Speciation of arsenic by ion chromatography and off-line hydride generation electrothermal atomic absorption spectrometry

Abstract

Arsenic species including arsenite (As^III), arsenate (As^V), arsenobetaine (AB), monomethylarsenate (MMA), dimethylarsenate (DMA), p-aminophenylarsenate (p-APA) and arsenocholine (AC) have been separated on an anion- and a CAS 1 ion-exchange column connected in series. Arsenite co-elutes with arsenobetaine. The mobile phase used was 5 mmol l^–1 NH₄H₂PO₄ solution (pH 5.8) at the flow rate of 1 ml min^–1. Arsenic species in the fractions that eluted from the columns were determined by hydride generation atomic absorption spectrometry (AAS). A thermal decomposition procedure was introduced to convert the organoarsenicals, such as arsenobetaine and arsenocholine, into As^V, which can be reduced by sodium tetrahydroborate to form arsenic hydride, and then trapped on a palladium-coated graphite tube, followed by AAS detection. Arsenobetaine which co-elutes with As^III can be determined by substracting the amount of As^III determined from the total amount of As^III plus arsenobetaine after thermal decomposition. Detection limits (3σ signal-to-noise) were 1.6 ng ml^–1 for As^III, As^V, MMA and p-APA and 1.9 ng ml^–1 for DMA, AB and AC for a 0.1 ml injection. The proposed method has been applied to the determination of arsenic species in spiked lake water and river water.