Background-Free Detection of Spin-Exchange Dynamics at Ultra-Low Magnetic Field

Ultra-low field nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) inherently suffer from a low signal-to-noise ratio due to the small thermal polarization of nuclear spins. Transfer of polarization from a pre-polarized spin system to a thermally polarized spin system via the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) could potentially be used to overcome this limitation. SPINOE is particularly advantageous at ultra-low magnetic field, where the transferred polarization can be several orders of magnitude higher than thermal polarization. Here we demonstrate direct detection of polarization transfer from highly polarized Xe-129 gas spins to H-1 spins in solution via SPINOE. At ultra-low field, where thermal nuclear spin polarization is close to background noise levels and where different nuclei can be simultaneously detected in a single spectrum, the dynamics of the polarization transfer can be observed in real time. We show that by simply bubbling hyperpolarized Xe-129 into solution, we can enhance H-1 polarization levels by a factor of up to 151-fold. While our protocol leads to lower enhancements than those previously reported under extreme Xe gas pressures, the methodology is easily repeatable and allows for on-demand enhanced spectroscopy. SPINOE at ultra-low magnetic field could also be employed to study Xe-129 interactions in solutions.

Automated summary

Ultra-low field nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) suffer from low signal-to-noise ratio due to small thermal polarization of nuclear spins. A technique called Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) can transfer polarization from a pre-polarized spin system to a thermally polarized spin system, potentially overcoming this limitation. In this study, we demonstrate the direct detection of polarization transfer from highly polarized Xe-129 gas spins to H-1 spins in solution using SPINOE. We show that by bubbling hyperpolarized Xe-129 into the solution, H-1 polarization levels can be enhanced by up to 151-fold.