Speciation of arsenic and selenium by capillary electrophoresis

Abstract

Capillary electrophoresis (CE) with direct UV detection, both on-capillary and with a high-sensitivity detection cell (HSDC), were used for the simultaneous determination of arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), selenate [Se(VI)], selenite [Se(IV)], selenocystine (SeC), selenomethionine (SeM) and selenocystamine (SeCM). These anionic and cationic species were separated with negative separation voltage polarity in a capillary coated with poly(diallydimethylammonium chloride) (PDDAC). The background electrolyte (BGE) providing optimal separation was 15 mM phosphate buffer at pH 10.6. Arsenic and selenium species were detected at 195 and 200 nm, respectively. The limit of detection (LOD) values with on-capillary detection (75 μm i.d.) for the individual analytes were 1.02, 1.50, 1.08, 1.35, 19.5, 0.36, 0.32, 1.11 and 1.47 mg/L, respectively (expressed as arsenic or selenium). The method precision for peak area was from 2.1 to 3.4% relative standard deviation. HSDC was applied to increase the detection sensitivity and gave LOD improvement factors from 4.7 to 8.2, yielding LODs for the individual target analytes ranging from 0.049 to 2.38 mg/L. When the normal sample stacking mode was employed using a sample plug up to 4.6% of capillary volume, both the cationic and anionic analytes were stacked simultaneously with LOD improvement factors of 2.6–4.5 yielding LODs for the individual analytes ranging from 0.11 to 7.42 mg/L. The method was applied for the speciation of arsenic in sediment and determination of SeM in a selenium nutrition supplement. The achieved separation selectivity also gives the method a more general application potential for simultaneous arsenic and selenium speciation when hyphenated with inductively coupled plasma MS.