A comparison of supercritical partition and supercritical adsorption chromatographies

Implications of the partial molar volume of solute in the stationary-phase material in both superciritical partition chromatography and supercritical adsorption chromatography are discussed within the framework of a thermodynamic model of solute retention. Using experimental retention data and the thermodynamic model it is illustrated that, in supercritical partititon chromatography, relationships among solubility, sorption and retention are confounded by the effects (pressure dependence) of the partial molar volume of solute in the stationary-phase material and possible swelling of the stationary-phase material by the supercritical mobile-phase fluid. For the first time in the literature, a supercritical sorption isotherm is imbedded in the thermodynamic model of solute retention in order to simulate temperature, pressure and density dependence of solute retention in supercirical adsorption chromatography. Unlike the behavior predicted by the model of solute retention in supercritical partition chromatography, simulation of pressure dependence of solute retention in supercritical adsorption chromatograpy with the aid of supercritical sorption isotherm does not show a retention minimum, and the retention coefficient exhibits an asymptotic decrease to aero with increasing pressure as expected. It is demonstrated that in supercritical adsorption chromatograpy the direct inverse relationship present between the solute's solubility and sorption also exists between solubility (solvent power) and retention but as weighted by the molar volume of the mobile phase.