Accurate measurement of translational diffusion coefficients: A practical method to account for nonlinear gradients

For MMR probes equipped with pulsed field gradient coils, which are not opt imized for gradient linearity, the precision and accuracy of experimentally measured translational diffusion coefficients are limited by the linearity of the gradient pulses over the sample volume. This study shows that the a ccuracy and precision of measured diffusion coefficients by the StejskaI-Ta nner spin-echo pulsed field gradient experiment can be significantly improv ed by mapping the gradient z-profile and by using the mapped calibration pa rameters in the data analysis. For practical applications the gradient dist ribution may be approximated by a truncated linear distribution defined by minimum and maximum values of the gradient. By including the truncated line ar gradient distribution function in the Stejskal-Tanner equation, the syst ematic deviation between the fitted curve and the experimental attenuation curve decreases by an order of magnitude. The gradient distribution may be calibrated using an intense NMR signal from a sample with a known diffusion coefficient. The diffusion coefficient of an unknown sample may then be de termined from a two-parameter fit, using the known gradient distribution fu nction, (C) 2001 Academic Press.