Practical Considerations

The merit of GC enantiomer separation is the great range of resolvable classes of compounds. With a few exceptions, enantiomer separation by GC is characterized by low separation factors a, and, as a (beneficial) consequence, reduced separation times. The use of highly efficient capillary columns is recommended. Chirasil-Val [IV], Chirasil-Dex [XXIV], and most cyclodextrin derivatives, [XI]-[XXIII], coated onto fused silica capillary columns, are commercially available. Factors such as availability, price, performance and reproducibility should guide the analyst when selecting chiral stationary phases for GC. Immobilized chiral stationary phases, such as Chirasil-Dex [XXIV], have the advantage of solvent compatibility, resistance to temperature shock and longevity. Enantiomer separation on Chirasil-type stationary phases can be performed in the usual temperature range 25-220°C. For special applications it is also possible to use cryogenic temperatures down to — 20°C.

The dimensions of commercial columns are typically 10-25 m x 0.25 mm (i.d.) and the film thickness of the chiral stationary phase is 0.25 |im. Column miniaturization has important merits. Since enan-tiomer separation represents a binary separation system, the whole elution window required for multi-component mixtures need not be exploited unless enantiomers are detected in complex matrices. With shorter columns, the elution temperature can be decreased, so that the chiral separation factor a is increased in the common enthalpy-controlled region of enantioselectivity. The loss of efficiency is compensated by the gain of selectivity leading to comparable resolution factors Rs. The shorter analysis times increase the sharpness of peaks and hence the detectability of the enantiomers. Recommended are 2 m x 0.25 mm (i.d.) columns with a film thickness of 0.25 | m. Further miniaturization via reduction of the internal diameter of the columns to 0.1 and 0.05 mm (i.d.) requires thinner films of the stationary phase in order to keep the phase ratio constant. The reduced amount of stationary phase decreases the sample capacity. The reduced signal-to-noise ratio may become critical in regard to the precision of the enantiomeric excess determination.

Unfortunately, there is no universal CSP available and column selection is a matter of trial and error. Chirasil-Val [IV], Chirasil-Dex [XXIV] and per-methylated ^-cyclodextrin dissolved in polysiloxane [XI] represent the most popular CSPs. On a given enantioselective column, the parameters column length, temperature, film thickness, concentration of CSP, mobile phase velocity and their influences on the resolution factor Rs (which is governed by the retention factor k, separation factor a and efficiency N) have to be carefully balanced. Some variables cannot be freely selected when commercial columns are used.

While Chirasil-Val [IV] and Chirasil-Metal [X] are available in both enantiomeric forms, Chirasil-Dex [XXIV] and other modified cyclodextrins, [XI]-[XXIII], occur only in the d form. The strategy of peak inversion (first kind), employed to elute the minor enantiomer as the first peak, is precluded with carbohydrate-based CSPs.

See also: Il/Chromatography: Gas: Derivatization. Ill/Chiral Separations: Chiral Derivatization; Cyclodextrins and Other Inclusion Complexation Approaches; Ion-Pair Chromatography; Ligand Exchange Chromatography; Liquid Chromatography; Supercritical Fluid Chromatography; Synthetic Multiple Interaction ('Pirkle') Stationary Phase.

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