Diffusional Attenuation

Abstract

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Molecular diffusion has a significant impact on the outcomes of NMR experiments featuring magnetic field gradients. This impact can be due to either imperfect static magnetic field homogeneity or the use of pulsed field gradients. In both cases, diffusion will result in an attenuation of the measured signal. Although this phenomenon has been extensively exploited and has led to numerous applications, in many cases it is undesirable. Moreover, diffusional attenuation can also interfere in more complex ways than just affecting sensitivity, such as introducing additional line broadening, or affecting the accuracy of experiments intended to measure relaxation time constants. Even in experiments intended for the measurement of diffusion, additional diffusional attenuation caused by imperfect magnetic field homogeneity can affect accuracy. In this article, a general procedure is introduced that allows the calculation of diffusional attenuation for nearly any pulse sequence. After a general introduction to the theoretical background, this procedure will be demonstrated on various case studies, exploring how diffusional attenuation can be determined for a pulse sequence. This will also serve to illustrate how pulse sequences can be designed to avoid, as far as possible, the undesirable effects of diffusion.