Chiral Separation by Capillary Electrochromatography Using Cyclodextrin Phases

Abstract

Cyclodextrin and its derivatives are well known chiral selectors in gas chromatography (GC), high-performance liquid chromatography (HPLC), capillary electrophoresis (CE), and also in capillary electrochromatography (CEC), a hybrid method of CE and liquid chromatography (LC). The separation of enantiomers by CEC on cyclodextrin stationary phases (1–4) can be achieved by three different methods: (i) open tubular capillary electrochromatography (o-CEC); (ii) packed capillary electrochromatography (p-CEC); and (iii) rod-CEC. In o-CEC, the internal capillary wall is coated with the chiral stationary phase (see Fig. 1), e.g., with Chirasil-Dex (see Fig. 2), which is a permethyl-β-cyclodextrin covalently linked via a spacer to dimethylpolysiloxane (4–6). For p-CEC, using capillaries filled with typical chiral HPLC packing materials, two different techniques for fixing cyclodextrins or its derivatives to the silica are described: (i) the cyclodextrin or its derivatives are covalently bound via a spacer to the silica particles (4,7) (e.g., Chira-Dex-silica [see Fig. 2]); or (ii) Chirasil-Dex is immobilized on the silica material (4,8). The packing bed is held in position by frits, and detection occurs in the empty part of the capillary (see Fig. 1). In rod-CEC, columns consisting of a porous solid prepared by an in situ sol-gel-process (9) or by sintering of silica (10) were used. For enantiomer separation, the resulting monolith is derivatized with Chirasil-Dex (9,10) (see Fig. 1).