Principles and Practice of Chromatography - Chromatography Applications > Gas Chromatography Applications > Chiral Separations > Page 79

Chiral Separations

An example of a chromatogram of a test mixture used by Supelco to demonstrate the chromatographic characteristics of their a- DEX column is shown in figure 41. The stationary phase is claimed to have a high shape selectivity for positional isomers (e.g. xylenes, menthols, cresols etc.) and the small internal cavity of the permethylated a-cyclodextrin gives it a rigid character and unique chiral selectivities.

The use of the different solutes illustrates the retentive character of the stationary phase. The relative elution times of the normal paraffins (which are solely retained by dispersive interactions with the stationary phase) indicate a moderate dispersive character, but it is interesting to note that the propane diols are eluted between the C10 and C11 hydrocarbons. This indicates that the C3 chain alcohol is retained by polar forces to an almost equivalent extent as the C11 hydrocarbon retained by dispersion forces only. This implies a fairly strong overall polar character to the stationary phase. It is also seen that the m and p xylenes are well separated showing good spatial selectivity and the separation of diol enantiomers good chiral selectivity.

Courtesy of Supelco

The a-DEX™ column was 30 m long, 0.25 mm I.D., carrying a film of stationary phase 0.25 mm thick. The temperature was 90˚C and the helium flow velocity was 30 cm/s.


Figure 41 Chromatogram of a Test Mixture for a Permethylated a-Cyclodextrin Stationary Phase

As the two enantiomers are strongly polar it would also appear that the greater retention of the (–)-1,2-propane diol was due to polar interactions between the OH groups with the neighboring polar moieties on the a-cyclodextrin.