The role of organic modifiers on polar group selectivity in reversed-phase liquid chromatography

Abstract

The influence of the organic modifiers methanol (MeOH), acetonitrile (AN) and tetrahydrofuran (THF) on polar group selectivity in reversed-phase liquid chromatography has been studied. In order to elucidate polar group effects, we have first explored the influence of the surface coverage of bonded phase on selectivity. Using a series of synthesized n-octy bonded phases, we have been able to observe significant differences in group contribution with bonded phase coverage, the largest differences arising from MeOH H 2O as mobile phase and the least from THF H 2O as mobile phase. The importance of using phases that minimize accessible silanol groups in order to study the influence of the mobile phase has been emphasized. We have selected a high coverage n-octyl phase that is silanized for these studies. In order to examine polar group effects, we have normalized the methylene group increment in the MeOH H 2O, AN H 2O and TH H 2O binary phases. As the hydrophobic selectivity is thus roughly normalized, meaningful relative polar group contributions are observed. Plots of log k′ (THF H 2O) vs. log k′ (MeOH H 2O) reveal particularly striking polar group differences. The practical usefulness of the plots is shown in the peak reversal of solute mixtures with the two mobile phases. Further studies reveal that polar group selectivity can be powerfully controlled using ternary phases of MeOH THF H 2O. Thus, the choice of mobile phase can greatly influence separation in reversed-phase liquid chromatography.