RESOLUTION OF DISCRETE AND CONTINUOUS MOLECULAR-SIZE DISTRIBUTIONS BY MEANS OF DIFFUSION-ORDERED 2D NMR-SPECTROSCOPY

Diffusion-ordered 2D NMR (DOSY) experiments have been developed for the analysis of mixtures. These experiments yield conventional chemical shift spectra in one dimension and spectra of diffusion coefficients (or approximate molecular radii) in the other. At the heart of this method are the analysis and display of data obtained with pulsed field gradient (PFG) NMR techniques. At each point on the chemical shift axis, a set of intensities is obtained as a function of K2 (K = gammagdelta, where gamma is the magnetogyric ratio and g and delta are the gradient pulse amplitude and duration, respectively). These diffusion-dependent data sets are inverted by means of the computer programs SPLMOD, when discrete diffusion coefficients are expected, and CONTIN, when continuous distributions are present. Since the inversion is ill-conditioned, it is necessary to introduce additional information to limit the range of solutions. In addition to assumed prior knowledge of the decay kernels and the nonnegativity of amplitudes and decay constants, a set of criteria has been constructed for the discrete analysis case that takes into account physical limits on diffusion coefficients, experimentally accessible values, and artifacts resulting from variations in effective decay kernels resulting from instrumental nonlinearities. Examples of analyses of simulated data and experimental data for mixtures are displayed. Also, 2D spectra generated by CONTIN are displayed for polydisperse polymer samples. These results demonstrate ''at a glance'' qualitative analysis of molecular size distributions in mixtures and, with proper care, quantitative determination of diffusion coefficients.