PREREDUCTION OF ARSENIC(V) TO ARSENIC(III), ENHANCEMENT OF THE SIGNAL, AND REDUCTION OF INTERFERENCES BY L-CYSTEINE IN THE DETERMINATION OF ARSENIC BY HYDRIDE GENERATION

L-Cysteine reduces arsenic(V) to arsenic(III) rapidly on warming. In addition, the signal from arsenic(III), after reduction by tetrahydroborate(III), is enhanced by the presence of L-cysteine in the arsenic solution. Interferences from transition elements and other hydride-forming elements (with the exception of selenium and tellurium) are substantially reduced by the presence of L-cysteine. Because of the different rates of reaction at different pHs, speciation of arsenic(III) and arsenic(V) is possible. The detection limit, by dc plasma atomic emission spectrometry, for arsenic(III) is 0.62 ng mL-1 for a 5-mL sample. The relative standard deviation for 10 determinations of a solution containing 50 ng mL-1 arsenic(III) was 1.1%. A major advantage of this technique is that it requires low acid concentrations and, furthermore, the prereducing agent fulfills several functions and is relatively innocuous.