Chiral selectivity can be further improved by bonding other chirally active groups onto the secondary hydroxyl groups of the cyclodextrin (see again figure 45). as might be expected, much of the chemistry used to derivatize these cyclodextrin compounds is proprietary and so synthetic details are not readily available. Unfortunately, some of the derivatized cyclodextrins are vulnerable to hydrolysis when in contact with water vapor. Consequently, it is imperative that all carrier gases are completely dry and that careful steps are taken to ensure that any sample that is placed on the column is also completely dry.

A number of derivatized cyclodextrin stationary phases have been described in the literature. In one product, the positions 2 and 6 are alkylated (pentylated) which results in the generation of a number of very dispersive (hydrophobic) centers that can interact strongly with any alkyl chains contained by the solutes. After the 2 and 6 positions have been pentylated, the 3 position hydroxyl group can be trifluoroacetylated producing another type of stationary phase with greater polar interactivity. These cyclodextrin based stationary phases have a very wide field of application. In practice it would appear that the derivatized g cyclodextrins are more broadly selective than the b material. These materials have been successfully used for the separation of a very wide range of different compound classes.

Yet another type of stationary phase involves the substitution of the cyclodextrin hydroxyl groups with pure 'S' hydroxypropyl groups that is then followed by permethylation. This procedure reduces the size selectivity of the phase but, at the same time, introduces more polar (hydrophilic) interactive groups. The beta material has been shown to be more selective than the alpha or gamma phases. This stationary phase provides a good general purpose column and from the structure, it is obvious that there are many possible derivatizing procedures for the cyclodextrins. Many products that provide unique interactive character are now commercially available and many more are likely to be synthesized in the future. Other cyclodextrin stationary phases will be considered when the applications of chiral GC are discussed.