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 phot o-oxidation hydride generation with ICP-MS detection. This sample pre-oxida tion method eliminates As(III) and As(V) preservation concerns and simplifi es the chromatographic separation. Four oxidants, Cl-2, MnO2, H2O2 and I-3( -), were investigated. Chlorine (ClOaq-) and MnO2 selectively converted As( III) into As(V) in pure water samples, but the conversion was inefficient i n 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 diffic ult. The most appropriate oxidant for the selective conversion of As(III) i nto As(V) with minimal interference from the urine matrix is I-3(-) at pH 7 . Unlike H2O2, excess oxidant can be easily removed by the addition of S2O3 2-. The I-3(-)-S2O32- treatment on a fortified sample of reconstituted NIST SRM 2670 freeze dried urine indicated that arsenobetaine (AsB), dimethlyar sinic acid (DMA), monomethylarsonic acid (MMA) and As(V) were not chemicall y degraded with recoveries ranging from 95 to 102% for all arsenicals. Samp le clean-up involved pH adjustment prior to C-18 filtration in order to ach ieve efficient As(III) conversion and quantitative recoveries of AsB and DM A. The concentrations determined in NIST SRM 2670 freeze dried urine were A sB 17.2 +/- 0.5, DMA 56 +/- 4 and MMA 10.3 +/- 0.3 with a combined total of 83 +/- 5 mu g L-1 (+/- 2 sigma).