Exploring the Limits of Overhauser Dynamic Nuclear Polarization (O-DNP) for Portable Magnetic Resonance Detection of Low γ Nuclei

Abstract

Nuclear magnetic resonance (NMR) spectroscopy in portable, permanent magnet-based spectrometers is primarily limited to nuclei with higher gyromagnetic ratio, γ, such as ¹H, ¹⁹F, and ³¹P due to the limited field strength achievable in these systems. Overhauser effect dynamic nuclear polarization (O-DNP), which transfers polarization from an unpaired electron to a nucleus by saturating an electron paramagnetic resonance transition with an oscillating radio frequency magnetic field, B_1e, can increase the polarization of low γ nuclei by hundreds or even thousands, enabling detection in a portable system. We have investigated the potential for O-DNP to enhance signals using (4-amino-2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO hereafter) as a source of unpaired electrons in a homebuilt ultra-low field (ULF) O-DNP-NMR spectrometer. We have found, in general, that larger concentrations of TEMPO are required for effective O-DNP with low γ nuclei, which has a number of important effects. Spin exchange effects cause the EPR lines to overlap and ultimately merge at high concentrations of TEMPO, fundamentally increasing the maximum possible enhancement, while the electron–electron dipolar interaction reduces both longitudinal and transverse relaxation times for the electrons, dramatically increasing the required B_1e strength. The relationship between TEMPO concentration, B_1e magnitude and O-DNP enhancement is quantified, and strategies for achieving these fields are discussed.