Study of retention mechanisms in Hydrophilic Interaction Chromatography by Nuclear Magnetic Resonance Spectroscopy

Abstract: This thesis deals with investigations of the retention mechanisms in hydrophilic interaction liquid chromatography (HILIC) using nuclear magnetic resonance spectroscopy. The aims are to understand how the different types of stationary phases can influence the retention of various solutes and the adsorption abilities of solvent on stationary phases.The thesis encompasses the development of a saturation transfer diffe­rence nuclear magnetic resonance (STD-NMR) spectroscopy method by which the mechanisms in HILIC were probed on hydrophilic and polar stationary phases with varying charge properties and water-retaining abilities under high-resolution magic angle spinning (HR-MAS) condit­ions. By applying the developed method, results show that toluene is in­deed capable of traversing the water-enriched layers of all the three tested stationary phases. In addition, the STD-NMR method was applied to study interaction mechanisms using set hydrophilic compounds rang­ing from small organic acids, nucleobases, nucleosides, and other neutral molecules to further elucidate the differences in HILIC selectivity caused by a dipolar interaction, hydrogen bonding, and electrostatic interaction.Finally, the solvent adsorption on the stationary surface was studied by applying a nuclear magnetic relaxation dispersion (NMRD) technique in combination with a relaxation model in order to resolve the deuterium T1-NMRD profile of acetonitrile-d3 in aqueous solutions confined in the pores of modified HILIC silicas with nominal pore diameters ranging from of 6 to 10 nm. It was found that the acetonitrile-d3 had a strong field dependence at low magnetic fields, which was attri­buted to surface sites at which the molecules were trapped with residence times in the range of 0.1–3 µs.