MULTIPLE ENANTIOSELECTIVE RETENTION MECHANISMS ON DERIVATIZED CYCLODEXTRIN GAS-CHROMATOGRAPHIC CHIRAL STATIONARY PHASES

2,6-Di-O-pentyl-3-O-(trifluoroacetyl) (DP-TFA) derivatized beta- and gamma-cyclodextrins are able to resolve a wide variety of volatile racemic compounds. The enantiomeric recognition mechanism of these phases was investigated. The retention behavior of homologous series showed that lengthening the side alkyl chain increases the retention time but does not affect enantioselectivity. The thermodynamic parameters, free energy, enthalpy, and the difference in free energy, enthalpy, and entropy between enantiomers were evaluated for 24 enantiomeric pairs. From this data, it appears that the compounds can be arranged in two groups. One group has high values for enthalpy, entropy, free energy, and the corresponding difference parameters between enantiomers. The second group has significantly lower values for all parameters. It is shown that compounds belonging to the second group follow an enthalpy-entropy compensation regression (in k' vs DELTA-H) while the group I compounds do not. Also on a given column the mass capacity for group II compounds is significantly higher than that for group I compounds. A small difference was found In the mass transfer behavior of the two groups of compounds. It Is believed that there may be at least two different chiral recognition mechanisms with the derivatized cyclodextrin gas chromatographic stationary phases. It is postulated that one mechanism involves cyclodextrin (CD) inclusion complex formation and the other does not. Currently, more enantiomers seem to resolve through external or multiple association than through an inclusion process. The fact that derivatized CD chiral stationary phases can resolve different enantiomers via different mechanisms (and probably by combination or intermediate mechanisms) provides the practical benefit of increasing the number and types of compounds resolved on these columns.