Evaluation of superficially porous particle based zwitterionic chiral ion exchangers against fully porous particle benchmarks for enantioselective ultra-high performance liquid chromatography

Abstract

Zwitterionic chiral ion-exchange selectors (ZWIX) obtained by conjugation of quinine and 2-aminocyclohexanesulfonic acid via a carbamate bond were immobilized on three different silica particle types, viz. 120 Å 3 μm fully porous particles (FPP), 200 Å 3 μm FPP and 160 Å 2.7 μm superficially porous particles (SPP). Selector densities were determined by elemental analysis and the porosities of packed columns measured by inverse size exclusion chromatography with polystyrene standards. Liquid chromatographic tests with a set of chiral zwitterionic, acidic and basic analytes showed that the surface chemistry was successfully transferred to the distinct particle morphologies. The chromatographic performance of the three columns was evaluated by acquiring van Deemter curves. The results showed that the column packed with the SPP particles gives the best performance and kinetic plots further demonstrated that they represent the most favorable compromise in terms of speed, efficiency and pressure drop. Sub-minute separations could be accomplished at much lower pressure drop on the core-shell column, e.g. 2-amino-2-phenylbutyric acid was baseline separated in less than 15 s on a 5 cm long column. The Maxwell effective medium theory with second order approximation was applied to calculate effective diffusion in the mesoporous zones of SPP and FPP, which allowed eventually to deconvolute the individual peak dispersion contributions (ha, hb, hc,m, hc,s, hc,ads). The efficiency gain of the 160 Å SPP column compared to the 120 Å FPP (benchmark) column was mainly due to lower eddies (ha), smaller c-term accounting for slow adsorption-desorption kinetics in enantioselective chromatography (hc,ads), and also due to lower stationary mass transfer resistance (hc,s). Enhanced effective diffusion (Deff) in the SPP column contributed to a lower longitudinal diffusion (hb). In contrast, the mobile phase mass transfer coefficient was similar in the two columns leading to comparable hc,m contributions. This study discloses some options for improvement of the efficiency of ZWIX-type chiral columns such as replacing narrow pore (120 Å) by wide pore (200 Å) particles, substituting FPP by SPP and reducing the selector density on the surface.