Solvent considerations in the thermodynamic driving forces for reversed-phase liquid chromatography.

摘要

Reversed phase liquid chromatography (RPLC) is one of the most widely used analytical tools, but little is known about the fundamental driving force of retention in an RPLC system. The original intent of this work was to determine the ultimate driving force of retention and fundamental retention mechanism for RPLC. By looking at mobile phase solvation as well as stationary phase interactions with a variety of solutes over the entire mobile phase composition range from 0:100 to 100:0 (v/v) hydro-organic mixtures, one would hope to elucidau the driving force of separation. We have gained knowledge of the fundamental driving force for an RPLC separation, but there is no definitive answer for all solutes, mobile phase conditions nor stationary phase bonding densities.; Much work has been done to correlate methylene selectivity of different solutes. Instead a small hydrophobic model may be a better descriptor of solute partitioning into alkyl chains of the stationary phase. We looked at partitioning of a small nonpolar molecule, methane, and found that there was a change in retention mechanism over the temperature range studied, as evidenced by a change in slope for the van't Hoff plot. Even at 50/50 MeOH/H{dollar}sb2{dollar}O, the hydrophobic effect seems to be dominant for the small hydrophobic molecule. Larger molecules typically studied show linear van't Hoff relationships due to their inability to interact with stationary phase alkyl chains. This chromatographic system for methane was also determined to be entropically driven.; There have been many studies that look at shape selectivity effects on retention with large polycyclic aromatic hydrocarbons, but no studies have looked at the placement of an organic functional group on a benzene ring. These geometrical positional isomers are assumed to have the same oil-water partitioning thermodynamics, but we found that there are differences in thermodynamics of solute transfer, most notably the free energy, based upon where the functional group is placed. All isomer pairs studied showed similar results of an increase in free energy of solute partitioning with an increase in bonding density and the separation was found to he enthalpically driven under all conditions studied.