Rapid arsenic speciation using ion pair LC-ICPMS with a monolithic silica column reveals increased urinary DMA excretion after ingestion of rice

Abstract

Rapid separation of arsenic species is described by application of a commercial monolithic silica column (Chromolith™). Arsenic species were separated within 3 min, greatly reducing the time required with conventional columns. The separation was achieved by ion-pair liquid chromatography using 2.5 mM tetrabutylammonium bromide, 10 mM phosphate buffer (pH 5.6) and 1.0% (v/v) methanol as the mobile phase. Detection limits of 0.107, 0.084, 0.120, 0.121 and 0.101 μg L⁻¹ As for As(III), arsenobetaine (AsB), dimethylarsinate (DMA), monomethylarsonate (MMA) and As(V), respectively were achieved. The precision of the method, based upon analysis of 15 μg L⁻¹ As, was better than 5.9% for all species. The technique was applied to the speciation analysis of urine and food samples in an ingestion experiment to investigate the effect of rice consumption on arsenic exposure in humans. For the first time this study reveals that ingestion of American long grain rice, which is widely consumed in the UK and purchased from a local supermarket, significantly increases the excretion of DMA in human urine. This is consistent with a recent study which reported that DMA was the predominant arsenic species in rice from the USA, which also happened to contain the highest mean arsenic level in the grain compared to rice grown in Bangladesh and India.¹