Retention mechanism on phosphodiester stationary phases in hydrophilic interaction liquid chromatography and purely aqueous mobile phase part II: Overloading with limited soluble samples

The adsorption behaviour of caffeine and theophylline under hydrophilic interaction chromatography and purely aqueous conditions was investigated on four phosphodiester stationary phases. Solute adsorption isotherms were determined by frontal analysis or inverse method. The bi-Langmuir model was found to be the best choice to describe the behaviour of caffeine and theophylline adsorption in purely aqueous conditions, whereas the biMoreau model describes the adsorption phenomena in HILIC conditions. The results obtained demonstrate that the interaction of caffeine and theophylline with the stationary phase surface varies depending on the mobile phase composition. Both in pure aqueous mobile phase and in HILIC mode, the heterogeneity of the surface of the studied stationary phases is confirmed. In hydrophilic solutions, the sample molecules interact with the stationary phase only. In hydrophobic conditions, a lateral interaction occurs between caffeine or theophylline molecules, which are poorly soluble in acetonitrile-rich solvents. This confirms that the same compound on the same stationary phase can behave rather differently, depending on the mobile phase composition. Thus, the mobile phase may govern and control the retention mechanism.