Capillary electrochromatography (CEC) is a quickly developing high-performance analytical separation method based on a combination of principles of liquid chromatography (LC) and capillary electrophoresis (CE). In CEC, the driving force dragging the mobile phase through the separation column is electroosmotic flow (EOF), which is induced by applied electric field. The method enables the separation of neutral analytes with selectivity typical of LC, whereas the charged analytes' migration velocity, high separation efficiency, and simplicity of instrumentation correspond to CE. Important progress was made in the column technology, allowing a burst of applications, particularly in pharmaceutical chemistry and biochemistry. In spite all of this, CEC at the present time is not considered a mature technique. Intensive development and associated lack of specialized instrumentation characterize the current situation. Nevertheless, it is obvious that CEC is a promising analytical tool in many important application areas.
part of the electric double layer and its magnitude, measured as linear velocity ueo by a neutral marker, are given by the Helmholtz-Smoluchowski equation:
ueo = (fioeCE)/z where eo is the permittivity of vacuum, e is the dielectric constant of the mobile phase, z is zeta potential, E is electric field strength, and z is viscosity. In comparison with the pressure-driven parabolic flow in LC, the EOF is less dependent on the pore diameter and its flow profile is flatter, which both lead to a decrease of peak broadening and to a higher separation efficiency of CEC. Because the whole column operates as an electroosmotic pump, smaller sorbent particles and longer columns could be used in comparison with LC, which also facilitates a higher efficiency. The direction of EOF is from anode to cathode in the negatively charged silica-based packing. EOF of the opposite direction is also possible in packings with a positively charged surface (e.g., in polymeric monoliths containing quaternary ammonium groups).
The roots of CEC can be traced back to several decades (e.g., Ref. ), but only the experiment of Jorgenson and Lukacs demonstrated the real separation power of the new emerging analytical method. After that, Knox and Grant[3,4] used very small silica particles to gain extremely high separation efficiencies, and a steep growth of interest documented by an exponential increase in the number of articles began, giving a cumulative number of some 1300, to date. All this information is considered in several discussed general reviews[1,5-14] and monographs.[15-17]
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