Comparison of different electroosmotic flow modifiers in the analysis of inorganic anions by capillary electrophoresis

The determination of inorganic anions using capillary electrophoresis with non-treated silica capillaries requires reversal or elimination of the cathodic electroosmotic flow (EOF). The behaviour of different monocharged [tetrabutylammonium (TEA), cetyltrimethylammonium (CTA) and tetradecyltrimethylammonium (TTA)] or multicharged [hexamethonium (HM) and hexadimethrine (HDM)] alkylammonium quaternary ions as electroosmotic flow modifiers has been compared. Indirect spectrophotometric detection using chromate as carrier ion has been used. The effect of EOF modifiers and chromate concentration, pH and applied potential on the separation of bromide, chloride, nitrite, nitrate, sulfate and phosphate has been studied. Optimal conditions for the separation of these anions have been established and resolution has been compared. Reproducibility in areas (R.S.D. 1.8-8.5%) and in migration times (R.S.D. 2.3-7.5%), and detection limits (200-1000 mu g l(-1)) were similar for all the EOF modifiers. For HDM very low concentrations, down to 0.0001%, can be used successfully.