Impact of electroosmosis on isotachophoresis in open-tubular fused-silica capillaries: analysis of the evolution of a stationary steady-state zone structure by computer simulation and experimental validation.

A dynamic computer model for simulation of open-tubular capillary electrophoresis that includes in situ calculation of electroosmosis along the fused-silica capillary column has been applied to the characterization of an anionic isotachophoretic system in presence of a cathodic electroosmotic flow. For each column segment, electroosmosis is calculated with the use of a wall mobility, the voltage gradient and the degree of dissociation of the silanol surface groups of the capillary wall. Then, the bulk capillary flow is taken to be the average of all of the segment flows and considered to represent a plug flow. This simple approach enables the combined simulation of the temporal behavior of an isotachophoretic zone structure in presence of electroosmosis. For a model anionic isotachophoretic configuration at pH 6, simulation data reveal the asymptotic formation of a stationary steady-state zone configuration in which electrophoretic and electroosmotic zone displacements are opposite and of equal magnitude. The position of the stationary boundaries are predicted to be dependent on the selected wall pK and mobility values. For two different instruments, qualitative agreement between experimental data and simulation results obtained with a wall pK between 5 and 6 is demonstrated. However, for the two experimental setups, significant differences in electroosmotic pumping (i.e. wall mobility values) are noted.