INFLUENCE OF TEMPERATURE ON THE MECHANISM BY WHICH COMPOUNDS ARE RETAINED IN GAS-LIQUID-CHROMATOGRAPHY

The influence of temperature on the retention mechanism and solvation interactions of 46 varied solutes in 10 representative stationary phases of different polarity within the temperature range of 60 to 140 degrees C is discussed. Gas-liquid partition is shown to the dominant retention mechanism for most solutes with inrterfacial adsorption of increasing importance at low phase loadings, low temperatures and for solutes of different polarity to that of the stationary phase. Guidelines are presented for predicting those conditions for which interfacial adsorption is likely to be a significant retention mechanism. A cavity model is used to characterize the solvation process in terms of the free energy contributions to solvation from the cavity-dispersion interactions and the sum of the remaining polar interactions. As a function of temperature it is shown that the contribution from polar interactions are only weakly temperature dependent over the temperature range studied while the cavity-dispersion interactions term shows a much more significant variation becoming less favorable for solute transfer at higher temperatures. In all cases, the contribution of the cavity-dispersion interactions term is favorable for solute transfer from the gas phase to the liquid phase. Principal component analysis is used to identify the factors contributing to the solvation process and their individual temperature dependence. In the case of the cavity-dispersion interactions term one factor accounts for more than 99.7% of the total variance. Three factors are identified as contributing to the polar interactions term. The first principle component accounts for more than 95% of the total variance at all temperatures and by correlation with other independent scales of dipolarity/polarizability is identified as representing the contribution from orientation and induction interactions. The two remaining principal components are shown to represent hydrogen-bond formation and charge-transfer complexation involving systems with pi-electrons. The temperature dependence of the principal components provides insights into the general role of polar intermolecular interactions on the solvation process and their temperature variation.