Heisenberg spin exchange effects of nitroxide radicals on Overhauser dynamic nuclear polarization in the low field limit at 1.5 mT

Abstract

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) at very low magnetic fields (0.05–20 mT) have gained interest due to the simple and portable magnet design and newly emerging applications outside of the usual laboratory setting. A method to enhance the NMR signal is needed due to the low thermal polarization of nuclear spins at these fields; dynamic nuclear polarization (DNP) via the Overhauser effect from free radicals is an attractive option. In this report we describe a DNP-enhanced NMR system operating at a fixed field of 1.5 mT and measure 1H signal enhancements of up to −350 fold during the saturation of a selected electron spin resonance (ESR) transition of dissolved nitroxide radicals. This −350 fold enhanced polarization is equivalent to what would be obtained by prepolarization in a 0.53 T field. The ESR spectra at varying radical concentrations are indirectly found through DNP-enhanced NMR detection. Here, ESR line broadening at higher radical concentrations due to Heisenberg electron spin exchange is observed. Enhancements in the limit of maximum power are reported as a function of concentration for three ESR transitions, and are found to increase with concentration. The >300 fold 1H NMR signal amplifications achievable at 1.5 mT will reduce experimental time by several orders of magnitude, permitting NMR relaxation, imaging or pulsed-field gradient diffusion experiments that are inaccessible without using the DNP effect at 1.5 mT. We demonstrate the potential benefit of such large signal amplification schemes through T 1 and T 2 relaxation measurements carried out in a much shorter time when employing DNP. Finally, we compare our results to those obtained in the earth’s magnetic field and find that the signal to noise ratio (SNR) of DNP-enhanced signal at 1.5 mT is much greater than that obtained by previous studies utilizing DNP enhancement in the 0.05 mT earth’s magnetic field.