Early History

Initially, development of suitable methods for chiral separations by GC was seriously impeded by the small separation ratios that were obtained when using the standard GC stationary phases. Considerable effort was made in the late 1950s to develop a method for the separation of the amino acid optical isomers but, unfortunately, they were futile. There were some successes reported in the literature (18,19) but, these separations could not be reproduced (20,21).

The first successful separation of the amino acid enantiomers by a GC separation procedure which was found to be reproducible was reported by Gil-Av et al.(22), in 1966. Gil-Av et al. was quick to recognize, that the almost inevitably small separation ratios that occurred between the optical isomers, could only be exploited using the high resolving power obtainable from the open tubular column. It also became apparent that if the separation was to be achieved in a reasonable time then relatively high operating temperatures would also be necessary.

Gil-Av et al. considered that if a chiral form of stationary phase was used, then the two antipodes of a mixture would form diastero isomeric interaction with somewhat different intermolecular distances between the antipode molecules and those of the stationary phase, Hence the polar, dispersive and/or steric interactions, between substituents on the asymmetric centers of the solute and stationary phase molecules respectively, would differ. Such differences would modify the standard energy of distribution and, thus, the magnitude of the distribution coefficients of the individual enantiomers.

Gil-Av et al. used a N-TFA-D-isoleucine lauryl ester and N-TFA-L-isoleucine lauryl ester mixture as the stationary phase which were coated on the walls of a capillary column 100 m long, 250 m I.D. and was shown to have an efficiency of about 98,000 theoretical plates. The samples of the derivatized amino acids were injected with a split ratio of 1 100. The separation was carried out isothermally at 90 C and the analysis time was just over 4 hours. The results obtained for the separation of some derivatized amino acids on both the (L) and (D) column are shown in figure 38. The chromatograms are shown broken at specific intervals in order to contract the chromatogram and provide for clear presentation. It is seen that on the (L) phase the separation appears to be slightly better than that on the (D) phase. In addition, the (L) isomers are eluted first on the (D) phase and the (D) isomers first on the (L) phase. Thus, in this particular system, a given enantiomer would appear to interact more closely with its opposite enantiomer in the stationary phase and, consequently, be more retained.