Steady-State Free Precession (SSFP) NMR Spectroscopy\nfor Sensitivity Enhancement in Complex Environmental and Biological\nSamples Using Both High-Field and Low-Field NMR

Nuclear magnetic resonance (NMR)\nspectroscopy is a valuable and\ncomplementary tool in environmental research, but it is underutilized\ndue to the cost, size, and maintenance requirements of standard “high-field”\nNMR spectrometers. “Low-field” NMR spectrometers are\na financially and physically accessible alternative, but their lower\nsensitivity and increased spectral overlap limit the analysis of heterogeneous\nenvironmental/biological media, especially with fast-relaxing samples\nthat produce broad, low-intensity spectra. This study therefore investigates\nthe potential of the steady-state free precession (SSFP) experiment\nto enhance signal-to-noise ratios (SNRs) of fast-relaxing, complex\nsamples at both high- and low-field. SSFP works by obtaining steady-state\ntransverse signal using a train of equally spaced radiofrequency pulses\nwith the same flip angle and a time between pulses less than the transverse\nrelaxation time, allowing for thousands of scans to be summed in a\nshort time period. Here, <sup>13</sup>C-SSFP is applied to samples\nof varying complexity (egg white, dissolved organic matter, and crude\noil) at low-field and at high-field for testing and comparison. The\npotential of <i>in vivo</i> SSFP NMR is additionally investigated\nby applying <sup>31</sup>P-SSFP to live Eisenia fetida at high-field. In some samples, SSFP increased <sup>13</sup>C SNR\nby over 2000% at both high-field and low-field compared to standard <sup>13</sup>C NMR and enabled detection of peaks that were not observable\nby standard <sup>13</sup>C NMR. Ultimately, SSFP holds great potential\nfor improving analysis of fast-relaxing, complex samples, which could\nin turn make low-field NMR spectroscopy a more effective tool not\nonly in environmental/biological research but also in numerous other\ndisciplines.