Gas Chromatography - GC Columns > Chiral Stationary Phases > Page 34

Chiral Stationary Phases

Modern organic chemistry and pharmaceutical research are becoming increasingly interested in methods of asymmetric syntheses. This enthusiasm has been provoked by the differing physiological activity that has been shown to exist between the geometric isomers of pharmaceutically active compounds. A tragic example being the drug Thalidomide, which was made available as a racemic mixture of N-phthalylglutamic acid imide. The important physiological activity resides in the R-(+)-isomer and it was not found, until too late, that the S-enantiomer was probably tetratogenic and caused serious fetal malformations. The separation and identification of isomers can, clearly, be very important and chromatography can be very effective in the resolution of such mixtures. The use of GC for the separation of asymmetric isomers is not as common as LC, but nevertheless there some very effective optically active stationary phases that can be used in GC for the separation of enantiomers.

Some of the more useful GC stationary phases are based on the a- and b-cyclodextrins already described. The a-cyclodextrin structure is depicted in figure 20. The columns are usually 30 or 60 m long 0.25 mm I.D. and have an operating temperature range of 30˚C to 250˚C. Both the a and b forms are commercially available and both have been used very satisfactorily for the separation of the optical isomers of different flavors and fragrances. In order to employ the cyclodextrins as stationary phases for GC the permethylated a- or b-cyclodextrins are often embedded in a siloxane matrix (e.g. 35% phenyl-65% methyl polysiloxane) which is deposited on the walls of fused quartz capillary tubes.