May 26, 2006
NMR Study of Xenon Atoms to Probe Molecular Interactions with Chromatographic Column Materials
Department of Chemistry and Biochemistry
California State University, Los Angeles
Chromatography is widely used in research laboratories and chemical industry to separate molecular mixtures. Chromatographic column materials are silicate based porous particles with surface coatings. The purpose of this study is to understand the retention mechanism of chromatographic column material at the molecular level. The knowledge of retention mechanisms is necessary for designing improved column materials and developing experimental methods for separating mixtures. We have proposed to use Xe-129 NMR for this study. The large polarizibility of Xenon’s electron cloud makes xenon a sensitive probe to local environments as it gives wide NMR chemical shift range. The interaction of Xe atoms with the stationary phase of a C18 HPLC column material had been studied using Variable Temperature (VT) NMR. Lower temperature slows down the dynamics of xenon molecules and the hydrocarbons surface coatings of the column material, which allows us to probe their interactions. Two-dimensional NMR exchange spectroscopy (2D EXSY) can be used to study the dynamics of xenon atom’s diffusion within the porous spaces of the material of interest. Using H-1 and Xe-129 NMR on the column material with solvents of different polarity such as cyclohexane, ethylene glycol and ethyl acetate, helped us understand how to improve the retention mechanism. The more polar molecules did not mix well with the SB-C18 stationary phase while the non polar molecule mixed well, but the xenon can still be stuck into the stationary phase. The fact that the xenon molecules remain means that gases can be trapped in the column causing air bubbles. Enhancement in signal to noise of Xe-129 NMR will also be discussed.
1. Structure, Dynamics, and Interaction of the Stationary Phase and Xenon Atoms in the Zorbax SB-C18 HPLC Column Material Studied by Solid State NMR and 129Xe NMR. Y. Ba, D. Chagolla, J. Phys. Chem. B,106, 2002, 5250-5257
2. Solvation, interaction and dynamics of xenon atoms in HPLC column materials studied by variable-temperature dependent 129Xe, 1H129Xe cross-polarization,and two-dimensional exchange NMR experiments D. Chagolla, G. Ezedine, Y. Ba, Microporous and Mesoporous Materials, 64, 2003, 155.
3. Probing the Interaction of Solvents with the Stationary Phase of C18 High-Performance Liquid Chromatographic Column Material by Variable-Temperature Dependent 129Xe Nuclear Magnetic Resonance Spectroscopy. D. Chagolla, E. V. Mathias and Y. Ba, Journal of Chromatography A. 2006 Accepted.