New statistical approach to a gas chromatography retention model: Application to dichlorophenol isomers

The retention mechanism of a series of six positional isomers of dichlorophenol in gas chromatography was investigated using pure alpha-, beta-, and gamma-cyclodextrin (CD) stationary phases. For each CD, the values of enthalpy (Delta H) and entropy (Delta S) of transfer of an isomer from the gas to the CD stationary phase were determined. These data demonstrated the leading role of enthalpy according to entropy. Enthalpy-entropy compensation revealed that the retention mechanism was independent of both the chloro group position on the phenol ring and the size and shape of the CD cavity. A model based on the number of CD inclusion complexes with an isomer was also presented. This model explains current experimental data including the reverse elution order observed for certain pairs of isomers.