CONTROLLED CHANGES OF SELECTIVITY IN THE SEPARATION OF IONS BY CAPILLARY ELECTROPHORESIS

The selectivity of low-molecular-weight inorganic and organic anions is evaluated on a capillary electrophoresis system utilizing a negative power supply and indirect UV detection at 254 nm. The background electrolyte chromate was found to be most useful for highly mobile inorganic anions and short chain carboxylates. Incorporated in this electrolyte is an additive that reverses the direction of electroosmotic flow (EOF) in the capillary, so that the EOF augments the mobility of the analytes. This results in exceedingly short analysis times, under 5 min, with efficiencies approaching 600 000 theoretical plates. From migration time data, a correlation between ionic equivalent conductance and analyte mobility in the electrolyte is established. This empirical correlation equation aids in the prediction of migration order (selectivity) of a separation. Other parameters evaluated on the chromate electrolyte are ionic strength, pH and EOF modifier concentration. It was found that pH and the EOF modifier provided significant selectivity control over the analytes while the other electrolyte parameters permitted only subtle changes in selectivity.