Capillary electrochromatography packed stationary phases

Capillary Electrochromatography Another approach to separating neutral species is capillary electrochromatography (CEC). In this technique the capillary tubing is packed with 1.5-3-pm silica particles coated with a bonded, nonpolar stationary phase. Neutral species separate based on their ability to partition between the stationary phase and the buffer solution (which, due to electroosmotic flow, is the mobile phase). Separations are similar to the analogous HPLC separation, but without the need for high-pressure pumps, furthermore, efficiency in CEC is better than in HPLC, with shorter analysis times. 

Desiderio, C., Aturki, Z., and Eanali, S., Use of vancomycin silica stationary phase in packed capillary electrochromatography I. Enantiomer separation of basic compounds. Electrophoresis, 22, 535, 2001. 

Capillary electrochromatography (CEC) is a miniaturized separation technique that combines aspects of both interactive chromatography and capillary electrophoresis. In this chapter, the theory of CEC and the factors affecting separation such as the stationary phase and mobile phase parameters have been discussed. The chapter focuses on the types and preparation of columns for CEC and describes the progress made in the development of open-tubular, particle-packed, and monolithic columns. The detection techniques in CEC such as the traditional UV detection and improvements made in coupling with more sensitive detectors such as mass spectrometry are also described. The chapter provides a summary of some applications of CEC in the analysis of pharmaceuticals and biotechnology products. 

Zimina, T. M., Smith, R. M., and Myers, P. (1997). Comparison of ODS-modified silica gels as stationary phases for electrochromatography in packed capillaries. /. Chromatogr. A 758, 191-197. 

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. 

Capillary electrochromatography (CEC) — A special case of capillary liquid chromatography, in which the mobile phase motion is driven by -> electroosmotic volume flow through a capillary, filled, packed, or coated with a stationary phase, (which may be assisted by pressure). The retention time is determined by a combination of -> electrophoretic mobility and chromatographic retention. 

Taylor et al. [39,56] analyzed a mixture of corticosteroids in extracts of equine urine and plasma. Gradient elution was used to facilitate trace enrichment at the head of the column. Huber et al. [36] presented another illustration for the use of capillary electrochromatography with gradient elution. Five steroid hormones were separated by using a capillary column packed with 6-pm Zorbax ODS stationary phase. 

Figure 28 Capillary electrochromatography of four t-RNAs. Capillary column, packed with 2-pm nonporous ODSS stationary phase, 20.5/27 cm x 100 pm i.d. running voltage, 20 kV, electrokinetic injection, 1 kV for 2 s mobile phase in (a), hydroorganic eluent containing 1 5 mM phosphate and composed of 40% (v/v) methanol. Solutes 1, t-RNAGIU 2 t-RNAVal 3, t-RNALys 4, t-RNAphe. (Reprinted from Ref. 119, with permission.)...

Capillary electrochromatography uses electroendosmotic flow (EOF) to perform highly efficient separations in small-diameter fused-silica capillaries, packed with HPLC-type stationary phases. It can be considered as a combination of capillary electrophoresis (CE) and HPLC. The separation of solutes is based on electrophoretic mobility (for charged species) and interaction with the stationary pha.se, allowing the separation of both neutral and charged compounds. 

Another approach is electrochromatography with capillary columns packed with an achiral stationary phase, preferentially a reversed-phase type material. The chiral SO is added to the background electrolyte, and may be adsorbed onto the stationary phase by a secondary equilibration process. Enantioseparations in this additive mode have been reported with cyclodextrin type SOs )504-507) and with a chiral ion-pair agent derived from quinine 1508) as mobile phase additives. 

Capillary electrochromatography (CEC) is a high-efficiency microseparation technique in which mobile-phase transport through a capillary (usually 50- to lOO-pm I.D., packed with stationary-phase particles) is achieved by electroos-motic flow instead of a pressure gradient as in HPLC (342-349). The absence of backpressure in electroosmotic flow allows the use of smaller particles and longer columns than in HPLC. In the reversed-phase mode, CEC has the potential to yield efficiencies 5 to 10 times greater than reversed-phase HPLC. 

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