Modeling of the impact of ionic strength on the electroosmotic flow in capillary electrophoresis with uniform and discontinuous buffer systems.

A new dynamic computer model permitting the combined simulation of the temporal behavior of electroosmosis and electrophoresis under constant voltage or current conditions and in a capillary which exhibits a pH-dependent surface charge has been constructed and applied to the description of capillary zone electrophoresis, isotachophoresis, and isoelectric focusing with electroosmotic zone displacement. Electroosmosis is calculated via use of a normalized wall titration curve (mobility vs pH). Two approaches employed for normalization of the experimentally determined wall titration data are discussed, one that considers the electroosmotic mobility to be inversely proportional to the square root of the ionic strength (method based on the Gouy-Chapman theory with the counterion layer thickness being equal to the Debye-Hückel length) and one that assumes the double-layer thickness to be the sum of a compact layer of fixed charges and the Debye-Hückel thickness and the existence of a wall adsorption equilibrium of the buffer cation other than the proton (method described by Salomon, K.; et al. J. Chromatogr. 1991, 559, 69). The first approach is shown to overestimate the magnitude of electroosmosis, whereas, with the more complex dependence between the electroosmotic mobility and ionic strength, qualitative agreement between experimental and simulation data is obtained. Using one set of electroosmosis input data, the new model is shown to provide detailed insight into the dynamics of electroosmosis in typical discontinuous buffer systems employed in capillary zone electrophoresis (in which the sample matrix provides the discontinuity), in capillary isotachophoresis, and in capillary isoelectric focusing.