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

The thermodynamic parameters responsible for the formation of diastereomeri c associates between enantiomers of methyl lactate and two Lipodex E column s [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 t he direct determination of Delta(Delta G(R,S)(0)) from experimental values of the chiral separation factor a obtained at different temperatures for th e separation of the enantiomers of methyl lactate on two Lipodex E columns. Method B relies on the determination of the relative retention of the enan tiomers of methyl lactate in respect to an inert reference standard on a re ference column containing only the solvent and that of the enantiomers of m ethyl lactate to the same reference standard on the reactor column containi ng 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 s tandards. Obtained results for Delta(R,S)(Delta G(0)) show that the data ca lculated by Method A are too small and are dependent on the concentration o f 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 Delt a(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 co ncentration of the chiral selector in the mixed stationary phase. (C) 1999 Elsevier Science B.V. All rights reserved.