Separation of organic and inorganic arsenic species by capillary electrophoresis using direct spectrophotometric detection

Abstract

Capillary electrophoresis (CE) with UV detection was used for the determination of arsenite, arsenate, monomethylarsonic acid, dimethylarsinic acid, p-aminophenylarsonic acid, 4-hydroxy-3-nitrobenzenearsonic acid, 4-nitrophenylarsonic acid, phenylarsonic acid, and phenylarsine oxide. The electrophoretic mobilities of these anionic species were determined in a 20 mM phosphate buffer in a pH range from 4 to 11, which established pH 10 as the optimum for the separation. The target analytes were then separated in a fused-silica capillary using 20 mM NaHCO3-Na2CO3 buffer, pH 10, as electrolyte and detected at 192 nm. Both normal- and reversed electroosmotic flow (EOF) separation modes were investigated and in the latter case, poly(diallydimethylammonium chloride) (PDDAC), was used for dynamic coating of the capillary and to provide a stable and reproducible reversed EOF (relative standard deviation RSD, 0.39%). The influence of electrolyte pH and composition, applied voltage, as well as EOF reversal protocols upon the method performance criteria were investigated. The optimised method provided limits of detection for the target analytes of 1.62, 6.22, 1.45, 1.83, 0.34, 0.40, 0.40, 0.18, and 0.30 mg/L As, respectively. Linearity was obtained in the range of 0.5-40 mg/L As (for aryl compounds) and from 5-100 mg/L As (for the remaining analytes). Reproducibility of peak areas was in the range of 0.8-5.5% RSD. The method was applied to the determination of four aryl arsenic compounds used as additives in animal feed. Analytes were extracted with 40 mM hydrochloric acid - acetonitrile 4:1 v/v, and then cleaned up by passing through a C18 solid-phase extraction cartridge before analysis by CE with detection at 200 nm. Recoveries for the four analytes were in the range of 78.8-108.3%.