Cyclodextrin stationary phases: synthesis, characterization and applications in liquid chromatography

Abstract

Since the creation of the first octadecyl bonded phase for High Performance Liquid Chromatography (HPLC), liquid chromatograhers have investigated a wide variety of bonded phases in their ongoing search for increased selectivity. These studies have led to a number of useful phases which appear to separate by different mechanisms such as hydrophobicity, ionic properties, electron density, and so forth. A highly selective chiral bonded phase material for HPLC and Thin Layer Chromatography (TLC), was produced by bonding cyclodextrin to silica gel particles through a spacer "arm" via a reproducible process which yields a stable, non-hydrolytic, non-nitrogen containing bonds. The synthesis, characterization and applications of these new stationary phases are described. The basic property of cyclodextrin molecules, which allows them to effect numerous chemical separations, is their ability to form selective inclusion complexes with a wide variety of organic and inorganic molecules. The formation of these inclusion complexes may occur by a combination of factors, such as hydrophobic effects, dipole-dipole interaction, hydrogen bonding (between the cyclodextrins hydroxyl groups and guest molecules) or the release of high energy water or modifier during complex formation. In general, binding to the cyclodextrin is governed by the molecule's ability to closely fit inside the cavity of the cyclodextrin, although the polarity of the molecule also plays an important role. Besides their ability to separate different kinds of enantiomers by HPLC and TLC, cyclodextrin stationary phases also proved to be the media of choice in separation of dlastereomers, geometrical and structural isomers, and many nonisomeric compounds. The effects of temperature, mobile phase composition, and flow rate upon chromatographic selectivity and resolution are described. The results indicate that cyclodextrin stationary phases are versatile, flexible and effective compared to conventional normal or reversed phase stationary phases.