Comparison of the kinetic performance and retentivity of sub-2 μm core-shell, hybrid and conventional bare silica phases in hydrophilic interaction chromatography

The separation performance and retention properties of four sub-2 μm underivatised silica materials were evaluated in the hydrophilic interaction chromatography (HILIC) mode. These included an organic/inorganic hybrid silica, conventional silica, narrow particle size distribution silica and a core-shell silica. Van Deemter characterisation was performed using conditions to give high retention factors (k = 5.5-6.0) with 10 cm columns to limit the contribution of extra-column dispersion. The core-shell 1.6 μm bare silica (Cortecs) was shown to be kinetically superior to fully porous particle types. Little column-to-column variation in the reduced b-coefficient was observed for the test analytes as corroborated by arrested elution experiments. However, the reduced b-coefficient was shown to be different between analytes, e.g. cytosine versus nortriptyline. It is speculated that the nature of the retention mechanism (hydrophilic versus ionic retention) and solute physiochemical properties perhaps influence the b-coefficient. Maxwell-Effective Medium Theory (EMT) applied to results for a wider range of solutes indicated that the intra-particle diffusion (Dpart) behaviour for individual compounds is broadly similar irrespective of the particle morphology in HILIC. Finally, the impact of varying buffer concentration for a test mix showed that retention and peak shape varied considerably between different silicas. High efficiency separations can be achieved for hydrophilic and basic solutes using a combination of sub-2 μm core shell bare silica particles and appropriate buffer concentrations.