Earth’s magnetic field (~50 uT) provides an abundant, cryogen-free magnetic field that can be leveraged for NMR spectroscopy. Without superconducting or bulky permanent magnets, fieldable spectrometers could be developed for applications ranging from benchtop or industrial analysis of chemical samples to meeting the needs for national security and forensics. Researchers have successfully used Earth’s magnetic field for NMR dating back to the 1950’s in the areas of diffusion measurements, MRI, and most recently spectroscopy. Note at this low field, differences in chemical shifts are often too small to measure, and the spectral data centers on heteronuclear and homonuclear J-couplings. To obtain this special spectral signature that provides molecular identification and structure, a heteronuclear J-coupling must be present in the molecules, which breaks the magnetic equivalence of the nuclei. Due to the limited Larmor frequency dispersion at the Earth’s 50 uT field, the strong coupling regime is entered for both homonuclei and heteronuclei, which provides rich structural details due to the emergence of additional peaks brought on by the strong coupling regimes. At present, the requirement of a heteronuclear J-coupling is fairly restrictive in the samples that can be spectroscopically analyzed without isotopic labelling schemes. However, small organophosphate and fluorinated molecules have shown considerable promise for Earth’s field NMR and are also of national security interest as they form the basis of chemical warfare agents, pesticides, and some pharmaceuticals. In this talk we will discuss the construction of Earth’s field NMR spectrometers, describe the definition of the strong coupling regime, and benchmark the NMR observables. What emerged from our research program is the ability to determine the sign and magnitude of the J-couplings, chemical shifts for non-H nuclei, quantitative measures, and new methods to address the current challenges in using Earth’s field NMR for small organophosphate molecules. The results will be contextualized to demonstrate the power of Earth’s field NMR that could be easily integrated into commercial chemical processing or laboratory settings.
This work is released for publication in accordance with Los Alamos National Laboratory LAUR-21-22193 by Triad National Security, LLC (Los Alamos, NM, USA), operator of the Los Alamos National Laboratory under Contract No. 89233218CNA000001 with the U.S. Department of Energy.