Cyclodextrins in HPLC Applications

Native cyclodextrin HPLC columns were deliberately designed to be used in the reserved-phase mode of operation, in order to take full advantage of the host-guest complexation capabilities of the molecule.

In a more recent and somewhat different experimental approach, the inclusion properties are suppressed by using a non-hydrogen bonding, polar organic solvent (e.g., acetonitrile) as the main component of the mobile phase. Acetonitrile has the tendency to occupy the cavity and seems to enhance hydrogen bonding between the hydroxyl groups on the cyc-lodextrin and hydrogen bonding groups on the chiral analyte.

In this so-called polar organic mode of operation, the addition of small amounts of glacial acetic acid and triethylamine is used as a tool to enhance enan-tioselectivity. On the other hand, the addition of a hydrogen bonding solvent such as methanol allows reduced retention of strongly retained molecules. This technique produces some unusual enantioselectivities that certainly enhance the usefulness of native cyclo-dextrin phases.

Furthermore, a variety of cylcodextrin derivatives has been immobilized on a chromatographic support, which can be used under normal as well as under reversed-phase conditions. The most popular commercially available derivatized cyclodextrin chiral selectors are listed in Table 1.

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