Enantiomer separation by high-performance liquid chromatography on polysiloxane-based chiral stationary phases

Abstract

The synthesis of two polysiloxane-based chiral stationary phases (CSPs) derived from a π-acidic N-(3.5-dinitrobenzoyl)-β-amino acid (JEM-1) and a π-basic N-(1-naphthyl)-leucine selector is described as is their systematical comparison with the corresponding “brush”- type CSPs. The enantioselectivity of the polysiloxane-based CSPs is higher under both normal- and reversed-phase conditions. In the normal- phase mode, the greater enantioselectivity stems from smaller retention factors for the least retained enantiomers, presumably because of a reduction of analyte interactions with the support silanols owing to effective shielding of the surface by the polymer. The retention factors of the second-eluted enantiomers are shifted to higher values on the π-basic CSP and to lower values on the π-acidic CSP. The latter CSP shows but a small increase in enantioselectivity relative to the corresponding “brush”-type CSP having a comparable selector loading. The silanophilic interactions can be further reduced by end-capping with hexamethyldisilazane (HMDS). When lower amounts of polar modifier are used, the resolution of the polymeric CSPs approaches that of the corresponding brush-type CSP. Under reversed-phase conditions enantioselectivity is reduced but not to the extent generally found for brush-type CSPS. The presence of the non-polar polymeric backbone can introduce hydrophobic interactions which may alter enantioselectivity. It would seem advantageous to use dimethylpolysiloxanes having a high selector concentration in order to reduce the extent of any non-chiral contribution by the polysiloxane backbone to analyte retention while enhancing the favorable chiral recognition properties of the polymer.