Combined Use of Pulsed Gradient Spin Echo and High Resolution Magic Angle Spinning to Investigate Solutes Diffusion in Presence of a Chromatographic Stationary Phase

1. Introduction We recently introduced a novel analytical method based on the Pulsed Gradient Spin Echo (PGSE) experiment and High Resolution Magic Angle Spinning (HRMAS), which combines the advantages of column chromatography separation and NMR structural analysis [1]. Specifically, we showed that, in a PGSE experiment, the separation of the NMR spectra of the components of a mixture could be enhanced by several orders of magnitude upon addition of a typical stationary phase used in HPLC, such as either a normal (silica gel) or a reversed (functionalized ODS silica gel) phase. HRMAS is then required to recover high resolution NMR spectra by removing magnetic susceptibility broadenings caused by the presence in solution of the solid support. This combined PGSE and HRMAS technique was subsequently applied to investigate indirectly crucial steps of reversed phase liquid chromatography, such as the partitioning of the analyte between different phases [2]. Interestingly, with respect to HPLC, this technique offers surprising perspectives for mixture analysis [3]. Indeed, while reversed phase liquid chromatography (RPLC) is required to separate the components of a model mixture (benzene, naphthalene, and anthracene dissolved in an acetonitrile/water mix), both normal and reversed stationary phases give excellent discrimination in NMR. Because of its unexpected potential for mixture analysis, this technique was hence referred to as 'Chromatographic NMR'. To investigate the discrimination process evidenced in Chromatographic NMR, the apparent diffusion coefficients (D) of the above mentioned 3-component mixture were measured in the presence of a constant weight of bare silica as a function of the amount of liquid solution in the MAS rotor.