MODELING FLOW PROFILES AND DISPERSION IN CAPILLARY ELECTROPHORESIS WITH NONUNIFORM ZETA-POTENTIAL

We have reported earlier that in capillary electrophoresis experiments at low pH values, broadening of the sample peak can be unusually high-higher than can be accounted for by diffusion alone. In this paper, theoretical now profiles inside the capillary have been examined as contributing to band broadening and raising the plate height. The theory is based on the expectation that the zeta potential changes along the length of the capillary, z, due to electrical fields that exist across the wall of the capillary. This paper discusses the particular case in which the radial voltage across the capillary wall is constrained to be a linear function of z. This case applies to the configuration in which the capillary is covered by a conductive shield. The configuration is of special interest because it can allow control of electroosmotic now under specific conditions. Given that the radial voltage is a linear function of z, it is reasoned that zeta(z) can also be assumed to be linear. The theory is developed for the resulting velocity profile as a function of z, and the effect on dispersion and plate height is presented. The theory includes addition of pressure and the corresponding influence on plate height.