Separation principle, capillary conditions

This overview concerns the new chromatographic method - capillary electrochromatography (CEC) - that is recently receiving remarkable attention. The principles of this method based on a combination of electroosmotic flow and analyte-stationary phase interactions, CEC instrumentation, capillary column technology, separation conditions, and examples of a variety of applications are discussed in detail. 

The first four terms on the right-hand side of Eq. (4.15) represent effects inherent in the principle of the method and cannot be suppressed to zero however, their influence on the separation efficiency can be controlled by instrument design and selection of appropriate working conditions. Interactions between the analyte and capillary wall is a concern primarily for the analysis of biomolecules, and they may be reduced or eliminated by coating the capillary wall. 

Different separation mechanisms, which determine selectivity, can be exploited in hpce by appropriate choice of operating conditions. There are four principle modes of operation (table 4.22) and it should be noted that in only one, micellar electrokinetic capillary chromatography (mecc), is it possible to separate neutral species from one another. 

With CZE, the normal polarity is considered to be [inlet—(+), detector—(—) outlet] as shown in Figures 1.4 and 1.5. As electrophoresis ensues, the analytes separate according to their individual electrophoretic mobilities and pass the detector as analyte zones (hence, the term capillary zone electrophoresis or CZE). The fact that, under appropriate conditions, all species (net positive, net negative, or neutral) pass the detector indicates that a force other than electrophoretic mobility is involved. If the applied field were the only force acting on the ions, net positively charged (cationic) substances would pass the detector while neutral components would remain static (i.e., at the inlet) and anionic components would be driven away from the detector. It is clear that, if this were the case, CE would be of limited use. Fortuitously, there is another force, electroosmotic flow (EOF), driving the movement of all components in the capillary towards the detector when under an applied field (and a normal polarity). EOF plays a principle role in many of the modes of CE and most certainly in CZE. This is discussed briefly in the next section. 

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