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 constan t voltage or current conditions and in a capillary which exhibits a pH -dependent surface charge has been constructed and applied to the desc ription of capillary zone electrophoresis, isotachophoresis, and isoel ectric focusing with electroosmotic zone displacement. Electroosmosis is calculated via use of a normalized wall titration curve (mobility v s 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 roo t of the ionic strength (method based on the Gouy-Chapman theory with the counterion layer thickness being equal to the Debye-Huckel length) and one that assumes the double-layer thickness to be the sum of a co mpact layer of fixed charges and the Debye-Huckel thickness and the ex istence of a wall adsorption equilibrium of the buffer cation other th an the proton (method described by Salomon, K.; et al. J. Chromatogr, 1991, 559, 69), The first approach is Shown to overestimate the magnit ude of electroosmosis, whereas, with the more complex dependence betwe en the electroosmotic mobility and ionic strength, qualitative agreeme nt between experimental and simulation data is obtained, Using one set of electroosmosis input data, the new model is shown to provide detai led insight into the dynamics of electroosmosis in typical discontinuo us buffer systems employed in capillary zone electrophoresis (in which the sample matrix provides the discontinuity), in capillary isotachop horesis, and in capillary isoelectric focusing.