Steps to achieving high-resolution NMR spectroscopy on solutions at GPa pressure

Recent theoretical advances and molecular-dynamic estimates of the dielectric constant of water have extended the HKF model for aqueous solution chemistry up to 6.0 gigapascals (GPa), which are conditions well beyond the capabilities of conventional NMR spectroscopy (Pan and others, 2013; Sverjensky and others, 2014, see also Wasserman and others, 1995). These developments provide strong motivation to design a simple NMR probe that allows experiments on solutions, at high resolution, to pressures of a few GPa (Pautler and others, 2014; Ochoa and others, 2015, 2016). Here we describe the performance of a compact NMR probe that can reach several GPa pressures. The probe is made by placing a solenoid microcoil inside of a standard piston-cylinder device used in solid-state physics. High pressures are achieved in the sample by applying force to a coaxial piston. Early designs of the probe, although useful, were limited in sample size to 10 to 15 L. Here we describe modifications that allow greatly improved resolution and sensitivity, including 1H-1H NMR correlation spectroscopy, on solutions at 2.8 GPa pressure. Sample sizes can be expanded if, instead of a standard NMR spectrometer that is built around a superconducting magnet, one employs a magnetic-resonance imaging (MRI) system that is built around a permanent magnet. The MRI systems can apply larger magnetic field gradients than conventional spectrometers, and thus have more robust shimming capability, which is needed because of the juxtaposition of different alloys in the pressure cell. More importantly, these systems can have magnetic fields that are oriented perpendicular to the magnet bore, which allows rotation of the coil within the axis of the pressure cell to increase sample volumes and pressure. Sensitivity is improved by replacing the traditional, but reduced-volume, closed sample container mounted in the center of the NMR detection coil with an open NMR coil that dangles freely in solution, thus making the sample solution itself the pressure-transmission fluid. The efficacy of these modifications are demonstrated by measuring 1H NMR spectra for ethyl alcohol and ethyl alcohol/methyl alcohol mixtures at pressures up to 2.8 GPa. Further developments are discussed that will allow geochemists to acquire aqueous solution NMR spectra at higher 3 to 4 GPa pressures