Thermodynamics of Modifier Effects in Supercritical Fluid Chromatography

Abstract

Thermodynamic properties underlying the shift in solute retention that results from a change in composition of a binary mobile-phase fluid in supercritical fluid chromatography (SFC) are identified and discussed. It is shown that the composition change in the mobile-phase fluid affects the retention properties of the stationary phase even when interfacial adsorption is neglected, and bulk partitioning is supposed to be the only mechanism of solute retention. In the resultant relationship, the effects of modifier (cosolvent) on density are explicitly separated from the effects on intermolecular interactions. The relative contributions of the separate terms to the observed shift in retention are 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-supercritical fluid mixture but that the corrections become less important as the cosolvent effect increases.

Cosolvent Effects of Modified Supercritical Carbon Dioxide on Cross-Linked Poly(dimethylsiloxane)

“…Capacity Factors. Roth 43 gives the relationships that express the effect of cosolvent concentration on the retention of a solute in supercritical fluid chromatography. Roth's work analyzes a two-phase, four-component system.…”

A review of retention mechanism studies for packed column supercritical fluid chromatography

“…Roth et al first described the pressure dependence of the compound retention in a hypothetical, isothermal, isobaric SFC system 135 . Later, the authors proposed another thermodynamic equation to explain the dependence of the retention change and the change in the composition of a binary mobile‐phase 136 . In the study, the retention change was described by the following equation: where k 1 is the retention factor; ∆x 4s is the change in modifier fraction in the stationary phase causing by changes in compositions of the mobile phase fluid; is the infinite‐dilution fugacity coefficients of the compound in the two phases; V s is the geometric volume of the stationary phase, and ζ 4m mixing expansivity.…”

Determination of thermodynamic properties by supercritical fluid chromatography

“…At a constant temperature and pressure, the change in solute retention factor with the mole-fraction of cosolvent in the mobile phase fluid is given by [118] ∂ln k 1 ∂x 4m T,P,n 2s…”

“…(12) is only valid if the principal component of the stationary phase (2) is a high-molar-mass polymer (M 2 → ∞); a more general form of Eq. (12) can be found elsewhere [118]. An experimental application of Eq.…”