THE NUCLEAR MAGNETIZATION AS THE ORIGIN OF TRANSIENT CHANGES IN THE MAGNETIC-FIELD IN PULSED NMR EXPERIMENTS

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(-iω0tt)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. © 1990.