Factors affecting the retention of quaternary ammonium ions in reversed-phase high-performance liquid chromatography

The influence of solute structure (charge and hydrophobic substitution), organic modifier (type and concentration) and ion-pairing agent on the retention of nine quaternary- and bis-quaternary ammonium ions has been investigated in reversed-phase HPLC on ODS-silica. A functional group approach was taken to elucidate the influences of substitution on the charged nitrogen and the addition of a second positive charge to the solute molecule. These and other factors contributing to solute retention are discussed within the context of solvophobic theory. Hydrophobic effects and the solvation of the charged centre are shown to be the major factors contributing to retention in non-ion pair eluents. In addition, various electrostatic interactions in the mobile and stationary phases may contribute to solute retention in eluents containing an ion-pairing agent. It was found that ternary eluents containing hexane sulphonic acid and two organic modifiers offer certain selectivity advantages when compared with binary eluents.