Differential pulse cathodic stripping voltammetric speciation of trace level inorganic arsenic compounds in natural water samples

A simple, fast and sensitive speciation method is described for inorganic arsenic in water at the (mu)g/l level, applicable in the laboratory and in the field, based on differential pulse cathodic stripping voltammetry (DPCSV). Only As(III) is deposited on a Hg electrode in the presence of Cu and Se in HCl medium. Determination of total As is performed by reducing As(V) to As(III) using sodium metabisulfite/sodium thiosulfate reagent stabilized with ascorbic acid. As(V) is quantified by difference. The detection limit (S/N>3) was 0.5 (mu)g/l with a linear range from 4.5 to 180 (mu)g/l. The relative standard deviation (n=6) was 2.4, 2.5, 4.2% for As(III) and 8.0, 6.8, 9.0% for As(V) at levels of 45, 10, and 5 (mu)g/l, respectively. Analysis of the NIST 1640 natural water standard yielded total arsenic concentration 26.5(plus-minus)3.4 (mu)g/l (n=3) compared to the certified value of 26.7 (mu)g/l. Results obtained on several natural water samples analyzed both in the laboratory and on-site compared well with those obtained by HR ICP-MS, GFAAS and IC-AFS. Ions (phosphate, iron, manganese) commonly found in groundwater containing arsenic were found to have negligible interference.