ABSTRACT
Interactions of protein with the inner wall of fused silica capillaries has been the major obstacle to successful application of capillary electrophoresis to protein separations. At pH values above about 2, the weakly acidic silanol groups on the capillary surface become ionized, and the charge density on the wall increases with pH to a maximum of about 10 at which point the silanol groups are fully dissociated. This characteristic of fused silica has two consequences for protein analysis. First, proteins with basic amino acid residues positioned on the protein surface can participate in electrostatic interactions with ionized silanols. Protein adsorption at the capillary wall can result in band broadening, tailing, and, in the case of strong interaction, reduced detector response or complete absence of peaks. Second, changes in the state of the wall during an analysis or from run to run can alter the magnitude of EOF, resulting in changes in analyte migration times and peak areas. Protein adsorption can alter the zeta potential of the capillary wall, changing EOF and degrading reproducibility. Three strategies have been employed to minimize protein-wall interactions: operation at extremes of pH, use of buffer additives, and use of wall-coated capillaries.
