Bidirectional isotachophoresis in open-tubular, untreated fused-silica capillaries

A dynamic computer model for simulation of open-tubular capillary electrophoresis, which includes in situ calculation of electroosmosis along the fused-silica capillary and allows the application of imposed flow, has been applied to the characterization of bidirectional isotachophoretic systems in the presence of a cathodic plug flow. Electroosmosis is calculated based upon the voltage gradient and a wall titration curve (mobility vs. pH, which takes into account the dissociation of surface groups of the inner capillary wall, a wall mobility at full dissociation (high pH) and the non-vanishing surface charge at low pH. For model bidirectional isotachophoretic configurations between pH 4 and 9, simulation data reveal the complete loss of the cationic zones at the cathodic column end and the asymptotic formation of a stationary steady-state anionic zone configuration in which electrophoretic and electroosmotic zone displacements are opposite and of equal magnitude. The position of the stationary boundary between the leading compound and sample is predicted to be dependent on the pH of the system, applied imposed co-flow, and sample composition. Cationic sample trains can readily be detected with a sensor placed towards the cathodic end of the capillary. However, without imposed co-flow, anionic structures are shown to be detectable only at elevated pH values. For detection at 73 or 90% of column length, net cathodic displacement rates per cm column length of about greater than or equal to 10 and greater than or equal to 15 mu m/s, respectively, are required. Electroosmosis-based displacement of these magnitudes are predicted only for pH greater than or equal to 6 and alkaline systems, respectively. For a detector placed at 73% of column length, qualitative agreement between experimental data and simulation results is obtained. Practical aspects of the bidirectional isotachophoretic systems are discussed with data of urinary salicylate and two of its metabolites.