Stationary Phase Limitation by Chiral Selectivity

The extent to which an enantiomer can interact with the stationary phase depends on how close it can approach the molecules of the stationary phase. 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 sterically 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 been relatively recent. Nevertheless, chiral separations by GC has now been developed into highly practical systems. 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 thermally stable chiral stationary phase produced by Frank, Nicholson and Bayer (8) in 1977 was by the co-polymerization of dimethylsiloxane with (2-carboxypropyl) methoxysilane and L-valine-t-butylamide .

Courtesy of ASTEC Inc.

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

This material was relatively stable up t o 220 o C 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 60 oC using nitrogen as the carrier gas.