Principles and Practice of Chromatography - The Control of Chromatographically Available Stationary Phase (V<sub>s</sub>) > Stationary Phase Limitation by Chiral Selectivity. > Page 39


Courtesy of ASTEC Inc.


Figure 17 The Separation of the Enantiomers of a-Halocarboxylic Acid Esters on a b-Cyclodextrin-Based Stationary Phase

A thermally stable chiral stationary phase was produced by Frank, Nicholson and Bayer (8) in 1977 by the co-polymerization of dimethylsiloxane with (2-carboxypropyl) methoxysilane and L-valine-t-butylamide. This material was relatively stable up to 220oC with little racemization but, was not made commercially available until 1989. Presently, there are a number of effective GC chiral stationary phases available, some of the more effective being based on cyclodextrin,. The separation of the enantiomers of an a-halocarboxylic acid ester on a fused silica open tubular column coated with a b-cyclodextrin product is shown in figure 17. The column was 10 m long and operated at 60oC using nitrogen 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).