) DRC gas DRC gas flow rate (ml min−1) DRC rejection parameter q (RPq)a

15 1.20 1200–1300 W Platinum, 1.1 mm orifice Platinum, 0.89 mm orifice DRC 52 Cr, 53Cr, 0.87–0.92 1.0

115

In

10 500 3 114 Ammonia 0.4 0.60

a The RPq is the q Mathieu parameter that is used to selectively filter out low mass interference ions in the DRC [29].

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Standards and Technology, Gaithersburg, MD, USA) was used to validate results. Instrument grade argon and ammonia gases (BOC, Australia) were used for DRC– ICP –MS. Polypropylene and ‘Teflon’ plastic volumetric-ware were used exclusively for preparation of all solutions. The silica gel column (particle size: 50 mm; average pore size: 60 A, ; 500 mg per 2.8 ml; length: 10 mm; internal diameter: 9 mm, Alltech, USA) was pre-conditioned with 3 ml of methanol (Merck, Darmstadt, Germany) followed by 10 ml of deionized water. The anion exchange column (particle size: 35–75 mm; 500 mg per 2.8 ml; length: 10 mm; internal diameter: 9 mm, Alltech, USA) was pre-conditioned with 10 ml of deionized water.

2.3. Sampling train The sampling train is shown in Fig. 1, and consisted of three 250 ml Greenberg– Schmidt impingers in series, a gas flow controller and a pump. KOH solution (0.025 M, 100 ml) was placed in each of the first two impingers. Approximately 100 g of dried silica gel was placed in the third impinger to protect the pump and test meter. The entire sampling device was enclosed in a portable refrigerator to prevent the loss of impinger solution from evaporation. Using similar apparatus, Sheehan et al. [5] had demonstrated that the chromium(VI) collection efficiency was 83– 87% in the first impinger at a flow rate of 15–16 l per min (lpm). Consequently, in this study, the air was sampled at a flow rate of 15–16 lpm for 24-h (23 m3). After collection, the samples were stored at 4 °C and analysis commenced within 24 h.

2.4. Chromium(VI) separation, extraction and determination Sample solutions from the first and second impingers were combined together. The sample pH was adjusted to 8 with 2% (v/v) nitric acid and eluted through a silica gel column. The eluent containing the chromium(VI) was collected. The chromium(III) was retained on the column and discarded. The pH of the eluted solution was

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Fig. 1. Air sampling rain.

adjusted to 2.9 with 2% (v/v) nitric acid. APDC (5% w/v, 2 ml) was added and shaken for 15 min at room temperature (22 °C) with a Janke & Kunkel HS 501D shaker at 300 rpm. After standing for 5 min, chloroform (5 ml) was added and shaken again for 15 min. The solution was then transferred into a 250 ml separating funnel to allow phase separation for 10 min and the chloroform phase was collected. The remaining aqueous phase was re-extracted with APDC (5% w/v, 2 ml) and chloroform (5 ml). The extracts were combined, evaporated to dryness, dissolved in nitric acid (20% v/v, 1 ml), diluted to 5 ml and analyzed by DRC –ICP –MS.

3. Results and discussion

3.1. Selection of the chromium speciation method It has been reported that both chromium(VI) and chromium(III) species could be complexed by APDC [6]. Methyl iso-butyl ketone (MIBK) has been reported as a suitable extraction solvent for chromium complexes [6]. However, it was found that MIBK was partially miscible with water, and the extent of miscibility varies with both temperature and ionic strength [30]. Significant error was found during extraction with MIBK, consequently, chloroform was used instead.

In this study, quantitative extraction of chromium(VI) from KOH solution was observed with APDC/chloroform. However, approximately 10% of chromium(III) was also co-extracted. Hence, this method could not be used, and a preliminary step was required to separate chromium(III) from chromium(VI). During preliminary experiments, an anion exchange column was used to separate chromium(VI) from chromium(III) and was found to sorb chromium(VI) completely. This is in agreement with previously published work [7,14]. However, our study found that complete elution of chromium(VI) could not be achieved, even with 25% (v/v) nitric acid. A silica gel column was investigated for the separation of chromium(III) and chromium(VI). It was found that chromium(III) was selectively sorbed onto the silica gel column without any retention of chromium(VI). These findings concurred with Boussemart et al. [18]. Consequently, a silica gel column was used in this study for selective removal of chromium(III) species.

3.2. Remo6al of chromium(III) by silica gel column The effect that pH had on the removal efficiency of chromium(III) from a silica gel column was investigated. A number of chromium(III) so-

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lutions (50 mg l − 1) were prepared in KOH (0.025 M) and the pH of each adjusted incrementally to cover the pH range 4– 12. Each solution was passed through a silica gel column, the pH adjusted to 2.9 and extracted with APDC/chloroform. It was found that at pH 8, 97– 100% of chromium(III) was removed. This was attributed to the silica gel behaving as a cation exchanger, since above pH 7 the exposed silanol groups of the silica gel are deprotonated and converted to siloxanyl ions [31]. The effect of pH on the removal efficiency of chromium(III) is shown in Fig. 2.

3.3. Optimum pH for chromium(VI) extraction by APDC/chloroform

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3.4. Optimum APDC concentration for chromium(VI) chelation The effect of APDC concentration was investigated by preparing a number of chromium(VI) solutions (0.05–100 mg l − 1) in KOH (0.025 M) and extracting with varying concentrations of APDC (0.005–0.1% (w/v)) in the sample solution. From Fig. 4, it is evident that full recovery can be achieved when the log{[APDC]/[Cr(VI)]} is greater than 4. However, this is conditional on the APDC concentration being 0.05% (w/v) or higher. These results demonstrate that there are two contributing factors to achieving high recoveries: the concentration of APDC and the concentration ratio of APDC to chromium(VI).

3.5. Intercon6ersion of chromium species (III/VI) The optimum pH for APDC/chloroform extraction of chromium(VI) was determined by investigating the effect of pH on recovery of chromium(VI). A series of chromium(VI) solutions (50 mg l − 1) were prepared in KOH (0.025 M). Prior to extraction, the pH of each solution was adjusted so that the series spanned the pH range, 2–4.5. The optimum pH interval for extraction was found to be 2.8– 3.0. Fig. 3 illustrates the influence of pH on the APDC/chloroform extraction of chromium(VI).

It is difficult to predict the interconversion of chromium species in environmental samples [27]. In this study, separate chromium(VI) and chromium(III) in KOH solutions were simultaneously exposed to ambient conditions to investigate the interconversion between chromium(VI) and chromium(III). Aliquots of each solution were taken daily and the concentration of chromium(VI) was measured. The pH was also monitored at the same time. There was no significant

Fig. 2. Removal efficiency of chromium(III) by silica gel column as a function of pH.

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Fig. 3. Recovery of chromium(VI) from the impinger solution with varying extraction pH.

Fig. 4. The influence of the concentration ratio of APDC to chromium(VI) on the recovery of chromium(VI).

change in the recovery levels of chromium(VI) solution. This suggested that chromium(VI) was stable in 0.025 M KOH solution, and conversion to chromium(III) is unlikely to occur under ambient conditions for at least 9 days. However, in the case of the chromium(III) solution, oxidation to chromium(VI) did occur. The rate of conversion was found to be 3% after 24 h exposure, 8% after

48 h, 10% after 72 h, then no further change was observed. In order to investigate the interconversion between chromium(VI) and (III) in actual air sample matrix, samples from a residential area in Sydney were collected, and spiked with chromium(VI) and chromium(III), respectively. It was found that there was no significant conversion of chromi-

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um(VI) to (III). However, within 6 h 9.3% of chromium(III) converted to chromium(VI), and during the course of a 11-day exposure period, 13% (range 8–17%) of chromium(III) converted to chromium(VI) in the air sample matrix. These results suggest that the conversion of chromium(III) in air sample matrix occurs faster and more readily than in KOH solution. This may be due to the presence of oxidizing species in the solution of air sample matrix. The initial oxidation of chromium(III) to chromium(VI) may be attributed to the relative ease of oxidation of the fresh Cr3 + and Cr(OH)3; after a certain time the aged Cr3 + and Cr(OH)3 were resistant to oxidation [25,27]. Fig. 5 shows the pH of the chromium(III) solution decreased from 12.1 to 11.8 after 3 days exposure, whilst there was no significant change in the chromium(VI) solution for the same 3-day period. The change in the pH of the chromium(III) solution may be partly due to the oxidation of chromium(III) to chromium(VI): 4Cr3+ + 3O2 +20OH− “4CrO24 − +10H2O However, this is unlikely because the concentration of chromium is negligible in comparison to the necessary neutralization requirements. From the 4th day onwards, the trends in pH for both

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solutions were similar. The decrease in pH was attributed mainly to the absorption of carbon dioxide from air. In the solution containing the air sample matrix, no change of pH was found. This was due to the carbonate buffering effect from absorption of ambient carbon dioxide. This study shows clearly that the analysis of samples must be carried out as soon as possible to minimize any interconversion effect.

3.6. Air matrix interferences Potential interferences from air matrix were also simulated by evaluating the recovery of chromium(VI) species from KOH solution, spiked with urban particulate matter (NIST 1648) and a mixture of metals. The spiking levels of the metals and urban particulate matter used in this study were in excess of the levels expected in the Sydney region. Fig. 6 shows that there was less than 10% interference from the residential area air sample matrix, spiked urban particulate matter and the mixture of metals.

3.7. Correction for ICP–MS interferences 3.7.1. Polyatomic interference Polyatomic interferences due to carbon and

Fig. 5. Variation of pH with respect to exposure time of 100 mg l − 1 chromium(VI) in KOH solution (A) and 100 mg l − 1 chromium(III) in KOH solution (B).

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Fig. 6. Recovery of 50 mg l − 1 chromium(VI) in 0.025 M KOH (A) in KOH (0.025 M) spiked with 100 mg l − 1 of Al, As, Ba, Be, Ca, Cd, Co, Cu, Fe, Mn, Mg, Mo, Ni, Na, Pb, Sb, Se, V, and Zn (B) and in KOH (0.025 M) spiked with 50 mg of NIST 1648 (C) and recovery of spiking 100 mg l − 1 chromium(VI) in air sample matrix (D).

Fig. 7. ICP –MS spectra of chromium in the presence of chloroform.

chlorine from APDC and chloroform were problematic for the quantitation of chromium by ICP – MS. The interfering ions include, 1 35 H Cl16O+ (m/z 52), 37Cl15O+ (m/z 52), 40 Ar12C+ (m/z 52), and 37Cl16O+ (m/z 53), which have the same m/z as 52Cr+ and 53Cr+, respec-

tively. To eliminate these interferences, a DRC was used with ammonia as the reactive gas. The effects of chlorine and carbon based interferences on 50Cr+, 52Cr+, 53Cr+ and 54Cr+ are shown in Fig. 7. In normal ICP–MS mode, without the use of DRC, the effects were very significant on the

Y. Li et al. / Talanta 57 (2002) 1143–1153 52

Cr+ and 53Cr+ ions, such that the results can be misleading. When DRC was used, the interferences from the polyatomic species were eliminated with ammonia gas via the following reactions: HClO+ + NH3 “ ClO +NH+ 4

ClO+ +NH3 “ ClO +NH+ 3 ArC+ + NH3 “ArC +NH+ 3

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The RPq was optimized (using ammonia gas) for the removal of low mass polyatomic interference species, including carbon and chlorine. The optimization of the ammonia gas flow rate and RPq are shown in Figs. 8 and 9, respectively. At the optimized ammonia gas flow rate (0.4 ml min − 1) and RPq value (0.60), the relative abundances obtained for 50Cr, 52Cr, 53Cr and 54Cr by ICP –MS were 3.5, 83.2, 10.5 and 2.8%, respec-

Fig. 8. Optimization of ammonia gas flow for chromium (m/z 52) in 0.1% CHCl3 and 2% HNO3 matrix. (A) Matrix only, (B) 10 mg l − 1 chromium in matrix.

Fig. 9. Ratio of intensity of analyte solution to matrix solution versus RPq at m/z 52. Matrix solution: 0.1% CHCl3 in 2% HNO3; analyte solution: 0.1% CHCl3 in 2% HNO3; analyte solution: 10 mg l − 1 chromium in matrix solution.

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Table 2 Elimination of quenching interference of potassium by the use of indium as an internal standard 20 mg l (VI)

−1

52

Cr intensity (counts s−1) 115 In intensity (counts s−1) Without internal standard (mg l−1) With internal standard (mg l−1)

Cr

20 mg l Cr (VI)+30 mg l−1 K −1

790 647

602 780

1 044 285

783 042

20

15.2

20

20.4

Table 3 Levels of airborne chromium(VI) in the Sydney Basin Location

Type of location Chromium (VI) (ng m−3)

Lidcombe Glenwood Rooty Hill Rooty Hill Wollongong Wollongong Matraville Matraville

Residential Residential Industrial Industrial Industrial Industrial Industrial Industrial

0.08 0.14 0.09 0.25 1.25 1.31 0.62 0.87

tively. These are in agreement with the chromium isotopic ratios reported in the literature [32].

3.7.2. Quenching effect A further complication in the quantitation of chromium(VI) species was the quenching effect on the plasma by potassium. This reduced the detector response and sensitivity for chromium. However, in this study it was found that these effects could be overcome by using indium as an internal standard. Table 2 illustrates the influence of quenching and the effectiveness of indium for quantitation of a 20 mg l − 1 chromium(VI) solution in the presence of 30 mg l − 1 potassium. Without the use of an internal standard, the recovery of chromium(VI) was only 76%. When an internal standard correction was applied, the recovery improved to 102%.

3.8. Method detection limit The method detection limit (MDL) was estimated from reagent blanks (KOH, 0.025 M) and KOH (0.025 M) spiked at the level of 0.15 mg l − 1 of chromium(VI). The MDLs were calculated as the product of the standard deviation (n= 7) and the one-sided t distribution value [33]. The reagent blank and the spiked KOH solutions gave estimated MDLs of 6.2× 10 − 3 and 2.0× 10 − 2 mg l − 1, respectively. A MDL of 2.0× 10 − 2 mg l − 1 in the impinger solution is equivalent to 0.17 ng m − 3 for a 23 m3 air sample (rounded to 0.2 ng m − 3) and is based on extraction of a 200 ml sample in KOH (0.025 M). In KOH solution, the average recovery for chromium(VI) spiked at the level of 0.15 mg l − 1 was 81 9 4% (n=7), and for spiking levels ranging from 0.5 to 100 mg l − 1, the average recovery was 949 4% (n= 10).

3.9. Analysis of airborne chromium(VI) A number of air samples were collected from residential areas and industrial areas around the Sydney basin. These were analyzed by the proposed method and the levels of chromium(VI) were found to be less than 0.2 ng m − 3 in the residential areas, and ranged from 0.2–1.3 ng m − 3 in the industrial areas. The results are shown in Table 3 and are comparable with literature values (0.15–0.6 ng m − 3) for a similarly industrialized city in the USA [34]. In order to investigate the performance of the method at concentrations above the MDL, eight samples were collected from a residential area, and the impinger solutions were spiked at the levels of 0.20, 0.25 and 0.5 mg l − 1 chromium(VI). The average recovery was found to be 9599%. This demonstrated that the proposed method is applicable to the determination of chromium(VI) in air.

4. Conclusions The results of this study have shown that the proposed method is applicable to the determina-

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tion of ultra trace levels of hexavalent chromium in air. Interconversion studies have shown that there was no conversion of chromium(VI) to chromium(III), and there was an average of 13% conversion of chromium(III) to chromium(VI) in solutions containing air sample matrix. Consequently, analysis should be commenced as soon as possible after sampling. It was also found essential to use DRC with ammonia as the reactive gas and an internal standard to eliminate interferences for the determination of chromium by ICP – MS. The method detection limit was found to be 0.2 ng m − 3 based on a 23 m3 air sample. Acknowledgements The authors are grateful to Russell Hopkins from the NSW EPA Atmospheric Science Section (Lidcombe) for collecting some of the air samples for this study, and to Peter Snitch from Perkin Elmer for his support with the ICP–MS instrumentation. References [1] D.E. Kimbrough, Y. Cohen, A.M. Winer, L. Creelman, C. Mabuni, Crit. Rev. Environ. Sci. Technol. 29 (1) (1999) 1–46. [2] Air Quality Guidelines for Europe, WHO Regional Publications, European Series No. 23, pp. 221 –228. [3] Health Effects Assessment for Hexavalent Chromium, US Environmental Protection Agency, EPA/540/1-86-019. Washington, DC Environmental Protection Agency, 1984. [4] M.C.R. Alavanja, C. Brown, R. Spirtas, M. Gomez, Appl. Occup. Environ. Hyg. 5 (8) (1990) 510 – 517. [5] P. Sheehan, R. Ricks, S. Ripple, D. Paustenbach, J. Am. Ind. Hyg. Assoc. 53 (1992) 57 –68. [6] K.S. Subramanian, Anal. Chem. 60 (1988) 11 –15. [7] M. Sperling, S. Xu, B. Welz, Anal. Chem. 64 (1992) 3101 – 3108. [8] Y.S. Fung, W.C. Sham, Analyst 119 (1994) 1029 – 1032. [9] M. Derbyshire, A. Lamberty, P.H.E. Gardiner, Anal. Chem. 71 (1999) 4203 –4207. [10] R.W. Bell, J.C. Hipfner, J. Air Waste Manage. Assoc. 47 (1997) 905 – 910. [11] USEPA Method c 218.4, Chromium, Hexavalent (AA, Chelation Extraction), Approved for NPDES (Issued 1978). [12] USEPA Method c 3060A, Alkaline Digestion for Hexavalent Chromium, SW- 846 update III revision 1, December 1996.

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