Capillary Electrochromatography CEC

There is much current interest in electrochromatogra-phy, which is a hybrid technique between HPLC and CE, operating with packed capillaries. It is based on partitioning the analytes between the stationary phase (in packed capillaries) and the mobile phase (electrolyte) under CE conditions with a high applied voltage. The procedure benefits from the large range of HPLC stationary phases and some specific phases which are beginning to be developed for CEC. It links the high efficiency of CE with the range of separations possible with reversed-phase HPLC. For transport of the analytes in CEC, EOF is still present, but now the effective EOF is mainly generated from the packed bed of stationary phase rather than at the walls of the capillary. This results from the electrical double layer

Figure 6 Schematic of the chiral analyte 5-alkyl-5-phenyl hydantoin distributed between the cyclodextrin, the aqueous phase and the micelle. SDS, Sodium dodecyl sulfate.

on the surface of the particles in contact with the mobile phase and produces a ( potential. The potential of CEC was first demonstrated in 1987, showing the presence of the partitioning effects and, for charged analytes, the continued effect of electromig-ration. In terms of the separation conditions in CEC and HPLC, the stationary and mobile phases are very similar, but the separation efficiency can be much improved. Many applications of electrochromatogra-phy are currently being studied and the range of capillary packing materials continues to expand.

Chiral stationary phases are being examined by a number of researchers. In one of the first reports of chiral separations by CEC 5 |im AGP, immobilized on to silica, was packed into a 50 |im internal diameter capillary. Cationic and neutral enantiomers were resolved by this method, which included benzoin and the drugs cyclophosphamide and hexobar-bital. This early work did not illustrate large improvements in peak efficiency over other procedures as the capillary packing methods required improvement, but it did illustrate the potential.

CDs have also been used as immobilized silica-based materials and the neutral CD hydroxypropyl-^-CD silica was used in resolution of the basic drug mianserin. In method design with the CDs, the choice of the background electrolyte has played an important part and pH, organic modifier and differences in field strength have all been considered. The direction of flow past the detector was shown to be dependent on the background electrolyte used. For ^-cyclodex-trin and a phosphate buffer the direction of flow is towards the cathode, but with triethylammonium acetate the flow is reversed. This can be beneficial in controlling the order of retention in CEC. The pH change in the electroosmotic mobility can be used to obtain a suitable overall retention time. Organic modifiers such as methanol and acetonitrile in the electrolyte have an effect on both the retention and enantioselectivity. The results from studies with CD-bonded silica phases indicate that a range of parameters and additives can affect the retention and enantioresolution in CEC, as they do with chiral stationary phases in HPLC.

Other stationary phases which have been shown to give enantioseparations are the cellulose and amylose derivatives which have been coated on to silica. These stationary phases are known as Chiralcel and Chiral-pak phases (Daicel, Japan), respectively. These materials have been successful in HPLC for enantioselectiv-ity with a wide range of chiral compounds and have recently been utilized as packing materials for CEC (Figure 7). Generally in CEC good resolution has been achieved for a number of chiral compounds resolved by HPLC, with improvements in separation efficiencies in many cases.

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