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).

Courtesy of ASTEC Inc.

Figure 18 The Separation of Warfarin Isomers on a CYCLOBOND Column

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 sizes. Three of the most widely characterized are alpha, beta and gamma cyclodextrins that contain 6, 7 and 8 glucose units respectively. Cyclodextrins are chiral structures and the beta- cyclodextrin has 35 stereogenic centers. CYCLOBOND is a trade name used to describe a series of cyclodextrins chemically bonded to spherical silica gel. The separation of the isomers of Warfarin is shown in figure 18. The column was 25 cm long and 4.6 mm in diameter packed with 5 micron CYCLOBOND 1. The mobile phase was approximately 90%v/v acetonitrile, 10%v/v methanol, 0.2 %v/v glacial acetic acid and 0.2%v/v triethylamine. It is seen that an excellent separation has been achieved with the two isomers completely resolved.