Field modulation effects induced by sample spinning: application to high-resolution magic angle spinning NMR.

High-resolution magic angle spinning (HRMAS) has become an extremely versatile tool to study heterogeneous systems. HRMAS relies on magic angle spinning of the sample and on pulse sequences originally developed for liquid state NMR. In most cases the outcome of the experiment is conform to what is expected from high-resolution liquid state NMR spectroscopy. However in some instances, experiments run under MAS can produce some very puzzling results. After reviewing the basic hardware which is at the heart of HRMAS spectroscopy, we show that the origin of this behavior lies in the natural time-dependence of some physical quantities imparted by the rotation. We focus in particular on the effects of B1 inhomogeneities on the nutation, the (90 degrees)+x-t-(90 degrees )-x and the MLEV16 experiments. Different models of radiofrequency distribution of B1 fields in a solenoidal coil are derived from simple geometrical considerations. These models are shown by NMR spin dynamics calculations to reproduce the experimental NMR results. They are also consistent with electromagnetic simulations of the B1 field distribution inside a solenoidal coil.