The Place of Capillary Electrochromatography Among Separation Techniques—A Review

Replacing pressure-driven flow with electroosmotic flow in chromatography with packed capillary columns results in a new and powerful analytical technique, combining the advantages of high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE): capillary electrochromatography (CEC). In essence, it involves packing CE capillaries with HPLC stationary phases and applying a voltage across the packed capillary, which generates an electroosmotic flow (EOF) that transports solutes along the capillary toward the detector; on the way, both differential partitioning and electrophoretic migration of the solutes occur, resulting in their separation. This dual separation mechanism can afford unique selectivities. Also, the plug-flow profile of EOF reduces flow-related band broadening, so that separation efficiencies of several hundred thousand plates per meter are often obtained (i.e., one or two orders of magnitude greater than those of present conventional chromatographic systems). There is no back pressure when EOF occurs, so small particle sizes (1–3 μm) and/or long columns can be used. Generally, carrier electrolytes containing high levels (40–80%) of organic solvents such as methanol or acetonitrile are employed, making it useful for water-insoluble compounds, which can be difficult to analyse by CE. CEC can be faster than HPLC, and gradient-elution CEC is being developed as an alternative to gradient-elution liquid chromatography of polymers.