Strategies for Improving Quantitative NMR Diffusion and Relaxation Measurements

Diffusion and magnetic nuclear relaxation are important for the study of molecular motion and structure, paramagnetism, solution dynamics, and resolution of mixtures.

Thermal convection and radiofrequency (rf) inhomogeneity adversely affect the accuracy of diffusion and relaxation. Rf inhomogeneity leads to partial excitation of signals in sections of the sample leading to incomplete refocusing and distortion of relaxation decays and diffusion intensities. Convection, combined with rf inhomogeneity leads to nuclei being excited then being transported to a region with different rf and gradient intensity before a refocusing pulse is applied, increasing the apparent decay rate.

The intensity of signals in a DOSY experiments are strongly influenced by relaxation. To obtain a quantitative determination of the amount ofmixture components, DOSY must be combined with relaxation measurements. Quantitative DOSY is especially sensitive to rf inhomegeneity and convection. The use of spacially volume-restricted samples improves the accuracy by improving rf homogeneity and reducing thermal convection.