Diffusion and field-gradient effects in NMR Fourier spectroscopy

Abstract

In NMR Fourier transform spectroscopy, rf pulses are periodically applied to a spin system to generate a sequence of free induction decays which form the basis of spectroscopic measurements. In inhomogeneous magnetic fields, distortions of the free induction decay arise from superimposed spin‐echo effects. They are strongly dependent on molecular diffusion which is also responsible for the nonexponential envelope of the echo train observed in a stopped pulse experiment. These effects are investigated by means of two independent methods, the Fourier expansion method and the partition method. The results are discussed and used to explain measurements made on a one‐spin system. The influence of echoes on the sensitivity of Fourier transform spectroscopy is considered, and it is concluded that the achievable gain does not normally outweigh the complications arising from retention or enhancement of echoes. Experimental parameters are derived for efficient suppression of echoes by means of a periodically pulsed magnetic field gradient.