A Polymer NMR Cell for the Study of High-Pressure and Supercritical Fluid Solutions

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Abstract
Nuclear magnetic resonance (NMR) offers researchers unique, highly localized molecular information. The importance of this technique is well established in studies using chemical shift, spin coupling, and relaxation times providing detailed structural information, determining chemical equilibria and kinetics, and understanding molecular dynamic processes. However, the widespread application of NMR spectroscopy to high-pressure liquids and supercritical fluids has been limited due to the complexity of the necessary instrumentation. One approach to these studies is to build a dedicated high-pressure probe. Another involves the utilization of a high-pressure cell designed to fit in commercially available probes. Here we present the design and implementation of a simple, three-piece, high-pressure NMR cell constructed of high-performance polymers. The present cell has pressure capabilities of up to 400 bar; however, the ultimate temperature and pressure limits will be determined by the specific polymer chosen. High-resolution NMR spectra of methanol modified and tributyl phosphate (TBP) modified supercritical CO2 are presented. An example of supercritical fluid phase behavior monitored with NMR is demonstrated for the TBP system in which the chemical shift changes in the 31P nucleus as a function of density are indicative of solution phase separation. The multinuclear NMR data demonstrate the utility of this cell for studying supercritical fluid solution systems relevant to analytical separations and extractions.