Effects of modifiers in subcritical fluid chromatography on retention with porous graphitic carbon

The effect of different modifiers in subcritical fluid chromatography (SubFC) on interactions between solute and porous graphitic carbon (PGC) and between solute and carbon dioxide-modifier mobile phases was studied by the use of linear solvation energy relationships (LSERs). This study was performed to allow efficient optimization of the composition of the carbon dioxide-modifier mobile phase in regard of the chemical nature of the solutes to be separated. With all modifiers tested (methanol, ethanol, n-propanol, isopropanol, acetonitrile, tetrahydrofuran and hexane), the solute/stationary phase interactions are greater than the solute/mobile phase ones. Dispersion interactions and charge transfer between electron donor solute and electron acceptor PGC mainly explain the retention on this surface, whatever the modifier. These interactions are quite constant over the range of modifier percentage studied (5-40%). For acidic compounds, the retention variation is mainly related to the change in the basic character of mobile and stationary phase due to the variation of modifier percentage. Changes in eluting strength are mostly related to adsorption of mobile phase onto the PGC with methanol and acetonitrile, and to the increase of dispersion interactions between the solute and the mobile phase for other modifiers. Relationships between varied selectivities and solvation parameter values have been studied and are discussed in this paper.