Comparison of grafting modes for the preparation of cholic acid-based stationary phases. Influence on enantiomer separations in high performance liquid chromatography

An original chiral selector, namely the prop-2-enyl 3 alpha-(N-phenylcarbam ate)-7 alpha, 12 alpha-dihydroxy-5 beta-cfiolan-24-oate, is bonded onto the silica surface by three different grafting modes. The influence of the tot al structure of the chiral stationary phases (CSPs) on enantioseparation is studied. These CSPs are prepared in two steps. In the first step, the sili ca surface is reacted with one of the following reagents: 1) (3-mercaptopro pyl)trimethoxysilane, yielding a rather monomeric support, 2) chlorodimethy lsilane, leading to a monomeric material, or 3) triethoxysilane, affording a polymeric packing. In the second step, the chiral selector is bonded onto the modified silica structures to provide the corresponding stationary pha ses SH, CDS, and TES, respectively. The CSP SH shows the poorest column per formance. CSPs CDS and TES show similar mass transfer kinetics; this highli ghts the good behaviour of the polymeric packing. CSP TES displays globally the highest enantioselectivity and is found to be extremely stable despite its use over months. The silanization reaction with triethoxysilane provid es a suitable support for chiral stationary phases.