Quantitative Confocal Raman Microscopy Study of Ion-Interaction Retention at Reversed-Phase Chromatographic Interfaces

Ion-interaction chromatography utilizes the addition of amphiphilic (surfactant) molecules to the mobile-phase solution in order to bring about the retention and separation of hydrophilic ionic analytes on reversed-phase liquid chromatographic stationary phases. The mechanism by which retention and separation in ion-interaction chromatography is accomplished remains unclear, although two primary mechanisms of this phenomenon have been proposed: preadsorption of the hydrophobic counterions to the hydrophobic stationary phase followed by interaction with the ionic analyte or adsorption of ion-paired species to the hydrophobic surface. There is evidence that both mechanisms contribute to ionic retention and separations. In this work, ion interactions at a C-18 stationary-phase interface are investigated by confocal Raman microscopy of individual stationary-phase particles, which is capable of quantifying surface concentrations of surfactants and model solutes. Raman spectra taken within the interior of the particle indicate that preadsorption of the hydrophobic surfactant cetylpyridinium to the stationary-phase interface is sensitive to ionic strength and concentration of organic modifier in the mobile phase. It was also determined that the presence of low concentrations of a model, ionic solute, nitrobenzenesulfonate, leads to greater adsorption of cetylpyridinium ion at the hydrophobic interface. As the concentration of nitrobenzenesulfonate is increased, the 1:1 increase in the cetylpyridinium ion interfacial concentration provides evidence of ion pairs forming at the stationary-phase surface.