Manipulation of phase and amplitude modulation of spin magnetization in magic angle spinning nuclear magnetic resonance in the presence of molecular diffusion

Nuclear magnetic resonance (NMR) experiments with a spinning sample [magic angle sample spinning (MASS)] are used to remove the line broadening in com posite systems, where the susceptibility contrast of its constituents gives rise to an inhomogeneous field that causes a line broadening and obscures chemical information. The NMR signal in these experiments has a phase and a n amplitude part. In the absence of diffusion, i.e., in the MASS spectra of solids, the amplitude of the signal from an isochromat is a constant indep endent of position and time and the phase is a periodic function of the rot or frequency nu (r). In fluids, the amplitude of a spin packet is a functio n of its position and time. The amplitude modulation and relaxation in diff usive MASS encodes the dynamics of motion and the landscape (geometry of po res and field gradients) probed by the motion. Here we use spin manipulatio n-total suppression of sidebands (TOSS)-to suppress the effects of phase wi th the goal of isolating the amplitude term. By the TOSS sequence the phase factor at time t for a spin packet at an azimuthal angle phi is made to de pend on phi only as a function of omega (r)t-phi, which suppresses the side bands in solids upon an integration over phi. Due to molecular diffusion, t he amplitude part depends on phi, and, thus, diffusive TOSS cannot suppress the sidebands. The residual sidebands carry the information of dynamics an d pore and magnetic field geometry, in addition, by reducing the size of th e sidebands, TOSS is of course, also useful in identifying various fluid co mponents in situ. The diffusive MASS gives a measure of the spread in local fields and diffusive TOSS gives a measure of the spread in local gradients . (C) 2001 American Institute of Physics.