ENANTIOMER SEPARATION BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY ON POLYSILOXANE-BASED CHIRAL STATIONARY PHASES

The synthesis of two polysiloxane-based chiral stationary phases (CSPs ) derived from a pi-acidic N-(3.5dinitrobenzoyl)-beta-amino acid (JEM- 1) and a pi-basic N-(1-naphthyl)-leucine selector is described as is t heir systematical comparison with the corresponding ''brush''-type CSP s. The enantioselectivity of the polysiloxane-based CSPs is higher und er both normal- and reversed-phase conditions. In the normal-phase mod e, 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 pi-basic CSP and to lower values on the pi-acidic CSP. The latter CSP shows bu t a small increase in enantioselectivity relative to the corresponding ''brush''-type CSP having a comparable selector loading. The silanoph ilic interactions can be further reduced by end-capping with hexamethy ldisilazane (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. Th e presence of the non-polar polymeric backbone can introduce hydrophob ic interactions which may alter enantioselectivity. It would seem adva ntageous to use dimethylpolysiloxanes having a high selector concentra tion in order to reduce the extent of any non-chiral contribution by t he polysiloxane backbone to analyte retention while enhancing the favo rable chiral recognition properties of the polymer.