Capillary electrochromatography separation buffer selection

The first and most often encountered separation mechanism in CE is based on mobility differences of the analytes in an electric field these differences are dependent on the size and charge-to-mass ratio of the analyte ion. Analyte ions are separated into distinct zones when the mobility of one analyte differs sufficiently from the mobility of the next. This mechanism is exemplified by capillary zone electrophoresis (CZE) which is the simplest CE mode. A number of other recognized CE modes are variations of CZE. These are micellar electrokinetic capillary chromatography (MECC), capillary gel electrophoresis (CGE), capillary electrochromatography (CEC), and chiral CE. In MECC the separation is similar to CZE, but an additional mechanism is in effect that is based on differences in the partition coefficients of the solutes between the buffer and micelles present in the buffer. In CGE the additional mechanism is based on solute size, as the capillary is filled with a gel or a polymer network that inhibits the passage of larger molecules. In chiral CE the additional separation mechanism is based on chiral selectivity. Finally, in CEC the capillary is packed with a stationary phase that can retain solutes on basis of the same distribution equilibria found in chromatography. 

Fig. 3.4. Effect of mobile phase selectivity on the CEC separation of barbiturates (1-6). Electrochromatography was performed at 15°C with an applied voltage of 30 kV on a 25 cm, 100 pm i.d., 3 pm Hypersil Phenyl packed capillary. Sample concentration was 170 pg ml"1 of each component with a 15 kV/5s injection. Detection was at 210 nm. a) ACN-50 mM phosphate buffer, pH 4.5-water (4 2 4 v/v/v), b) MeOH-50 mM phosphate buffer, pH 4.5-water (5 2 3 v/v/v). From Euerby et al [26], Journal of Microcolumn Separations, 1999. Reproduced with permission of John Wiley Sons, Inc.

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