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Arsenic compounds present in human urine provide information on daily arsenic exposure, the type of arsenic compounds ingested, and human methylating capacity. The arsenicals most frequently determined in urine by anion-exchange chromatography are arsenite, arsenate, methylarsonic acid, and dimethylarsinic acid (Chana and Smith, 1987; Heitkemper et al., 1989; Sheppard et al., 1990, 1992). Whereas the methylated arsenic compounds are unlikely to change chemically during storage of the samples or during the analytical procedure, arsenite might be oxidized to arsenate. For example, Thompson et al. (1993) recently reported significant oxidation of arsenite to arsenate during chromatography on a Dionex AS7 anion-exchange column with a mobile phase changing from 1mM to 1000 mM NaOH. Consequently, the results obtained for arsenite and arsenate by anion-exchange chromatography could be erroneous. The use of ionpairing reversed-phase chromatography was recommended to obtain reliable results. Should the oxidation of arsenite during anion-exchange chromatography be commonplace, then all results for arsenite and arsenate obtained by this method are incorrect. To define the conditions under which arsenite is likely to be oxidized, the influence of pH, time, and the arsenite concentration on the conversion of arsenite to arsenate in aqueous solutions in contact with air was investigated. |
