Selectivity adjustment in capillary electrochromatography with ternary mobile phases

Abstract

In capillary electrochromatography (CEC) the propulsion of the mobile phase is effected by electroosmosis, and the optimization of the mobile phase composition must take more factors into account than in pressure-driven LC: electroosmotic mobility (mobile phase velocity), retention, and selectivity. In this paper, a method is presented to improve selectivity for closely eluting solutes in CEC with capillaries packed with octadecylsilica gel. It is shown for mobile phases consisting of a mixture of acetonitrile with an aqueous buffer that the selectivity of the chromatographic system can be fine-tuned by a tertiary mobile phase component. N,N-Dimethylformamide, tetrahydrofuran, and dimethyl sulfoxide proved to be useful as selectivity-tuning modifiers. With these ternary mobile phases, there is a large impact on the selectivity (compared to a mobile phase containing only acetonitrile and the aqueous buffer) while the retention factors are only slightly reduced or increased and there is no intolerable loss in separation speed.