Construction and Characterization of an NMR Spectrometer Operating at Earth's Magnetic Field

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Spin is a property of many nuclei which can be exploited for imaging and spectroscopy with nuclear magnetic resonance (NMR) studies. While most medical applications of NMR require large magnets to create the resonance phenomenon, the natural magnetic field of the Earth can also serve the same purpose. Although spectral resolution is lower due to the relatively weak field of the Earth, the continuous presence of this natural field provides an opportunity for easy application of NMR techniques in the field. To access this opportunity, we build and characterize the performance of a low-cost, portable NMR spectrometer, based on the design published by Michal (2010). A standard microcontroller, fed with open-source pulse programming software from a typical laptop, operates the experiment's coil, removing the need for an expensive proprietary controller. We find that the system, despite its simplicity, is susceptible to unwanted feedback-driven oscillations in the output signal, which depends sensitively on the layout of the electronic components. We show that these oscillations may be due both to inductive coupling of ground wires to amplifier outputs, and to signal-ground coupling inherent to the microcontroller. After removing the feedback-driven oscillations, we demonstrate one potential use of the spectrometer as a one-dimensional imager.

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