Theory and Use of the Pseudophase Model in Gas−Liquid Chromatographic Enantiomeric Separations

Abstract

The theory and use of the "three-phase" model in enantioselective gas-liquid chromatography utilizing a methylated cyclodextrin/polysiloxane stationary phase is presented for the first time. Equations are derived that account for all three partition equilibria in the system, including partitioning between the gas mobile phase and both stationary-phase components and the analyte equilibrium between the polysiloxane and cyclodextrin pseudophase. The separation of the retention contributions from the achiral and chiral parts of the stationary phase can be easily accomplished. Also, it allows the direct examination of the two contributions to enantioselctivity, i.e., that which occurs completely in the liquid stationary phase versus the direct transfer of the chiral analyte in the gas phase to the dissolved chiral selector. Six compounds were studied to verify the model: 1-phenylethanol, alpha-ionone, 3-methyl-1-indanone, o-(chloromethyl)phenyl sulfoxide, o-(bromomethyl)phenyl sulfoxide, and ethyl p-tolylsulfonate. Generally, the cyclodextrin component of the stationary phase contributes to retention more than the bulk liquid polysiloxane. This may be an important requirement for effective GC chiral stationary phases. In addition, the roles of enthalpy and entropy toward enantiorecognition by this stationary phase were examined. While enantiomeric differences in both enthalpy and entropy provide chiral discrimination, the contribution of entropy appears to be more significant in this regard. The three-phase model may be applied to any gas-liquid chromatography stationary phase involving a pseudophase.

Separation of Enantiomers by Gas Chromatography on Chiral Stationary Phases

“…Another thermodynamic treatment of enantioselectivity based on the enthalpy/entropy compensation formalism, introduced by Melander et al for studies of hydrophobic interactions and separation mechanisms in reversed-phase HPLC [119], has also been advanced in cyclodextrin-mediated enantioselective GC [110,120]. A three-phase (pseudo) model for enantioselective GC, comprising a permethylated CD/polysiloxane CSP, was developed by Pino et al [121].…”

GC Separation of Enantiomers of Alkyl Esters of 2‐Bromo Substituted Carboxylic Acids Enantiomers on 6‐TBDMS‐2,3‐di‐alkyl‐ β‐ and γ‐Cyclodextrin Stationary Phases

“…Apart from the nonspecific ones, which generally influence the GC separation (i.e., column characteristics, mobile phase flow rate, and working conditions), the enantiomer separation depends particularly on the nature of the immobilized chiral selector and its immediate environment . The type of CD and its substituents, the degree of substitution and location of the substituents, as well as the concentration of a given CD derivative and the polarity of an achiral solvent, belong to the basic parameters which determine the quality of enantiomeric separations. It is assumed that inclusion into the CD cavity, interactions with inner or outer CD surface, as well as substituents attached to CD derivatives by hydrogen‐bonding, dispersion forces, dipole–dipole interactions, and electrostatic interactions are a prerequisite for a successful chiral recognition process .…”

A practical method for the quantitative assessment of non-enantioselective versus enantioselective interactions encountered in liquid chromatography on brush-type chiral stationary phase

“…If retentions are related to an inert internal standard, e.g., a saturated hydrocarbon, the corresponding relative retentions r render the approach as independent from the dimensions of the columns used. A method similar to the retention increment approach, based on a "pseudo-phase" model, was recently used in enantioselective GC by Armstrong et al [32] taking into account the partition equilibria occurring between the gaseous mobile phase and both components of the CSP, i.e., the achiral solvent and the cyclodextrin-containing SO in the liquid phase, and the equilibria of the racemic SAs between the achiral matrix and the cyclodextrincontaining SO in the liquid phase.…”