Axial nonuniformities and flow in columns for capillary electrochromatography

Unlike in capillary zone electrophoresis and microscale high-performance liquid chromatography, columns in capillary electrochromatography (CEC) have discontinuities of the electric field strength and the now velocity at the interface of the packed and open segments of the column. The goal of the present work is to offer a framework for measuring and interpreting the relevant electrochromatographic parameters such as the electric field strength, the potential drop, and the now velocity in the packed segment of a CEC column. This would help us gain further insight in the electrochromatographic process and facilitate the design of CEC separation technology and comparison of data from different sources. First, the now of ions that is governed by the conservation of current is analyzed and the potential drop across the packed and open segments of the column calculated. Then, conservation of volumetric now rate is used to calculate the now velocities through the two segments and, further, to estimate the net flow velocity through such a column. To satisfy the mass conservation law, in most cases a "flow-equalizing intersegmental pressure", which is different from the pressures at the two ends of the column, develops at the interface of the packed and the open segments. The intersegmental pressure, induced to equalize the now rates in the two segments, has been shown to have a significant effect on the magnitude as well as the radial distribution of the now velocity in the open segment, where the net now becomes a mixture of electroosmotic and pressure-driven flows.