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Architecture and Dynamics of C18 Bonded Interphases with Small Molecule Spacers



Lane C. Sander, Katrice A. Lippa, Catherine A. Rimmer, Maximilian Kuhnle, Volker Friebolin, Klaus Albert


The relationship between alkyl phase structure and chromatographic performance is investigated for a series of octadecyl (C18)- modified silica surfaces with defined spacing of the alkyl surface by a “pre-endcapping” technique. Stationary phases were prepared by a two step process with (1) reaction with less than stochiometric amounts of a small monofunctional silane, followed by (2) solution or surface polymerization with octadecyltrichlorosilane. The results of solid-state and suspension nuclear magnetic resonance (NMR) spectroscopy are correlated with the chromatographic behavior regarding shape selective separations. Two sets of 6 different stationary phases were prepared by solution and surface polymerization approaches, yielding materials with surface coverages from 2.7 to 5.6 mol/m2. 13C cross polarization magic angle spinning (CP/MAS) NMR spectra show a predominance of trans conformations for the set of surface polymerized phases with a C18 coverage greater than 4.5 mol/m2. For the solution polymerized phases, no predominance for the trans conformation was observed, even for surface coverages greater than 5.1 mol/m2. Proton spectra in suspension indicate the trend that a higher coverage for the surface polymerized materials correlates with a more rigid alkyl chain confirmation. The set of solution polymerized stationary phases confirms this tendency but minor deviations are observed for high coverages. These structural abnormities are confirmed by differences in the 29Si CP/MAS spectra. Furthermore, the 29Si CP/MAS spectra indicate a lower amount of cross-linking for the materials with the highest amount of placeholder (spacer). The use of the different spectroscopic and chromatographic methods provides a wealth of information on the surface morphology of the systematically prepared C18 materials and extends the understanding of surface morphology of alkyl modified silica and its influences of the molecular recognition process in liquid chromatography.
Analytical Chemistry


stationary phases, shape recognition, shape selectivity, retention mechanisms, NMR, SRM 869, synthesis, bonded phase


Sander, L. , Lippa, K. , Rimmer, C. , Kuhnle, M. , Friebolin, V. and Albert, K. (2009), Architecture and Dynamics of C18 Bonded Interphases with Small Molecule Spacers, Analytical Chemistry (Accessed April 13, 2024)
Created December 15, 2009, Updated February 19, 2017