1H NMR at Larmor frequencies down to 3 Hz by means of Field-Cycling techniques

Field-Cycling (FC) NMR experiments were carried out at H-1 Larmor frequencies down to about 3 Hz. This could be achieved by fast switching a high polarizing magnetic field down to a low evolution field which is tilted with respect to the polarization field. Then, the low frequency Larmor precession of the nuclear spin magnetization about this evolution field is registered by means of FIDs in a high detection field. The crucial technical point of the experiment is the stabilization of the evolution field, which is achieved by compensating for temporal magnetic field fluctuations of all three spatial components. The paper reports on some other basic low field experiments such as the simultaneous measurement of the Larmor frequency and the spin-lattice relaxation time in such small fields as well as the irradiation of oscillating transversal magnetic field pulses at very low frequencies as a novel method for field calibration in low field FC NMR. The potential of low field FC is exemplified by the H-1 relaxation dispersion of water at frequencies below about 2 kHz stemming from the slow proton exchange process. (C) 2017 Elsevier Inc. All rights reserved.