Type of Document Dissertation Author Tremblay, Lori Beth Author's Email Address firstname.lastname@example.org URN etd-01042007-190407 Title The Ultrahigh Resolution Mass Spectrometry of Natural Organic Matter from Different Sources Degree Doctor of Philosophy Department Chemistry and Biochemistry, Department of Advisory Committee
Advisor Name Title Alan G. Marshall Committee Member Jeffrey Chanton Committee Member John G. Dorsey Committee Member Timothy Logan Committee Member William T. Cooper Committee Member Keywords
- Electrospray Ionization
- Natural Organic Matter
- Mass Spectrometry
Date of Defense 2006-09-25 Availability unrestricted AbstractGas phase mass spectral analysis is now showing great promise in the characterization of natural organic matter. With the routine availability of the soft electrospray ionization (ESI) technique, volatilization and ionization of a sample is no longer a problem. Also, resolving power and mass accuracy of the mass spectrometers has increased greatly. Electrospray ionization combined with ultrahigh resolution Fourier transform-ion cyclotron resonance mass spectrometry at 9.4 Tesla has been shown to be an ideal tool for the molecular characterization of dissolved natural organic matter. Currently, high-field FT-ICR MS is the only technique capable of resolving and identifying individual elemental compositions in these complex mixtures. With the use of internal calibrants, the two techniques combined provided an ideal tool that was used to characterize organic matter from several different natural environments.
Leachates and dissolved organic matter (DOM) from forest floor woody debris are important mediators in metal cycling and mineral dissolution, microbial activity, and soil organic matter dynamics. A great deal of research has been directed at characterizing the chemical composition and molecular weight range of forest floor dissolved organic matter to better assess and predict its reactivity. Unfortunately, the chemical and structural properties of the dissolved components are extremely difficult to evaluate because of their chemical complexity and the wide variations in polarities and molecular weight. Direct analysis of DOM by high-field Fourier transform-ion cyclotron resonance mass spectrometry with electrospray ionization (ESI FT-ICR MS) was recently applied to a limited number of environmental organic matter samples. In Chapter 3 we tested the potential application of this technique to woody decay dynamics. Leachates from brown rot fungi (Gloeophyllum trabeum) degraded red spruce (Picea rubens), equivalent to a late stage decay, were analyzed using 9.4 Tesla ultra-high resolution FT-ICR mass spectrometry. The FT-ICR mass spectra revealed molecules at every nominal mass from approximately 250 to 1100 Daltons, some 6000 individual compounds in all, as well as molecular families containing ions that differ from each other in degree of saturation (number of double bonds), oxygen atoms and methylene units. Fungal decay did not appear to change the overall molecular weight range of leachable organic matter relative to that extracted from undegraded controls. However, a shift in location of the center mass to an average lower molecular weight and a modification of overall compound chemistry into molecular groupings that appear to be more structurally related was evident. Of significance also was the direct assessment of progressive demethylation (loss of CH2 units) among the full molecular weight spectrum of brown rot decayed leachate consistent with known fungal activity. Assessment of fine scale chemical properties of DOM can be obtained by this technique and used to project molecular dynamics in natural and laboratory systems.
In Chapter 4 the ultrahigh resolution FT-ICR technique was applied for tracing DOM as it moves from a terrestrial porewater into a Brazilian estuary. Coastal outwelling from intertidal zones, particularly in Northern Brazil, is one of the most important sources of DOM to the ocean. In this study the molecular composition of DOM in a mangrove porewater and two sites within an adjacent estuary by FT-ICR MS was characterized to assess the changes that occur in the initial stages of outwelling. Mass spacing patterns were found to be far more pronounced in the mangrove porewater DOM. Comparisons of molecular features of DOM from the three sampling sites based on double bond equivalents and Kendrick mass analysis normalized for both methylene (-CH2-) and oxygen (O) substitution indicated that the average molecular weights of estuarine DOM were lower than the average molecular weight of porewater DOM, due primarily to high molecular weight, highly unsaturated and/or aromatic components in the porewater DOM. Photodegradation was suggested as the mechanism that removes these high molecular weight components from porewater DOM once it reaches the estuary. The mass analyses were supported by UV-vis and 3-D synchronous scanning fluorescence spectroscopy measurements to aid in the characterization of DOM in this unique ecosystem.
Peatlands are significant carbon sinks. Carbon storage occurs because the peatlands are relatively undisturbed, and they promote very slow organic matter decomposition. They exist as the result of many sensitive feedbacks between hydrology and vegetation and are thusly responsive to disruptions in climate conditions. Persistent shifts in temperature and precipitation, possibly due to human activity (the Greenhouse Effect), could affect regional groundwater flow patterns, floral assemblages, ecosystem productivity, and methane fluxes. These shifts could alter the ability of the peatlands to act as efficient carbon sinks. It is thus important to characterize these ecosystems in order to predict effects from anthropogenic sources. Peatlands also contribute significantly to the global atmospheric methane budget, and therefore, it is important to identify the transport and transformation of dissolved organic carbon. In Chapter 5 we accomplished this by studying the composition of dissolve organic carbon (DOC) in soil porewaters from a fen and bog on the molecular level using ultra-high resolution ESI FT-ICR MS. Also, as human populations in the coastal zone increase, the impact of their activities on the environment has become a major issue. Coastal zones are heavily impacted by human activity, which can affect the food webs of the shelves, resulting in economic consequences. Understanding the role that intertidal zones play in global element cycles is necessary before determining any influence humans play in global change. Finally, this chapter focuses on the differences between UV and ESI TOF MS detection in order to compare the molecular weight results yielded from the two detection techniques.
Marine dissolved organic matter, which exhibits an average age of several thousand years, contains about the same amount of carbon as atmospheric CO2 and represents one of the largest pools of organic carbon in the global carbon cycle. Most marine DOM is found in deep sea and is generally made of up of refractory compounds, while surface marine DOM consists typically of more labile components. Polar oceans may be a source of DOM to the deep ocean because they are the only places where surface waters are capable of efficiently convecting down to the oceans bottom. When salt is ejected and dense brine-enriched waters infiltrate the deep ocean, deep water formation can be directly related to sea-ice formation. Sea ice is one of the most productive marine environments in the world, and the DOM concentrations in the brine are among the highest measured in marine waters. Unfortunately, despite the importance of marine DOM, the biogeochemistry of sea ice is largely unknown.
Detailed analyses of mass spectra of DOM collected from the Weddell Sea in Antarctica obtained on the 9.4 T FT-ICR instrument at the National High Magnetic Field Laboratory (NHMFL) is provided in Chapter 6 and revealed multiple peaks at every nominal mass unit separated by as little as 0.0364 Daltons. The ultra-high resolution mass spectra also revealed the presence of molecular families containing ions that differ from each other in degree of saturation, functional group substitution, and number of CH2 and CO2 groups. Using these ultrahigh resolution mass spectra, the composition of sea ice derived organic matter was characterized for the first time on a molecular basis.
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