Low-field NMR spectroscopy for samples with restricted mobility

This study presents a low-field NMR spectrometer featuring a multilayer Halbach magnet combined with mechanical and electrical shimming for improved magnetic field homogeneity and sensitivity. The magnet consists of three nested cylindrical layers, including an inner rotatable layer used for mechanical shimming. The electrical shimming system includes 7-channel shim coils coated with zirconia, thermal epoxy, and silver paste to enhance thermal stability and facilitate passive heat dissipation. Dividing the shim coils into two parallel parts reduced Joule heating from 96.2 °C to 32.6 °C. Without shimming, field inhomogeneity was about 140 ppm over the sample volume. The probehead integrates a solenoidal mini coil on a planar board to improve sensitivity, reduce B₁ inhomogeneity, and minimize impedance mismatches and signal losses, resulting in a water linewidth of 4.5 Hz for a 3 mm × 2.4 mm sample volume. To overcome limited spectral dispersion at low fields (0.5–0.7 T), the study employs optimal control pulses, calculated with the Krotov algorithm, for selective excitation of specific proton groups and compounds. The computational complexity was reduced by modeling mixtures as non-coupled spin systems, enabling compound-selective pulses. Demonstrations include selective excitation of cyclopentenone substructures and editing signals of benzoic acid and alanine mixtures. Additionally, the WAHUHA multiple-pulse sequence was applied to reduce homonuclear dipolar coupling, decreasing linewidths by approximately 15% in both low- and high-field NMR. Water suppression was achieved by incorporating water explicitly in the pulse design Hamiltonian. These advances show that benchtop low-field NMR can effectively analyze complex mixtures such as biofluids and metabolic samples, enabling portable, point-of-care NMR applications.