The nuclear magnetization as the origin of transient changes in the magnetic field in pulsed NMR experiments

Abstract The very measurement of the NMR signal in a pulsed NMR experiment disturbs the nuclear equilibrium magnetization. A small change in the magnetic field results, which can be of the order of −0.0025 ppm in protonated solvents. It is shown that the resonances in pulsed high-field NMR consequently are non-Lorentzian. The FID is properly described by S ( t ) = S (0)exp(− iω 0 t t )exp(− iΔωf { t }) is the small resonance shift which is experienced immediately following the pulse, and f { t } characterizes the subsequent return toward zero by the response of the lock system and by spin-lattice relaxation. In particular in samples containing large concentrations of protons, shifted and distorted resonance lines can be observed.