Solvation, interaction and dynamics of xenon atoms in HPLC column materials studied by variable-temperature dependent 129Xe, 1H–129Xe cross-polarization, and two-dimensional exchange NMR experiments

Department of Chemistry and Biochemistry, California State University, 5151 State University Drive, Los Angeles, CA 90032, USA
Microporous and Mesoporous Materials (Impact Factor: 3.45). 10/2003; 64(1-3):155-163. DOI: 10.1016/S1387-1811(03)00463-3


Xenon NMR is a useful method for probing structure and dynamics of micro-porous materials due to the sensitivity of xenon’s chemical shifts to its local interactions, and the diffusion property of xenon atoms. Here, we report a study of solvation, interaction and diffusion of xenon atoms inside the HPLC column materials, Zorbax SB-C18 and XDB-C18 which were made of siloxane surface coatings of porous silica, by variable-temperature dependent (VT) 129Xe, 1H–129Xe cross-polarization (CP), and two-dimensional exchange (2D EXSY) NMR experiments. The VT NMR experiment showed the solvation and dynamics of xenon atoms in the column materials. The CP experiment at low temperature provided evidence for probing the direct interaction of xenon atoms with the hydrocarbon chains of the stationary phase, and helped for assigning the 129Xe peaks in the VT NMR spectra. The 2D EXSY NMR experimental result showed the diffusion of xenon atoms within the accessible spaces in the column materials. Combined with our previous study, a full picture of xenon’s behavior inside the column materials has been described. This study provides a basic understanding of xenon NMR of the column materials, which enables us to conduct further investigation of retention mechanisms of column materials in terms of molecular interaction and diffusion by xenon NMR method.

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    ABSTRACT: VT (129)Xe NMR was applied to probe the interactions of solvents having different polarity indices with the stationary phase of a RP-C18 HPLC column material. It was observed that the highly polar ethylene glycol molecules do not mix with the alkyl chains of the RP-C18 stationary phase and the solvent is unable to enter the pores and the spaces between the particles. Three phases in this sample are defined as stationary/xenon phase, xenon gas phase (in the pores and the spaces between the particles) and ethylene glycol/xenon phase. In contrast to ethylene glycol, the nonpolar solvent cyclohexane was observed to be well mixed with the RP-C18 stationary phase. The capillary rise effect allows the solvent to enter the pores and the spaces between the particles. Two phases in this sample are defined as stationary/cyclohexane/xenon phase and cyclohexane/xenon phases. The properties of ethyl acetate are between those of ethylene glycol and cyclohexane. The (129)Xe NMR results show that the rational reversed phases should be conditioned from highly solvating to more polar solvents to remove the trapped air. The (129)Xe NMR results also show that pure stationary phase exists only when a highly polar solvent is used in reversed-phase chromatography. For a solvent with lower polarity, a stationary/solvent phase actually forms. This, together with the mobile phase, determines the selective factor for separating mixtures.
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