Diffusion Measurements by Magnetic Field Gradient Methods

Abstract

The development and application of magnetic field gradient methods for diffusion measurements are reviewed in detail from the theoretical foundations to the latest experimental methods. The development leads from the introduction of diffusion terms into the Bloch equations and the use of magnetic gradient pulses to the design of spin echo and stimulated echo pulse sequences for the measurement of diffusion. Attention is devoted to pulse sequences for heterogeneous samples, modulated gradients, radio field frequency gradients, and multiquantum experiments. Diffusion-ordered NMR spectroscopy (DOSY) experiments are also discussed along with the associated single-channel and multivariate analysis methods. Recent advances in this area include pulse sequences that suppress the effects of convection currents, heteronuclear DOSY experiments, and a variety of three-dimensional DOSY experiments designed to enhance the spectral resolution. Related studies of chemical exchange reactions, restricted diffusion, and anisotropic diffusion are described. The review ends with a brief discussion of the use of very large magnetic field gradients for the measurement of small displacements, and a consideration of polydispersity and size distributions that can be determined with diffusion measurements. Keywords: diffusion; PFG; DOSY; heterogeneous; restricted; anisotropic; exchange; polydispersity