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Document Type

Thesis - University Access Only

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Degree Name

Doctor of Philosophy (PhD)

Department / School



Fulvic acid is a natural occurring mixture of organic molecules that has proven difficult to characterize by most methods. This has been particularly true for the unambiguous determination of the molecular weight distribution of the components that comprise it. Two methods, gel filtration chromatography (GFC) and vapor pressure osmometry (VPO) have been extensively used. Both of these methods measure molecular weight distributions in aqueous solutions. It is believed that the components of FA undergo aggregation in aqueous solutions and consequently would yield molecular weight distributions that are significantly larger than that which FA’s components actually display. The purpose of this research was to reevaluate the molecular weight distribution of fulvic acid and test the hypothesis that fulvic acid is comprised of lower-molecular weight components than previously reported in the literature. To reevaluate the molecular weight distribution of fulvic acid the molecular weight distribution of standard fulvic acids obtained by the traditional GFC and VPO methods were compared to that obtained by fast atom bombardment, continuous-flow fast atom bombardment and laser-desorption Fourier-transform mass spectrometry (LD-FT MS). The results obtained by GFC and VPO yielded molecular weight distributions consistent with those obtained in the literature. Despite substantive experimental efforts the fast atom bombardment mass spectrometry experiments did not yield molecular weights of the fulvic acid samples. An explanation for this negative result is proposed. Laser-desorption fourier-transform mass spectrometry yielded molecular weight distributions that are less by as much as 50% of the values as obtained by GFC and VPO for most samples. These results will be used to provide a revised picture of FA. The molecular weight distribution of fulvic acid was also evaluated in the presence of multivalent cations by GFC. The presence of divalent cations at typical environmental concentrations caused the molecular weight distribution to double.

Library of Congress Subject Headings

Fulvic acids

Molecular weights -- Measurement



Number of Pages



South Dakota State University