DETERMINATION OF ARSENIC(III) AND ARSENIC(V) BY ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY AFTER-COMPLEXATION AND SORPTION ON A C-18 BONDED SILICA COLUMN

A flow injection procedure for the separation and pre-concentration of inorganic arsenic based on the complexation with ammonium diethyl dit hiophosphate (DDTP) and sorption on a C-18 bonded silica gel minicolum n is proposed. During the sample injection by a time-based fashion, th e As3+-DDTP complex is stripped from the solution and retained in the column. Arsenic(V) and other ions that do not form complexes are disca rded. After reduction to the trivalent slate by using potassium iodide plus ascorbic acid, total arsenic is determined by electrothermal ato mic absorption spectrometry (ETAAS). Arsenic(V) concentration can be c alculated by difference. After processing 6 ml sample volume, the As3-DDTP complexes were eluted directly into the autosampler cup (120 mu l). Ethanol was used for column rinsing. Influence of pH, reagent conc entration, pre-concentration and elution time and column size were inv estigated. When 30 mu l of eluate plus 10 mu l of 0.1% (w/v) Pd(NO3)(2 ) were dispensed into the graphite tube, analytical curve in the 0.3-3 mu g As 1(-1) range was obtained (r = 0.9991). The accuracy was check ed for arsenic determination in a certified water, spiked tap water an d synthetic mixtures of arsenite and arsenate. Good recoveries (97-108 %) of spiked samples were found. Results are precise (RSD 7.5 and 6%, for 0.5 and 2.5 mu g 1(-1), n = 10) and in agreement with the certifie d value of reference material at 95% confidence level. (C) 1998 Elsevi er Science B.V.