Electroosmotic flow modulation and capillary surface modification

Capillary electrophoresis (CE) technique has recently gained large interest in several fields of separation science. Electrophoretic separations are based on the differential migration between analytes due to their own mobility under an applied electric field. The movement of the entire running electrolyte from the anodic to the cathodic end of the capillary is due to the presence of the electroosmotic flow (EOF) generated by the presence of negatively charged silanol groups on the internal surface of the capillary. In some cases it is convenient to modulate the EOF in order to control the analyte migration as well as to minimize its interaction with the capillary wall. The mechanism involved in modulation or even reversal of the direction of the EOF in fused-silica capillaries is a specific adsorption of counter ions at the interfacial region between the capillary wall and the electrolyte. There are several ways to modulate and control EOF direction and strength: pH modulation, use of buffers with high ionic strength, and static or dynamic capillary coating. This chapter describes the theory and fundamentals on whichof EOF is based, and the several approaches recently employed for EOF its modulation in CE in the late years.