Open-tubular ion-exchange capillary electrochromatography of inorganic anions

The viability of open-tubular ion-exchange electrochromatography (IE-CEC) for the separation of inorganic anions was investigated. Open-tubular IE-CEC columns were prepared by coating the inner wall of a fused silica capillary with quaternary ammonium anion-exchange latex particles (approximately 75 nm in diameter) bound electrostatically to the wall. Dionex AS5A latex particles were used for this purpose. The coated capillary showed reversed electro-osmotic flow (EOF), allowing anions to be separated in the co-EOF mode. The latex particles on the capillary wall introduced anion-exchange interactions with the analytes, the strength of which could be moderated by the type and concentration of the competing ion used in the background electrolyte (BGE). With a relatively low concentration of a weak competing ion (e.g., chloride), the migration order followed that obtained in an anion-exchange system. Increasing the concentration of the competing ion resulted in a decrease in the ion-exchange interactions and a change in the migration order from that obtained by capillary electrophoresis using a high ionic BGE. A stronger competing ion (e.g., perchlorate or sulfate) suppresses the anion-exchange interaction at a lower BGE concentration. The effective ion-exchange capacity of the capillary, and also the resulting anion-exchange interactions with the analytes, could be increased by decreasing the capillary diameter. The approaches described in this paper provide a simple means to manipulate separation selectivity for inorganic anions.