Environmental NMR : diffusion ordered spectroscopy methods

Environmental samples generally contain a mixture of solutes of different but unknown sizes. Information on both the nature and the distribution of the species present is of the utmost importance. Diffusion provides a direct connection to molecular weight and intermolecular interactions. Thus, being able to measure diffusion provides a powerful means of investigating environmental samples of interest – especially when they contain multiple species. This article begins with a brief review of some basic concepts and relations for translational diffusion. Then, how pulsed-gradient spin-echo (PGSE) NMR can be used to measure diffusion is considered including some practical issues. PGSE NMR can measure diffusion over an enormous range of diffusion coefficients from the very high (e.g., in gas phase) to the very low (e.g., that of large polymers). In contrast to other analytical techniques, the PGSE NMR technique requires almost no sample preparation and is noninvasive. Further, as most isotopes contain an NMR-sensitive nucleus, the technique is applicable to just about any species. Applications of PGSE NMR diffusion measurements to environmental problems are then considered with a final section on future developments in the field.