Correlation between chromatographic and physicochemical properties of stationary phases in HPLC:: C30 bonded reversed-phase silica

In comparison to conventional C-18 phases, C-30 phases exhibit superior shape selectivity for the separation of isomers of carotenoids and vitamins. To obtain this enhanced recognition capability the HPLC separation must be performed at a well-defined temperature. At higher temperatures the capability of differentiating between different stereochemical isomers is lost, resulting in peak coelution, This separation behaviour is primarily dependent upon the organisation of the C-30 alkyl chains on the silica surface which can be visualised as two domains, the more ordered domain containing relatively rigid n-alkyl groups with trans conformations and the less ordered environment containing more flexible n-alkyl groups with gauche conformations. The ratio between trans vs. gauche conformations of the n-alkyl groups directs the shape selectivity of the C-30 phase. The temperature-dependent interconversion of trans to gauche conformations can be monitored by temperature-dependent solid-state nuclear magnetic resonance (NMR) and suspended-state NMR measurements and visualised by molecular modelling calculations. Thus a direct correlation between chromatographic and physicochemical properties of C30 bonded phases is possible. (C) 1998 Published by Elsevier Science B.V. All rights reserved.