Comparison of two methods for the gas chromatographic determination of thermodynamic parameters of enantioselectivity

The thermodynamic parameters responsible for the formation of diastereomeric associates between enantiomers of methyl lactate and two Lipodex E columns [with 5.37% or 10.08% of octakis(3-O-butanoyl-2,6-di-O-pentyl)-gamma- in SE 54] were calculated using two different methods. Method A was based on the direct determination of Delta(Delta G(R,S)(0)) from experimental values of the chiral separation factor a obtained at different temperatures for the separation of the enantiomers of methyl lactate on two Lipodex E columns. Method B relies on the determination of the relative retention of the enantiomers of methyl lactate in respect to an inert reference standard on a reference column containing only the solvent and that of the enantiomers of methyl lactate to the same reference standard on the reactor column containing the cyclodextrin derivative in the solvent S. SE 54 was used as solvent for Lipodex E and n-heptane, n-octane and n-nonane were used as reference standards. Obtained results for Delta(R,S)(Delta G(0)) show that the data calculated by Method A are too small and are dependent on the concentration of the chiral selector in the diluted stationary phase. Since the dependence of In alpha on 1/T for methyl lactate enantiomers on Lipodex E columns was nonlinear, it was not possible to calculate Delta(R,S)(Delta H-0) and Delta(R,S)(Delta S degrees) values by this method. Results obtained by Method B are essentially independent on the choice of reference compound and the concentration of the chiral selector in the mixed stationary phase. (C) 1999 Elsevier Science B.V. All rights reserved.