General retention parameters of chiral analytes in cyclodextrin gas chromatographic columns

Abstract

Two thermodynamic parameters – entropy (ΔS) and enthalpy (ΔH) – ideally describe the thermodynamics of how the retention of an analyte in a stationary phase depends on the temperature. The paper examines the conversion of an analyte's entropy and enthalpy into chromatographically more meaningful equivalents: its characteristic temperature and thermal constant. Thermodynamic and characteristic parameters of 29 enantiomer pairs of chiral analytes, analysed with four cyclodextrin stationary phases, were measured, tabulated, and investigated. The distribution of all newly-measured characteristic parameters was found to be similar to the known distribution of these parameters for some 12,000 pairs of analytes, analysed with several stationary phases. This similarity suggests that the peak widths of the investigated analytes in temperature-programmed analyses should be generally the same as the peak widths of other similarly retained analytes. It also suggests that the previously-known optimum general heating rate (about 10 ºC/tM, i.e. 10°C per hold-up time) is also the general optimum for temperature-programmed enantioselective GC analyses with cyclodextrins as stationary phases.The optimum general heating rate corresponds to the shortest analysis time for a predetermined peak capacity. It can substantially differ from specific optima corresponding to the best separation of particular peak pairs. Theoretical prediction of these specific optima requires more complex non-ideal thermodynamic models, and more accurate measurement of the parameters involved–these topics that are outside the scope of this report.