Viscous effects in capillary electrophoresis: Theory and experiment

Abstract

Simple theory which relates the viscous effects of additives used in capillary electrophoresis (CE) to resolution and resolution per unit time are developed for small molecule separations in the absence of sieving effects. The resolution theory shows that there is no advantage to using viscous additives for resolution improvement unless either a binding energy difference exists between the solutes and additive, or viscous effects cause a reduction in the vector sum of electrophoretic and electroosmotic velocities when these are of opposite sign. In general, increasing the viscosity is shown to result in a loss of resolution per unit time. Theory demonstrates that in cases where only binding is considered to be present, the maximum resolution increase is found to be at relatively small amounts of additive. In addition, specific regions of the electrophoreogram may demonstrate resolution enhancement at a specific concentration while other regions exhibit a resolution decrease, as compared to no additive present. CE separations of the components in a Triton surfactant, conducted using a polyethylene glycol (PEG) additive, demonstrate that improved resolution for some of the peaks exists at a specific PEG concentration. These results are mostly explained through the reduction in electroosmotic flow velocity that takes place through additive adsorption to the capillary wall, although some binding is present.