Studies On The Separation Performance Of VSPs Matrixed Chromatographic NMR Technique

The detection and structural characterization of the components of a mixture is a challenging task. Therefore, the development of a facile and general method practicable for both the separation and the structural characterization is desired. Diffusion-ordered NMR spectroscopy(DOSY) with the aid of a Virtual Stationary Phases(VSP) is a promising tool for this purpose. However, because the currently existing VSPs only separate limited components, the application of DOSY technique is restricted. Herein we introduce some VSPs, which can separate some mixtures of different structural types by liquid-state NMR spectroscopy. The main works in this thesis are investigating the separation performance of several different VSPs, and the effects of different factors on the separation. Then a general approach for the structural elucidation of the components in a mixture is hopefully established. The separation mechanism and the interaction ways for separation are possibly disclosed, thus providing important references for discovering new VSPs with an improved separation capability. The works in this thesis are summarized as follows:(1) Polyvinylpyrrolidone(PVP) was used as a VSP to separate p-xylene, benzyl alcohol, and p-methylphenol by the chromatographic NMR technique. The effects of concentration and weight-average molecular weight(MW) of PVP, sample temperature, solvent viscosity, and solvent polarity on the resolution of these components were investigated. It was found that higher PVP concentration, higher PVP MW and higher sample temperature could cause an increase in diffusion resolution for the three components. Moreover, the diffusion resolution did not significantly change in case that viscosity-higher solvents were employed. The three components showed different resolution in different solvents. It was also found that the polarity of the analytes played an more important role for the separation by affecting the diffusion coefficient.(2) First discovered by our lab, polydimethylsiloxane(PDMS) showed a powerful separation performance in organic phase. Moreover, it was a cheap, easily accessible and universal VSP. In this thesis, eight groups of different types of analytes were used to evaluate its separation capability: ① 1,2-propanediol, n-propanol and propylene oxide, ② ethanol, acetonitrile and acetic acid, ③ 1,5-dibromopentane, 1-bromopentane and n-pentane, ④ methanol, ethanol and formic acid, ⑤ benzene, naphthalene and anthracene, ⑥ phenylboronic acid, biphenyl and iodobenzene, ⑦ 1-decene, naphthalene and ethanol, ⑧ p-methylphenol, 1-octane and ethanol. It turned out that PDMS could baseline separate these eight group analytes. In addition, the several effects on separation performance of PDMS were preliminarily studied. The results exhibited that a higher PDMS concentration only caused a little increase on diffusion resolution for the components. Further, the raise of sample temperature, solution viscosity and analyte concentration caused different changes in diffusion resolution for different components of these mixtures.(3) Silica gel solution was found to be an effectual VSP for aqueous phase, and a preliminary evaluation of its separation capability was implemented. The results showed that silica gel solution demonstrated separation capability at some extent for the group consisting of fumaric acid, maleic acid and succinic acid, and for the group of n-butanol, n-butanoic acid and succinic acid with the aid of organic additives. An interesting phenomenon was observed that fumaric acid and maleic acid, a pair of cis- and transisomers, were separated by the VSP. In addition, the influence of various solvents, methanol content, sample temperature and solution pH on the diffusion resolution of fumaric acid, maleic acid and succinic acid was investigated, indicating that the influence of different organic additives on separation efficiency was distinct in aqueous solutions. Higher concentration of methanol and higher temperature were advantageous to increase the diffusion resolution for the analytes. In acidic conditions, the increase of solution pH improved separation efficiency further.