Effect of temperature on retention of enantiomers of beta-methyl amino acids on a teicoplanin chiral stationary phase

The isocratic retention of enantiomers of beta-methyl amino acids (beta-met hyltyrosine, beta-methylphenylalanine, beta-methyltryptophan and beta-methy l-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) was studied on a teicop lanin-containing chiral stationary phase at different temperatures and at d ifferent mobile phase compositions, using the reversed-phase mode. With var iation of both mobile phase composition and temperature, almost baseline se parations could be achieved for all four enantiomers of sterically hindered amino acids. The retention factors and selectivity factors for the enantio mers of all investigated compounds decreased with increasing temperature. T he natural logarithms of the retention factors (In k) of the investigated c ompounds depended linearly on the inverse of temperature (1/T). van 't Hoff plots afforded thermodynamic parameters, such as the apparent change in en thalpy (Delta H degrees), the apparent change in entropy (Delta S degrees) and the apparent change in Gibbs free energy (Delta G degrees) for the tran sfer of analyte from the mobile to the stationary phase. The thermodynamic constants (Delta H degrees, Delta S degrees and Delta G degrees) were calcu lated in order to promote an understanding of the thermodynamic driving for ces for retention in this chromatographic system. (C) 1998 Elsevier Science B.V. All rights reserved.