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

Abstract

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 S₂O₃²⁻. 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 μg L⁻¹ (±2ς).