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analytes exhibiting strong chiral selectivity. There are specific interactive sites that provide chiral selectivity, but there are many more sites that only contribute to general retention. These other sites can be deactivated by mobile phase additives (e.g. octylamine) which reduces the overall retention and increases the chiral selectivity. The second type consists of relatively small molecular weight chiral substances bonded to silica 9 Pirkle (37). Each bonded group has a limited number of chiral centers available but, due to their small size, there can be a large number of groups bonded to the silica (as opposed to much larger complex chiral moieties). It follows, that a relatively high probability is maintained of the solute interacting with a chiral center. The advantage of the Pirkle chiral phases is that, as the overall interacting molecule is small, the solutes are not strongly retained and thus the chiral selectivity becomes the dominant factor. The third type is based on

The second type of chiral stationary phase consisted of relatively small molecular weight chiral substances bonded to silica and were pioneered by Pirkle (20). Although each bonded group has a limited number of chiral centers available, due to their small size, there are a large number of them on the silica (as opposed to much larger complex chiral moieties), so, a relatively high interaction probability with a chiral center is maintained. The advantage of the Pirkle chiral phase is that the overall interacting molecule is small, and. so, the extra interactive contributions to retention are also small. It follows, that the chiral selectivity becomes the dominant factor controlling retention. The third type are the polymers of cellulose and amylose developed by Okamato (21) The polymers are derivatized to link appropriate interactive groups to the cellulose polymer which is then physically coated onto a silica support. The fourth type is based on the

. If the stationary phase is also chiral in nature, it is likely that one enantiomer in the sample will fit closely to the stationary phase surface whereas the other will be stearically excluded and thus have less stationary phase with which to interact. The first chiral separations in GC were reported by Gil-Av et al. as in 1966 (7), but, surprisingly, the use of GC for the separation of enantiomers has only recently been investigated and developed into a practical system. The use of chiral stationary phases in GC has been dogged by entantiomeric instability arising from the racemization of both the chiral stationary phase and the chiral solutes at elevated temperatures. In addition, at the elevated temperatures necessary to elute the solutes in a reasonable time, the chiral selectivity of the stationary phase can also be impaired

as the carrier gas. The use of LC for chiral separations is easier to carry out and generally more efficient. A number of racemic mixtures can be easily separated using a reverse-phase column and a mobile phase doped with a chiral reagent. In some cases, the reagent is adsorbed strongly on to the stationary phase, under which circumstances, the chiral selectivity resides in the stationary phase. Conversely, if the reagent remains predominantly in the mobile phase, then the chiral selectivity will be in the mobile phase. Camphor sulphonic acid and quinine are examples of mobile phase additives. The most common method used to achieve chiral selectivity is to bond chirally selective compounds to silica in a similar manner to a reverse phase (e.g., example of which is afforded by the cyclodextrins

g-cyclodextrin is distinctly more selective than the b material. It has been employed in the separation of a very wide range of compound classes, and from very small to very large molecules. Yet another stationary phase has been synthesized by substituting the cyclodextrin hydroxyl groups with pure the 'S' hydroxypropyl groups followed by permethylation. As a result, the size selectivity of the material is reduced but more polar (hydrophilic) groups are introduced. The b material has a greater chiral selectivity than the a or g phases. This material provides a good general purpose column. It is clear that there are many possibilities for derivatizing the cyclodextrins to provide unique interactive character; there are a large number commercially available and many more are likely to be synthesized in the future.   It is obvious, that the properties of the different chiral stationary phases available will differ considerably. Moreover, as the nature of many of the synthetic

of interactive possibilities ranging from weak and strong dispersive interactions, to polar interactions that span from induced dipole interaction, through dipole–dipole interaction, to strong hydrogen bonding. In addition, at the right pK, basic and acidic ionic interactions can also be invoked. More importantly, with 32 stereogenic centers the probability of interaction between chiral centers of solute and stationary phase is relatively high. Cyclodextrin The cyclodextrin based chiral stationary phases are some of the more popular materials used for contemporary chiral separations. One of their advantages lies in their use with all types of solvent. They can be used very effectively in the reversed phase mode and, as well as being usable as a normal phase. The cyclodextrins and their derivatives have been widely used for all types of chiral separations and can often be used for preparative separations. Cyclodextrin-based phases are readily available, covalently bonded