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).