Surface-anchored counterions on weak chiral anion-exchangers accelerate separations and improve their compatibility for mass-spectrometry-hyphenation

Abstract

In the present work we propose new variants of chiral stationary phases (CSP) with tert-butylcarbamoylquinine (tBuCQN) as chiral selector molecule. Four tBuCQN-CSPs with distinct bonding chemistries are compared in terms of their pH-dependent surface charge by ζ-potential determinations, by achiral and chiral liquid chromatographic tests and LC–ESI–MS hyphenation. In one embodiment tBuCQN was immobilized on 3-mercaptopropylmethylsilyl-modified silica by thiol-ene click reaction (brush type CSP with selector coverage of 0.38mmol/g). In another embodiment, poly-(3-mercaptopropyl)-methylsiloxane was coated onto vinylized silica particles in presence of tBuCQN and radical initiator. The tBuCQN selector was then immobilized onto the polysiloxane film which in turn was crosslinked to the vinyl-surface in a simultaneous double click reaction leading to a CSP with enhanced stability due to multiple linkages (0.29mmol/g tBuCQN). Aliquots of each of the two CSPs were further modified by oxidation of free residual thiol groups to sulfonic acid functionalities to obtain strongly acidic endcapping groups which act as immobilized counterions of the chiral WAX CSPs (0.2mmol/g sulfonic acid co-ligands for brush type CSP). This caused secondary repulsive interactions, hence balanced interactions of the target analytes (chiral acids) at the WAX site and decreased non-specific interactions. Furthermore, this rendered possible the use of milder elution conditions, i.e. lower ionic strength, for acidic compounds. Separation performance was maintained and slightly improved, respectively, when using polar organic or reversed-phase type elution mode in chiral separations which were significantly accelerated (isoeluotropic conditions could be achieved with ca. factor 40 lower counterion concentration in the mobile phase). Thus, LC–ESI–MS enantiomer separations could be readily performed at very low ionic strength conditions (10mM acetate) which is favorable due to less ion suppression. In addition to this the newly developed stationary phases showed complementary retention profiles in RP- and HILIC-mode which make these type of stationary phases also promising tools for achiral applications in pharmaceutical analysis, especially as orthogonal separation principle e.g. in 2D-LC and impurity profiling.