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

. 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

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

, but nevertheless there are a number of very effective optically active stationary phases that can be used in GC for the separation of volatile enantiomers. The first effective GC chiral stationary phase adopted derivatized amino acids to provide chiral selectivity (18) in 1966. These types of stationary phase had very limited temperature stability and the optimum temperature for separation can often be greater than that at which the stationary phase was stable. The first relatively stable chiral stationary phase was introduced by Bayer (19) who combined the derivatized optically active component in a polysiloxane gum

Chiral Polysiloxane Stationary Phases Bayer and his coworkers (19), synthesized a stationary phase that consisted of a chiral agent attached by an amide linkage to a carboxyl group of a polymer matrix of dimethylsiloxane or (2-carboxypropyl)-methylsiloxane. This combined the chiral selectivity of L-valine-t-butylamide with the high thermal stability and low volatility of the polysiloxanes. This stationary phase could be used over the temperature range of 30˚C to 230˚C. and would

sensitivity, that can be achieved when the capillary column is used in conjunction with a high sensitivity detector, can be a considerable advantage. The high speed capability of the capillary columns also makes them particularly useful in process control and in intermittent rapid monitoring, for example, the analysis of a patients breath under anesthesia. The following examples are taken from specific applications that illustrate some of the unique advantages of the capillary column.   Chiral Separations with Cyclodextrin Stationary Phases   Until relatively recently, interest in chiral chemistry had been largely academic and occupied a relatively minor position in the analytical chemistry syllabuses of most universities. However, in the early 1980s, the commercial interest in chiral substances suddenly increased, particularly in chiral drugs, and this interest proliferated very rapidly. This new enthusiasm was fostered by the recognition that the respective physiological