Stationary Phases for the GC Separation of Chiral Substances

The vast majority of chirally selective stationary phases employed in GC are based on a, b or g cyclodextrin. The underivatized cyclodextrin is now rarely used and a range of derivatives has been introduced that are designed to enhance the separation of enantiomers of different types and classes of compounds.

The more crystalline derivatized cyclodextrins can be physically incorporated into appropriate polysiloxanes that are then coated on the column walls or onto an inert support. In certain stationary phases the polysiloxane contains a proportion of phenyl groups that renders the mix more thermally stable and, thus, can be used at higher temperatures. The level of the cyclodextrin component in the stationary phase varies between about 5% to 20% depending to a large degree on the solubility of the cyclodextrin-based material in the particular polysiloxane.

Figure 53 The Cyclodextrin Molecule.

There are two parts of the cyclodextrin molecule that can be derivatized but only one part significantly affects its chiral selectivity. A cyclodextrin molecule is shown in figure 53 and it is seen that there are three positions that can be derivatized, positions 2, 3 and 6.

The OH group on the 6 position is usually alkylated and as the group is free to rotate, it blocks the base aperture to the cone and, thus, the 6 position offers no chiral selectivity. In addition the alkylation of the 6 position increases the solubility of the cyclodextrin in the polysiloxane matrix. Groups on the 2 and 3 positions however can strongly affect selectivity. The enantiomer that fits closest to the chiral centers will have much greater interaction with groups substituted on positions 2 and/or 3 and, thus, by choosing appropriate groups, specific chiral selectivity can be introduced for particular solute types. Groups with interactive character ranging from strongly polar (hydrophilic) to highly dispersive (hydrophobic), together with all the subtle combinations between, can be inserted offering an almost limitless range of unique selectivities.