Gel CEC

Sol-gels can also be used for more than one application at the same time. For instance, it has been shown that a sol-gel column can be produced that enables both preconcentration and separation of amino acids [9]. The column was prepared with a quaternary ammonium functionality, which gives the sol-gel a positive charge. This unique functionality in the sol-gel enabled amino acids to be preconcentrated and separated on the column with high enrichment and efficiency. Furthermore, the use of a positively charged surface for the silica surface enables the electroosmotic flow to be reversed to that which is normally observed in silica capillaries. This enables specialized separations to be performed on the sol-gel CEC column compared to that which would be normally achievable. 

Protein can be added to the solution of precursor in order to add some biocomponent to the CEC column. Chiral compormds, bovine serum albumin (BSA) and ovomucoid (OVM) have been trapped within such sol-gel CEC in order to check their chiral selectivity. These capillary columns showed good enantioselectivity toward selected enantiomers such as tryptophan, benzoin, eperisome or chlorpheniramine (Kato, 2002). 

CE was recently used for anthocyanin analysis because of its excellent resolution. This technique has different modes capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), micellar electrokinetic chromatography (MEKC), capillary electrochromatography (CEC), capillary isoelectric focusing (CIEE), and capillary isotachophoresis (CITP)."° CZE is the most popular method for anthocyanin... 

CEC capillary columns filled with hydrophilic polymer gels mimic those used for capillary gel electrophoresis [91]. Typically, the capillary is filled with an aqueous polymerization mixture that contains monovinyl and divinyl (crosslinking) acrylamide-based monomers as well as a redox free radical initiating system, such as ammonium peroxodisulfate and tetramethylethylenediamine (TEMED). Since initiation of the polymerization process begins immediately upon mixing all of the components at room temperature, the reaction mixture must be used immediately. It should be noted, that these gels are very loose, highly swollen materials that usually contain no more than 5% solid polymer. 

Although Fields already mentioned the possible preparation of monolithic silica-based CEC columns, the lack of experimental data leads to the assumption that this option has not been tested [111]. In fact, it was Tanaka et al. who demonstrated the preparation of monolithic capillary columns using a sol-gel transition within an open capillary tube [99,112]. The trick was in the starting mixture that in addition to tetramethoxysilane and acetic acid also includes poly(ethylene oxide). The gel formed at room temperature was carefully washed with a variety of solvents and heated to 330 °C. The surface was then modified with octadecyl-trichlorosilane or octadecyldimethyl-A N-dimethylaminosilane to attach the hy-... 

While only a few reports concern the in situ preparation of monolithic CEC columns from silica, much more has been done with porous polymer monoliths and a wide variety of approaches differing in both the chemistry of the monomers and the preparation technique is currently available. Obviously, free radical polymerization is easier to handle than the sol-gel transition accompanied by a large decrease in volume. 

Packed capillaries with a larger inner diameter may be useful in preparative separations. These will find an application in proteome research as a part of multidimensional separation systems that will replace 2-D gel electrophoresis. The preparative CEC will require solving of the problems related to heat dissipation since the radial temperature gradient negatively affects the separations, and sample injection. The fabrication of sintered frits in larger bore capillaries is also very difficult. However, in situ polymerized monolithic frits can be fabricated in capillaries of virtually any diameter [190]. 

In principle, many organosilanes can be used as a second precursor to form the organo-silica monolith. Various coprecursors reported for the fabrication of hybrid organo-silica monolithic columns for EC or CEC are listed in Table 8.1. The structure of the organo-silica material is affected by the ratio of the tetralkoxysilane to the alkyltrialkoxysilane in the sol-gel reactions, the type of precursor used, and the processing parameters. This can be seen, for example, in the phase separation behavior of... 

FIGURE 7 CEC-packed capillary with TEOS-sol-gel frits. (Reproduced with permission from reference 27.)... 

Fig. 3. IEC/CEC separation of anions and cations. Column TSK-Gel OA-Pak A (300 X 7.8 mm, 5 jam) eluent 5 mM malic acid-methanol (95 5) flow rate 1.2 ml/min sample volume 25 jal detection conductivity. Peaks (1) sulfate, (2) chloride, (3) nitrate, (4) fluoride, (5) sodium, (6) ammonium, (7) potassium, (8) magnesium, (9) calcium. Reprinted with permission from [19].

The use of antibiotic-based CSPs has been reported in capillary electrochromatography (CEC) for chiral resolution [60]. Teicoplanin CSP covalently bonded to silica gel was used to resolve the enantiomers of tryptophan and dinitrobenzoyl leucine by CEC [61]. Good levels of enantioselectivity were obtained with optimized separations. Vancomycin covalently bonded to silica gel was also evaluated in CEC for the chiral resolution of thalidomide and jS-adrenergic blocking agents under all the three mobile phase modes. The... 

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