Volker Schurig

Separation of enantiomers by gas chromatography

The separation of enantiomers by gas chromatography is performed on chiral stationary phases (CSPs) via hydrogen bonding, coordination and inclusion. Thus, typical chiral selectors are amino acid derivatives, terpene-derived metal coordination compounds and modified cyclodextrins. In Chirasil-type stationary phases the chiral selector is anchored to a polysiloxane backbone improving gas chromatographic performance. The present review article describes the state-of-the-art, scope and limitations, applications and mechanistic considerations at the advent of the millennium incorporating 16 figures and 168 references.

Dynamic phenomena during enantiomer resolution by complexation gas chromatography : A kinetic study of enantiomerization

Abstract A peak form analysis is presented which describes the elution profiles of interconverting enantiomers during their resolution on a chiral stationary phase in a gas chromatographic column. An equation has been derived which permits for the first time the quantitative assessment of peak distortions arising from enantiomerization occurring in the stationary phase. The simulated peak shapes represent a means of diagnosing configurational inversion during gas chromatographic enantiomer resolution, which is a source of error in the determination of enantiomeric compositions. For the invertomers of 1-chloro-2,2-dimethylaziridine the activation barrier in the presence of the resolving stationary phase nickel(II) bis[3-(trifluoroacetyl)-1R-camphorate] in squalane has been determined to be ΔG≠ = 104.9 ± 0.6 kJ/mol.

Enantiomer Separation by Electrochromatography in Open Tubular Columns Coated with Chirasil-Dex

Abstract The possibility of chiral interactions in a 50 μm i.d. open tubular column was created by coating the inner surface with an immobilizable dimethylpolysilo-xane containing chemically bonded permethylated β- or γ-cyclodextrin (CHIRA-SIL-DEX). This chiral stationary phase was employed for the separation of the enantiomers of NSAIDs and small molecules by the principle of electrochro-matography under various conditions.

Enantiomer separation by gas chromatography on chiral stationary phases

Abstract The analytical separation of enantiomers by capillary gas chromatography on chiral stationary phases (CSPs) is reviewed. Inherent principles and methodological advances are described. Three principal CSPs tailored for hydrogen bonding, coordination and inclusion are currently employed. Cyclodextrin derivatives have proved to be the most versatile and universal CSPs in gas chromatography. Anchoring the CSPs to a polysiloxane backbone leads to Chirasil-type stationary phases with improved temperature stability, efficiency and lifetimes. Immobilization of Chirasil-type stationary phases allows enantiomer separations also in capillary supercritical fluid chromatography and capillary electrochromatography. Thermodynamic parameters of enantioselectivity in gas chromatography, kinetic studies of enantiomerization and methods of enantiomer labelling are discussed.

Theoretical approach to the gas chromatogrphic separation of enantiomers on dissolved cyclodextrin derivatives

Abstract The theoretical concept described previously for enantiomer separation by coplexation gas chromatography was extended to polysiloxane phases containing dissolved cyclodextrin derivatives. A relationship between the chiral separation factor α and the cyclodextrin molality for various racemates. Retention increases R′ were determined as a measure of the enantiomer—cyclodextrin interactions and of the contribution of these interactions to the total retention time. By measuring R′ at different temperatures, Gibbs-Helmholtz parameters ( Δ R,S ΔG, Δ R,S ΔH and Δ R,S ΔS ) of enantiomer discrimination were obtained from the ln ( R′ R /R′ S ) vs. 1/ T plots. Because of the difficulty of finding a truly inert reference standard, these data are affected by a systematic error which restricts the interpretation of the observed enantioselectivities to a qualitative manner.

Chiral separations using gas chromatography

Abstract Chiral separations using gas chromatography (GC) are mainly performed on chiral stationary phases (CSPs) capable of hydrogen bonding, coordination and inclusion, such as amino-acid derivatives, terpene-derived metal-coordination compounds and modified cyclodextrins, respectively. In Chirasil-type stationary phases the chira l selector is anchored to a poly sil oxane backbone, thus improving GC performance. The present review article highlights method development, applications and ancillary techniques.

Enantiomer separation by pressure-supported electrochromatography using capillaries packed with a permethyl-β-cyclodextrin stationary phase

Abstract Efficient enantiomer separation by pressure-assisted, micro-packed capillary electrochromatography (CEC) has been carried out using a permethyl-β-cyclodextrin-modified silica support (PM–β-CD–silica). When comparing this method with micro-packed-high-performance liquid chromatography in the single-column-mode, CEC displays higher column efficiencies (about three times higher theoretical plate numbers at comparable elution times). The pressure support (about 10 bar), applied to avoid bubble formation, has a negligible influence on elution times in CEC. The influence of the type and composition of organic modifiers is described.

Unified enantioselective capillary chromatography on a Chirasil-DEX stationary phase. Advantages of column miniaturization

Immobilized Chirasil-DEX (mono-6-O-octamethylenepermethyl-beta-cyclodextrin chemically linked to dimethylpolysiloxane) can be employed as a versatile chiral stationary phase in chromatography. The chiral polymer has a long lifetime and is configurationally and thermally stable. The concept of unified enantioselective chromatography has been demonstrated for the enantiomer separation of hexobarbital by gas chromatography, supercritical fluid chromatography, liquid chromatography and capillary electrochromatography on a single open-tubular column (1 m x 50 microns I.D.) coated with Chirasil-DEX. The advantages of miniaturization in contemporary chromatographic enantiomer separation are demonstrated. Chirasil-DEX coated on porous silica is also useful for enantiomer separation in high-performance liquid chromatography.

Resolution of Enantiomers and Isotopic Compositions by Selective Complexation Gas Chromatography on Metal Complexes

SummaryThe high selectivity of complexation gas chromatography has been employed for enantiomer resolution and isotope separation. Thus, a chiral olefin, 3-methylcyclopentene, has been resolved analytically on optically active dicarbonyl-rhodium(I)-3-trifluoroacetyl-1R-camphorate in squalane. The deuterated ethylenes C2H4−nDn have been separated on the same rhodium(I)-containing stationary phase. The chiral aliphatic oxiranes epoxypropane andtrans-2,3-epoxybutane have been resolved on optically active nickel(II)-bis-3-trifluoroacetyl-1R-camphorate in squalane.

Recent progress in enantiomer separation by capillary electrochromatography

Enantiomer separation by electrochromatography (CEC) can be performed in three modes: (i) open‐tubular capillary electrochromatography (o‐CEC), in which the chiral selector is physically adsorbed coated, and thermally immobilized or covalently attached to the internal capillary wall; (ii) packed capillary electrochromatography (p‐CEC), in which the capillary is either filled with chiral modified silica particles or with an achiral packing material, and a chiral selector is added to the mobile phase; and (iii) monolithic (rod)‐capillary electrochromatography (rod‐CEC) in which the chiral stationary phase (CSP) consists of a single piece of porous solid. We present an overview on methods and new trends in the field of electrochromatographic enantiomer separation such as CEC with either nonaqueous mobile phases or stationary phases with incorporated permanent charges, or with packing beds consisting of nonporous silica particles or particles with very small internal diameters .