Speciation of inorganic and methylated arsenic compounds by capillary zone electrophoresis with indirect UV detection - Application to the analysis of alkali extracts of As₂S₂ (realgar) and As ₂S₃ (orpiment)

A capillary zone electrophoresis (CZE) method with indirect UV detection was developed to simultaneously separate inorganic and organic arsenic compounds including arsenite (iAs^III), arsenate (iAs^V), monomethylarsonate and dimethylarsenic acid (DMA^V). 2,6-Pyridinedicarboxylic acid (PDC) and n-hexadecyltrimethylammonium hydroxide (CTAOH) were selected to compose a background electrolyte (BGE), where PDC was used as chromophore and CTAOH functioned as electroosmotic flow (EOF) modifier to reduce/eliminate EOF. The choice of detection wavelength, the optimization of BGE pH, and effects of applied electric field strength and temperature on separation were further investigated. The limits of detection for the targeted analytes were between 0.19 and 0.23ppm as molecule. Good linearity of more than three orders of magnitude was obtained. Repeatability of migration times and peaks areas were 0.8-1.7 and 3.4-6.9% R.S.D.; whereas reproducibility were 1.2-2.2 and 3.6-7.1% R.S.D., respectively. The established CZE method was then applied to analyze the alkali extracts of realgar (As₂S₂) and orpiment (As₂S₃). The main components in both alkali extracts were identified to be iAs^III and iAs^V.