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

Abstract

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.

Determining the molecular weight, aggregation, structures and interactions of natural organic matter using diffusion ordered spectroscopy

“…For the NOM, CONTIN, which can deal with continuous distributions of diffusivities, is the most appropriate when the overall distribution of diffusion coefficients is required. Morris et al 57 carried out a detailed study on humic materials and demonstrated the application of CONTIN to probe the polydispersivity of humic material. However, the drawback associated with CONTIN is that it tends to 'over-smooth' the diffusion data and separation is rarely observed in the 2-D DOSY plot for NOM.…”

“…8(F)]. It may be possible to extract semiquantitative information from such projections 57 and in this example it is clear that even with a WS-FSFA/Cd ratio as small as 1 : 20 by weight, the majority of the Cd ions undergo exchange to some extent with the WS-FSFA components. Such findings have strong implications for understanding the behavior and toxicity of cadmium in the environment.…”

Interactions between natural organic matter and organic pollutants as revealed by NMR spectroscopy

“…[106] As discussed by Simpson, [47] there are different methods to process DOSY spectra. The CONTIN technique was introduced by Morris et al [107] in NOM studies as a means to solve the inverse of the Laplace transform and provide a distribution of diffusivities for a number of soil fulvic acid and HA. [107,108] Thus, for mixtures containing NOM and a contaminant, DOSY-NMR has the potential to map the changes in diffusion of both the contaminant and the surrounding molecules, thus providing a direct evidence of the occurred interactions.…”

“…The CONTIN technique was introduced by Morris et al [107] in NOM studies as a means to solve the inverse of the Laplace transform and provide a distribution of diffusivities for a number of soil fulvic acid and HA. [107,108] Thus, for mixtures containing NOM and a contaminant, DOSY-NMR has the potential to map the changes in diffusion of both the contaminant and the surrounding molecules, thus providing a direct evidence of the occurred interactions. [98] However, analogously to relaxation parameters, also in case of self-diffusion, one has to take into account that the self-diffusion measured by DOSY-NMR corresponds to an intermediate value between those of both the free and bound ligands.…”

A New Two-Dimensional Pulse Sequence for T2* Measurements of Protons in 13C Isotopomers

“…NMR is a valuable tool for the quantitative analysis of mixtures of organic compounds covering a wide range of molecular sizes such as humic substances (1)(2)(3) or mineral oils (4). In these systems, NMR delivers results unobtainable by other spectroscopic or chemical techniques.…”