Analysis of diffusion coefficient distributions in humic and fulvic acids by means of diffusion ordered NMR spectroscopy

The use of the computer program CONTIN to analyze pulsed-field gradient NMR (PFG-NMR) data for several standard humic and fulvic acids is described. A n advantage of PFG-NMR analysis is that integration of different spectral r egions provides a picture of how the diffusion coefficients vary with funct ional group composition for a given sample. Using prior knowledge of the sa mple and the principle of parsimony, CONTIN approximates a solution to the inverse Laplace transform applied to the decay of peak intensity with gradi ent area in the PFG-NMR experiment. Thus, a continuous distribution of diff usion coefficients is resolved for the polydisperse humic and fulvic acids. The results of the CONTIN analyses are in the form of a distribution funct ion and a two-dimensional DOSY plot. The 2D DOSY spectrum displays chemical shifts along one axis and diffusion coefficients along the other, while a number-average diffusion coefficient, D-N, a weight-average diffusion coeff icient, D-W, and a most probable diffusion coefficient, D-p, are realized f rom the diffusion coefficient distribution. For all spectral regions of eac h humic sample, D-w was greater than D-N, which in turn was greater than or equal to the D-p, suggesting that the diffusion coefficient distribution i s weighted toward smaller, more rapidly diffusing molecules. Polydispersiti es, estimated from the ratio D-W/D-N, were less than the reported M-W/M-N v alues for similar humic substances. Thus, the D-W/D-N ratio obtained by CON TIN analysis of PFG-NMR data can be at least a qualitative, and at best a s emiquantitative, indication of the polydispersity of the humic sample, but should not be used as a quantitative measure of polydispersity.