Indirect Zero Field NMR Spectroscopy

Abstract: This study pioneers the two-field correlation spectroscopy (COSY) in zero to ultralow field (ZULF) liquid state NMR, employing hyperpolarized [1-$^{13}$C]pyruvate as a model system. We demonstrate the successful integration of signal amplification by reversible exchange (SABRE) for hyperpolarization, enabling the detection of ZULF COSY spectra with increased sensitivity. The use of field cycling allows the acquisition of two-field COSY spectra at varying magnetic field strengths, including zero-field conditions. This enables insight into both J-coupling and Zeeman-dominated regimes benefiting from ULF field observation sensitivity and mitigation of low-frequency noise by conducting readout at higher fields (>5 $\mu$T). Our study explores the effects of polarization transfer, apodization techniques, and the potential for further application of ZULF NMR in chemical analysis exemplified for three X-nuclei and three corresponding molecules: [1-$^{13}$C]pyruvate, [$^{15}$N]acetonitrile and [3-$^{19}$F]pyridine. These findings pave the way for more sensitive and cost-effective NMR spectroscopy in low-field regimes.