Gas-chromatographic enantioseparation of unfunctionalized chiral hydrocarbons: an overview

Analytical gas-chromatographic enantioseparations of unfunctionalized chiral hydrocarbons (cycloalkanes and alkanes) on modified cyclodextrins with high-resolution capillary columns are reviewed. Due to the lack of functional groups of the hydrocarbons, enantiorecognition in the presence of cyclodextrins is ascribed to weak van der Waals forces. Thermodynamic parameters of enantiorecognition between four chiral alkanes and octakis(6-O-methyl-2,3-di-O-pentyl)-γ-cyclodextrin (Lipodex G) have been determined. The possible role of molecular inclusion is indicated by the complete loss of enantioselectivity when the cyclodextrins are replaced by the corresponding linear dextrins. The substitution pattern and cavity size of the modified cyclodextrins have a pronounced effect on the degree of enantiorecognition observed. On mixed binary cyclodextrin stationary phases, all C7 and C8 chiral alkanes possessing one stereogenic center can be enantioseparated simultaneously. The methodology has been used to determine enantiomeric excesses, ee, of alkanes formed by the enantioselective catalytic hydrogenation of prochiral alkenes. The novel online coupling of enantioselective gas chromatography with proton nuclear magnetic resonance spectroscopy is demonstrated for 2,4-dimethylhexane and Lipodex G.

Expanding the enantioselectivity of the gas-chromatographic chiral stationary phase Chirasil-Val-C11 by doping it with octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-γ-cyclodextrin

Combination of the enantioselective properties of the two versatile gas-chromatographic chiral stationary phases (CSPs) octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-CD (Lipodex E) 1 and L-valine-diamide-based CSP Chirasil-Val-C11 2 has been realized by doping the chiral polymer 2 with the nonpolymeric selector 1. The resulting mixed-mode CSP Chirasil-Val(gamma-Dex) 3 was found to have a greatly improved enantioselectivity toward proline and aspartic acid (as N-trifluoroacetyl ethyl or methyl esters) in comparison to the single-mode CSP 2. The presence of the CD selector in 3 extended the scope of gas-chromatographic enantioseparations achievable on 2 to underivatized alcohols, terpenes, and other chiral compounds that are exclusively enantioseparated on 1.

Extraordinary chiral discrimination in inclusion gas chromatography. thermodynamics of enantioselectivity between a racemic perfluorodiether and a modified gamma-cyclodextrin

The enantiomers of the perfluorodiether "compound B" [2-(fluoromethoxy)-3-methoxy-1,1,1,3,3-pentafluoropropane], a decomposition product of the inhalational anesthetic sevoflurane [2-(fluoromethoxy)-1,1,1,3,3,3-hexafluoropropane], were separated by gas chromatography on octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin (Lipodex E), dissolved in polysiloxane PS 255 (30% w/w), with an unexpectedly high separation factor of alpha = 10.6 at 26 degrees C. Using the concept of the retention increment R', non-enantioselective and enantioselective contributions to retention were separated and thus reliable thermodynamic parameters of enantioselectivity, i.e. - deltaS,R(deltaG) = 5.7 (0.05) kJ/mol at 303 K, - deltaS,R(deltaH) = 20.1 (0.64) kJ/mol, deltaS,R(deltaS) = -47.4 (2.0) J/K mol and T(isoenant) = 424 (30) K or approximately 150 degrees C, were determined by temperature-dependent measurements. The enantiomeric bias represents the largest values ever measured in enantioselective gas chromatography. An equation is presented which allows calculation of the non-enantioselective contributions to retention from measurements at two arbitrary concentrations of Lipodex E in polysiloxane. Surprisingly, the enantioselectivity is greatly reduced when employing the beta-cyclodextrin analogue and breaks down completely with the alpha-cyclodextrin analogue of Lipodex E.

Comprehensive two-dimensional gas chromatography with fast enantioseparation

The development of fast chiral analysis for use in comprehensive two-dimensional gas chromatography in which a short second dimension enantioselective capillary column provides a route to precise measurement of chiral ratios of enantiomers is described. Retention times as short as 8 s are reported for (+/-)-limonene, with adequate enantioseparation maintained (Rs approximately 1.0) on a 1-m cyclodextrin derivative-coated capillary column. Sufficiently fast elution on the second column was achieved by using GC/ MS in which the subambient pressure (vacuum outlet) conditions promote increased diffusion coefficients and higher component volatility; a 4-fold reduction of second-dimension retention time was observed, as compared with ambient pressure outlet conditions. The enantiomeric distribution of several monoterpene compounds in bergamot essential oil is reported as a demonstration of the method. Total analysis time of the target components was approximately 8.5 min.

Gas chromatographic technologies for the analysis of essential oils

Essential oil analysis has basically had one technical goal: to achieve the best possible separation performance by using the most effective, available technology of the day. The result achieved from this may then be used to answer the research or industrial analysis questions which necessitated the analysis. This may be for comparative purposes, where one oil is contrasted with other(s) for quality control or investigation of adulteration, to discover new components, or to characterise the chemical classes of compounds present. Clearly, today the analyst turns to chromatography as the provider of separation and then may supplement that with mass spectrometry to aid identification. The power of GC-MS means that advances in both the separation technique, and improvements in mass spectrometry detection - along with improved data handling tools - will immediately be relevant to the essential oil area. This present review outlines the developmental nature of instrumental approaches to essential oil analysis using gas chromatography. Mass spectrometry will be included to the extent that it represents the hyphenation of choice for most analysts when analysing essential oils. Thus single-column and multi-dimensional analysis will be covered, as will sample handling or introduction techniques prior to the analysis step, where these techniques provide some measure of separation. The recent demonstration of comprehensive gas chromatography will be discussed as the potentially most powerful separation method for essential oils. This brief review is not intended to be a comprehensive dissertation on the field of essential oil analysis since that would require sufficient space to occupy a book in its own right. Rather, it will outline selected considerations and developments, to help explain where new technology has been applied to advantage in this field.