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 to spherical silica gel particles 5 mm in diameter. The cyclodextrins are produced by the partial degradation of starch followed by the enzymatic coupling of the glucose units into crystalline, homogeneous toroidal structures of different molecular size. The molecular structure of a, b, and g cyclodextrins are shown in figure 54. The alpha-, beta- and gamma-cyclodextrins and have been shown to contain 6 (cyclohexamylose), 7 (

presently available for the GC separations. When the a, b, or g cyclodextrins are derivatized, the hydroxyl group on the 2-position reacts first. Nevertheless, the derivative is still size selective and interaction will be determined by the size and neighboring functional groups on the interacting molecule.   To give an example, very small molecules might be separated on the derivatized a-cyclodextrin whereas, in contrast, larger molecules might be better separated on the derivatized g-cyclodextrin. It follows, that the derivatized b-cyclodextrin might be selected for separating the isomers of solute molecules of intermediate size. In contrast, if the 3-position hydroxyl group is derivatized all the cyclodextrins completely loose their size selectivity. Derivatizing the 6-hydroxyl position has little or no effect on chiral selectivity but does appear to enhance the loading capacity of the stationary phase. This position is also used to anchor the cyclodextrins to the surface

of the 2 and 6 positions, the hydroxyl group in the 3 position can then be trifluoroacetylated which produces a very different type of stationary phase. which has a wide field of application. It has been reported that the derivatized 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

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 activity of the isomers

There is considerable evidence supporting the concept of solute inclusion in the cyclodextrin cavity during chromatographic development including a number of NMR studies (23). Cyclodextrins derivatize selectively and the reaction sites are distributed about  a mean. The 2-OH and 6-OH groups are the most reactive while the 3-OH group is the least reactive. Armstrong et al.  (24) obtained a plasma desorption mass spectrum for a mixture of O-(S)-2-hydroxypropyl-derivatized b-cyclodextrin which is shown in figure 40. The number above each peak denotes the number of substituted hydroxy propyl groups per cyclodextrin moiety. It is seen that there is a (more or less) symmetrical distribution of substituents about a mean of 6 hydroxyl groups reacted per cyclodextrin structure. The minimum appears to be about 2 and the maximum  about 12 substituents per moiety. This distribution resulting from substitution reaction, shows that the substituted cyclodextrin phases are

Courtesy of Supelco   Figure 27. A Molecular Model of Cyclodextrin   The thermal stability of the mixed stationary phase can be improved by incorporating a phenylpolysiloxane into the coating material. Phenylpolysiloxane also significantly improves the stability of the coating to oxidation, particularly at elevated temperatures (as cyclodextrin is basically a sugar, is will be very susceptible to oxidation at high temperatures). Some methysiloxane, however, must still be present to render the cyclodextrin soluble in the polymer matrix. Chiral selectivity can be further augmented by bonding other chirally active groups onto the secondary hydroxyl groups of the cyclodextrin (see again figure 27). Unfortunately, much of the chemistry used to derivatize these cyclodextrin compounds is considered proprietary and so synthetic