Current-monitoring method for measuring the electroosmotic flow rate in capillary zone electrophoresis

Abstract

Capillary zone electrophoresis (CZE) is attracting much attention (1-4) as a new separation technique that can complement high-performance liquid chromatography (HPLC). In CZE a migration channel of capillary dimensions is filled with electrolyte; a sample to be analyzed is injected at one end of the channel; and a high voltage is applied across the channel. When the electrolyte contacts the walls of the capillary, the inner surface of the capillary becomes charged, either through the ionization of surface groups on the capillary walls or through the adsorption of charged species from the electrolyte onto the inner surface. In either case, the electrolyte inside the capillary is no longer electroneutral but acquires a net charge, which may be positive or negative. Under the action of the applied electric field, the electrolyte moves toward one end of the capillary, and this movement is referred to as electroosmotic (electroendosmotic) flow. In addition to the bulk flow of the electrolyte, electrophoresis also takes place; that is, the applied electric field exerts a force on positively charged species to cause them to move to the negatively charged electrode and on negatively charged species to cause them to move to the positively charged electrode. As a result, the components in the injected sample separate into distinct zones, based on their different mobilities. However, in many cases, the rate of electrophoretic flow is typically less than the rate of electroosmotic flow. Consequently, species in the injected sample move in one direction-the direction of the electroosmotic flow-and thus the different species can be detected as each zone passes through some suitable detector located downstream from the capillary inlet. Clearly, a precise characterization of the electroosmotic flow is highly desirable not only for understanding CZE but also for optimizing the operation of CZE in analyzing a given sample. One way to measure the electroosmotic flow rate is to record the elution time of an injected uncharged marker solute, which will be carried through the capillary under the action of only electroosmotic flow (4-6). For this purpose it is necessary that the marker solute be truly neutral, that it have negligible interaction with the capillary walls, and that it be readily detected. Another way is to weigh the mass of electrolyte transferred from the capillary inlet to the capillary outlet over a timed interval (7). For this purpose, losses caused by evaporation must be eliminated and the use of a digital balance appears to be recommended. Both of these procedures have been demonstrated to give reliable measurements of the electroosmotic flow rate, provided that some care is taken. We describe here what we believe might be a simpler procedure for measuring the electroosmotic flow rate. I t is based on recording the time history of the current during CZE operation. Thus the new method requires no special type of injected solute or detector, and it can be used by anyone carrying out CZE separations.