Analysis of Diffusion Coefficient Distributions in Humic and Fulvic Acids by Means of Diffusion Ordered NMR Spectroscopy

• Published in: Analytical chemistry (Washington) 1999-12, Vol.71 (23), p.5315-5321
• Main Authors: Morris, Kevin F, Cutak, Benjamin J, Dixon, Ann M, Larive, Cynthia K
• Format: Article
• Language: English
• Subjects: Acids; Analysis; Analysis methods; Analytical chemistry; Applied sciences; Chemistry; Diffusion index; Exact sciences and technology; Pollution; Scientific imaging; Soil and sediments pollution; Spectrometric and optical methods
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac9907585

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. An advantage of PFG-NMR analysis is that integration of different spectral regions provides a picture of how the diffusion coefficients vary with functional group composition for a given sample. Using prior knowledge of the sample and the principle of parsimony, CONTIN approximates a solution to the inverse Laplace transform applied to the decay of peak intensity with gradient area in the PFG-NMR experiment. Thus, a continuous distribution of diffusion coefficients is resolved for the polydisperse humic and fulvic acids. The results of the CONTIN analyses are in the form of a distribution function 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 coefficient, D W, and a most probable diffusion coefficient, D P, are realized from the diffusion coefficient distribution. For all spectral regions of each 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 is weighted toward smaller, more rapidly diffusing molecules. Polydispersities, estimated from the ratio D W/D N, were less than the reported M W/M N values for similar humic substances. Thus, the D W/D N ratio obtained by CONTIN analysis of PFG-NMR data can be at least a qualitative, and at best a semiquantitative, indication of the polydispersity of the humic sample, but should not be used as a quantitative measure of polydispersity.