Journal of Chromatography & Separation Techniques
Open Access

Abstract

Retention Mechanism in Hydrophilic Interaction Liquid Chromatography New Insights Revealed From the Combination of Chromatographic and Molecular Dynamics Data

Fabrice Gritti

The retention mechanism of polar/charged analytes in hydrophilic interaction liquid chromatography (HILIC) remains ambiguous from the sole measurement of their retention factors. It is because thermodynamic properties only relate to the equilibrium relationship between the concentration in the mobile phase and that in the stationary phase. They do not provide any insights regarding their microscopic distribution across the mesopore volume. Chromatographers cannot unambiguously conclude whether analytes are adsorbed onto the surface of HILIC adsorbents, partitioned between the bulk and the water-rich interfacial layer or if both adsorption and partitioning mechanisms participate to the retention mechanism. In order to solve this ambiguity, it is proposed to combine chromatographic data (retention factor, inverse-size exclusion, sample diffusivity along the bed) with molecular dynamics (MD) data. The latter provide microscopic information regarding the structure of the eluent and the average mobility of the analyte across the mesopore. This enables 1) the clear delimitation between three pore regions: the rigid water layer adsorbed onto the solid surface, the interfacial diffuse water layer, and the bulk region, 2) the measurement of the adsorption and partitioning equilibrium constants of the analyte between these three pore regions, and 3) the concentration distribution of the analyte in the pore volume. The benefits of this new approach are demonstrated for a weakly retentive HILIC adsorbent (3.5 μm hybrid organic/inorganic silica particles) in contact with ternary eluent mixtures (acetonitrile/water pH 5/third solvent, 90/5/5, v/v/v). The third solvent has various polarities from water to n-hexane. The results show that, despite having nearly identical retention factors, the retention process of nortriptyline is essentially controlled by a partitioning mechanism while that of cytosine is governed by an adsorption mechanism. On the application side, it is shown how to significantly increase sample retention in HILIC by adding a third solvent in the mobile phase.