Gabriela Leu

Amplitude modulation and relaxation due to diffusion in NMR experiments with a rotating sample

Abstract The simultaneous effects of diffusion and coherent averaging by magic angle sample spinning (MASS) are investigated both theoretically and experimentally for spins diffusing in a spatially varying susceptibility field. The diffusion introduces a periodic modulation and a relaxation of the magnetization amplitude. The attenuation exponent for the n th echo at an early time is given by D 0 γ 2 g 2 πn /(4 ω r 3 ), where ω r is the rotor frequency, D 0 the diffusion coefficient, γ is the gyromagnetic factor, and a factor g 2 which is related to, but different from, the volume average of the gradient squared.

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 composite 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 an 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 independent of position and time and the phase is a periodic function of the rotor frequency νr. In fluids, the amplitude of a spin packet is a function of its position and time. The amplitude modulation and relaxation in diffusive MASS encodes the dynamics of motion and the landscape (geometry of pores and field gradients) probed by the motion. Here we use spin manipulation—total suppression of sidebands (TOSS)—to suppress the effects of phase with the goal of isolating the amplitude term. By t...

Detection of motion through susceptibility fields in two-dimensional exchange diffusive-MASS experiments

Abstract We show that the off-diagonal coherence peaks in two-dimensional Fourier transform NMR spectroscopy of fluids contained in porous media undergoing magic angle sample spinning (MASS) arise from amplitude modulation of the fluid’s magnetization. The amplitude modulation originates from the combined effect of MASS and the molecular diffusion through the inhomogeneous magnetic fields created by the susceptibility contrasts in the porous medium. The magnitude of the off-diagonal peaks provides information on the porous medium’s structural length scales, which give rise to correlation length scales of the magnetic susceptibility.

Selectively observing the amplitude modulation under magic angle sample spinning

Magic angle sample spinning (MASS) averages the inhomogeneous magnetic field due to the susceptibility contrast in porous media by modulating the local magnetic field (frequency modulation). Molecular diffusion introduces a homogeneous broadening, which modulates the amplitude of the signal (amplitude modulation). The depth of amplitude modulation is determined by the interplay of molecular diffusion and MASS averaging and contains rich information on local magnetic fields, through which the spatial structure of the sample may be obtained. In this paper, we present two methods to quantify the amplitude modulation: a phase suppressed method and a more conventional two-dimensional (2D)-exchange method. The phase suppressed method directly observes the amplitude modulation in the time domain. The approximate equations are derived to extract the physical information from the time domain data. The conventional 2D-exchange spectrum contains both the frequency modulation and the amplitude modulation terms. The a...

A new probe of pore geometry: diffusive MASS NMR

NMR methods are widely used to probe the structure and fluid dynamics of porous materials, as they are uniquely suited to these studies since NMR records the correlation of changing local magnetic fields over a time scale of ns to seconds. The local magnetic fields are established by local variations in the bulk magnetic susceptibility of the sample (and so are directly tied to the samples local structure). The fluctuation in field that a spin sees is due to molecular transport (including molecular diffusion) through these local fields, and so reports on the length scales of structures and impediments to transport. We have developed a new set of methods DIFFUSIVE-MASS to provide a means of systematically varying the effective time scale of the measurement and thus the effective length scale. This new handle permits a detailed, microscopic picture of the structure and dynamics. Diffusive MASS NMR methods will permit a systematic set of methods and analysis for characterizing the chemistry, structure and fluid dynamics of the mobile phase in porous materials. The approach will be applicable to any diamagnetic material. In particular, the industry of oil discovery depends on understanding heterogeneous porous media.