Effect of radial RF field on MAS spectra

When, in the method MAS, part of the sample is at the edge of the coil so as to experience a partly radial RF field, this part gives rise during the sample spinning to spectra shifted by –1 and +1 times the spinning frequency, which are superimposed on the normal spectra with possible sidebands due to the modulated interactions, anisotropic chemical shifts and dipolar or quadrupolar interactions. The amplitude of these shifted spectra depends on the RF field distribution over the sample and on the amplitude of the RF pulse preceding the observation of the FID. On the other hand, it is independent of the spinning frequency. The quantitative description of this effect necessitates the use of the Reciprocity Theorem. Following a simplified account of the relevant theory, experimental illustrations of this effect are presented. The analysis of these contributions to the amplitudes of the sidebands makes it possible to determine the conditions under which the perturbation of the spectra is negligible.