When,in the method MAS,part of the sample is at the adge 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 guardrupolar interactions.The amplitude of these shifted spectra depends on the RF field distrbution over the sample and on the amplitude of the RF pulse precending the observation of the FId.On the other hand,it is independent of ths spinning frequency.The quantitative description of the effect necessitates the use of the Reciprocity Theorem.Following a simplified account of the relevant theory,experimental illustrations of the 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.
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