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Document Type

Thesis - University Access Only

Award Date

1998

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

Abstract

Fulvic acid is a complex mixture that has many roles in the chemistry and geochemistry of natural environments. Although a variety of average chemical characteristics are known, its exact structural composition has remained elusive because of its heterogeneous nature. While mass spectrometry has proven to be a powerful analytical technique for the structural identification of the components of mixtures, it has been traditionally limited to the study of low-mass, volatile materials because the sample is required to be in the gas phase for analysis. Recently, however, the development of novel ionization techniques such as desorption ionization methods and electrospray ionization has led to the study of high-mass materials. In this report the application of laser desorption (LD) ionization and ESI to the mass spectrometric analysis of FA is presented. The mass spectrometry studies reported here were performed on Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers which are capable of ultrahigh resolution and sensitivity. Desorption wavelength was found to significatly affect the laser desorption Fourier-transform mass spectrometry of fulvic acid. At infrared desorption wavelengths (10.6μm and 1.06μm) where conditions favor thermal desorption mechanisms the spectra appeared similar with number average molecular weights (Mn) ranging from 400-600 daltons in the positive 10n mode. Desorption under high power density conditions resulted in significantly different positive-ion distributions for the FA samples (200-300Da at 1.06 μm). Under these conditions it appears that shock-wave type desorption mechanisms dominate. At 355 nm, the ability of the fulvic acid samples to absorb the incident wavelength increased the probability of fragmentation to such an extent that positive-ion distributions were determined to not be representative of the sample. Positive-ion formation in the LD experiment was found to occur predominantly through cationization of desorbed FA neutrals. Negative-ion LD mass spectra of the FA samples resulted in Mn values ranging from 350-580Da under thermal desorption conditions and 240-480Da under shock-wave driven desorption conditions. Negative-ion formation was favorable for the FAs under all LD conditions employed and most likely resulted from proton loss at carboxylic acid sites. Electrospray ionization studies resulted m the highest positive-ion mass-to-charge distributions ever observed for FA by mass spectrometry (1000-2400Da). Solution conditions were found to affect the mass-to-charge distributions for the ions as well as the ability to detect the ions. Evaluation of solution conditions determined that the optimum spray conditions for these samples was a 70% H2O:30% MeOH (v:v) solution. The effect of solution variables such as pH and ionic strength that are known to influence the inter and intramolecular associations of FA were evaluated, but in most cases could not be determined due to the formation of solvent-salt adduct ions that interfered with ion measurement. Negative-ion ESI analysis of the FA samples resulted in significantly lower Mn values (570-700Da). Multiply charged ions were determined to be present in the negative-ion distribution and as a result the calculated molecular weight averages for these spectra are not representative of the molecular masses of the FA components. The formation of such multiply-charged negative-ions in aqueous spray solutions reflects the polycarboxylic acid nature of FA. Positive-ion ESI studies of methylated FA samples resulted in lower mass-to-charge distributions and suggest that the FA ions measured in the positive-ion mode are due, to some extent, to intermolecular aggregates. In the methylated ion distributions, which ranged from 200-2200Da, it was determined that there were at least four singly-charged molecular components at each nominal mass inspected. The complexity of these distributions inhibited the application of MSn experiments for structural analysis, but accurate mass determinations were performed for several samples. To simplify the molecular composition, a two-stage silica gel fractionation was used. All the isolated fractions were found to be similar in mass-to-charge distribution, as well as molecular complexity, to the unfractionated FA precursor.

Library of Congress Subject Headings

Fulvic acids

Fourier transform spectroscopy

Cyclotron resonance

Format

application/pdf

Number of Pages

240

Publisher

South Dakota State University

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