Packed capillaries enantioselective separation

Lammerhofer and Lindner [62] reported on the enantiomer separation of deriva-tized amino acids on an ODS-packed capillary with a chiral quinine-derived selector as buffer additive in two different modes (i) in an electrophoretically dominated mode at high electrolyte concentration and (ii) in an electroosmotically dominated mode at a low electrolyte concentration. Enantiomer separation in the electrophoretically dominated mode (i) leads to high efficieny (about two to three times higher than in LC) but to a moderate enantioselectivity (about the same as in LC). In the electroosmotically dominated mode (ii) a higher enantioselectivity but a lower efficiency (even inferior to LC) occurs. The separations can also been performed in a non-aque-ous buffered mobile phase. Pressurization (8-10 bar) of the flow system on both ends of the separation capillary was applied. 

Column pressure usually has little effect on enantioselectivity in SFC. However, pressure affects the density of the mobile phase and thus retention factor [44]. Therefore, similar to a modifier gradient, pressure or density programming can be used in fast separation of complex samples [106]. Later et al. [51] used density/temperature programming in capillary SFC. Berger and Deye [107] demonstrated that, in packed column SFC, the effect of modifier on retention was more significant than that of pressure. They also showed that the enhanced solvent strength of polar solvent-modified fluid was nof due fo an increase in densify, caused by fhe addition of fhe liquid phase modifier, buf mainly due fo fhe change in composition. 

Enantioselective polyelectrolyte materials suitable for use as capillary tube and chromato-graphic packing material and consisting of polyvinyl pyridinium salts, (I), were prepared by Schlenoff [2] for use in analytical and membrane separations of chiral agents. 

Chiral resolution was also achieved by means of CE chromatographic techniques, with an enantioselective stationary phase, as reported by Li and Lloyd (1993), who used a,-acid glycoprotein as stationary phase packed in fused silica capillaries of 50 mm i.d. These authors reported the optimization (by varying pH, electrolyte, and organic modifier concentration in the mobile phase) of the separation of the enantiomers of hexobarbital, pentobarbital, isofosfamide, cyclophosphamide, diisopyramide, metoprolol, oxprenolol, al-prenolol, and propranolol. 

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