THERMODYNAMICS OF MODIFIER EFFECTS IN SUPERCRITICAL-FLUID CHROMATOGRAPHY

Thermodynamic properties underlying the shift in solute retention that results from a change in composition of a binary mobile-phase fluid i n supercritical fluid chromatography (SFC) are identified and discusse d. It is shown that the composition change in the mobile-phase fluid a ffects the retention properties of the stationary phase even when inte rfacial adsorption is neglected, and bulk partitioning is supposed to be the only mechanism of solute retention. In the resultant relationsh ip, the effects of modifier (cosolvent) on density are explicitly sepa rated from the effects on intermolecular interactions. The relative co ntributions of the separate terms to the observed shift in retention a re partly illustrated with the literature experimental data available at the moment. It appears that, in general, several corrections should be applied when using SFC to probe cosolvent effect in a solute-super critical fluid mixture but that the corrections become less important as the cosolvent effect increases.