THEORETICAL AND EXPERIMENTAL STUDIES OF THE EFFECT OF PRESSURE ON SOLUTE RETENTION IN LIQUID-CHROMATOGRAPHY

Pressure is often assumed to have a negligible influence on solute ret ention in liquid chromatography because of the small compressibility o f the mobile and stationary phases. The range of pressures commonly en countered in reversed-phase separations is considerable, however, and may give rise to significant changes in solute capacity factor. In thi s study, the retention of model solutes is measured directly along the chromatographic column as a function of the local pressure. The model solutes, a homologous series of derivatized fatty acids, exhibit a si gnificant increase in capacity factor ranging from +9.3% for n-C-10 to +24.4% for n-C-20 for inlet pressures from 1500 to 5000 psi. These ex perimental results are compared with a thermodynamic model derived fro m regular solution theory. This model suggests that state effects alon e are not sufficient to describe the measured change in solute retenti on and that variations in interaction energy with density must also be considered. By using the simple relationship of van der Waals for the interaction energy (E proportional to 1/V), the change in capacity fa ctor with density is slightly underestimated. However, by using an ext ended relationship that better describes polar fluids (E proportional to 1/V-2), good agreement is observed. Finally, the correlation of exp erimental results with this thermodynamic model reveals that all compo nents in the chromatographic system, including the solute, mobile phas e, and stationary phase, must be considered compressible. The results of this study have clear implications for the determination of fundame ntal physicochemical parameters, as well as for the everyday practice of liquid chromatography.