An Examination of Factors Influencing the Thermodynamics of Correlation-Gas Chromatography as Applied to Large Molecules and Chiral Separations

The effects of temperature and flow rate on enthalpies of transfer and the resulting vaporization enthalpies measured by correlation-gas chromatography are examined at the mean temperatures of measurement for a series of alkanes. The study is an effort to understand the parameters affecting the magnitude of the enthalpy of transfer of the analyte from the stationary phase of the column to the gas phase. An endothermic enthalpy of solution or association of the analyte with the stationary phase of the column, ∆slnHm(Tm), is observed which appears quite sensitive to temperature. A considerably less endothermic ∆slnHm(Tm) value is observed with (D)and (L)-menthol, and this becomes weakly exothermic with (D)and (L)-limonene on chiral columns containing cyclodextrin and its derivatives. The endothermicity and sensitivity to temperature observed on achiral columns offers an explanation of how it is possible to measure enthalpies of vaporization of large hydrocarbons that exceed the strength of the weakest bond in the molecule.