Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
Browse
2 results
Search Results
Item INVESTIGATION OF THE PRODUCTION AND DECAY PATHWAYS OF SUPEROXIDE BY CHROMOPHORIC DISSOLVED ORGANIC MATTER(2022) Le Roux, Danielle Marie; Blough, Neil; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chromophoric dissolved organic matter (CDOM) in natural waters absorbs sunlight which leads to the production of a suite of reactive intermediates and reactive oxygen species (ROS) such as superoxide (O2⦁-) and hydrogen peroxide (H2O2). A significant amount of research over the years has investigated the sources and sinks of these two ROS. The currently accepted sequence of reactions for their production involves photochemically produced one-electron reductants (OER) within CDOM reacting with dissolved oxygen to form O2⦁-, which undergoes self-dismutation to produce H2O2. A previously used method to detect radical species with CDOM has been modified herein to be conducted simply using a fluorometer. Production rates of OER and H2O2 were measured for a variety of samples and correlations between the rates and optical/structural properties of the samples indicate that lower molecular weight species produce more OER and H2O2. Based on the stoichiometry of the mechanism above, the ratio of the production rate of OER to that of H2O2 should be two. However, ratios from five to sixteen were obtained, which suggests that O2⦁- undergoes oxidative reactions that compete with dismutation. The possibility of a light-dependent pathway for O2⦁- decay has been proposed but had yet to be explicitly demonstrated. Herein this sink is directly shown through O2⦁- spiking experiments. Rapid consumption of the O2⦁- spike occurs if injected into a sample during irradiation, as compared to a spike introduced into the sample in the dark, suggesting the presence of a light-dependent sink. Extensive data analysis and kinetic modeling of the O2⦁- decay data has allowed for approximations as to the extent of the sink and its decay rate constant. O2⦁- and H2O2 are environmentally important species, and a significant amount of work has been done on modeling their concentrations in natural waters. Based on the work here, O2⦁- is produced at higher concentrations than previously believed, which has implications on the modeling of O2⦁- and H2O2 in natural waters. Additionally, the light-dependent oxidative sink of O2⦁- could be with moieties within CDOM, providing further insight to the photochemical transformation of DOM during transit from terrestrial sources to marine waters.Item A Deuterium Labeling Method for the Characterization of (Chromophoric) Dissolved Organic Matter Using Ultrahigh Resolution Electrospray Ionization Mass Spectrometry(2015) Baluha, Daniel Robert; Blough, Neil; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dissolved organic matter (DOM) is a complex ensemble of naturally occurring organic compounds found in virtually all aquatic environments. The overwhelming diversity of DOM makes it extremely difficult to understand the relationship between its bulk physicochemical properties and its molecular structure and composition. This dissertation describes the development of a novel method to identify ketone/aldehyde-containing species within DOM, which are known to contribute substantially to the ultraviolet/visible (UV-vis) absorption and emission of chromophoric DOM. In this method, an aqueous sample is treated with sodium borodeuteride (NaBD4) and is analyzed via ultrahigh resolution electrospray ionization (ESI) mass spectrometry. Ketone/aldehyde-containing species (at mass m) in the untreated sample are identified by searching the mass spectrum of the reduced sample for peaks corresponding to deuterated derivatives (at mass m+3.021927n). Initial experiments demonstrated that this method reliably discriminates among mass spectral peaks in an untreated DOM sample that comprise species with zero, one, and/or two reducible moieties. The reactivity and optical properties of reducible species within Suwannee River fulvic acid (SRFA) were studied by treating an aqueous sample with several amounts of NaBD4. This study demonstrated that most species with at least one ketone/aldehyde moiety were reduced a single time under low [NaBD4], while higher [NaBD4] resulted primarily in additional reductions on multi-ketone/aldehyde species. Furthermore, the changes in UV-vis absorption and emission of the reduced aliquots relative to that of the untreated were correlated with the number of ketone/aldehyde-containing species reduced and identified by this method. The fully developed protocol was used to compare DOM extracted from several aquatic environments. Two pools of ketone/aldehyde-containing species were tentatively identified: A terrestrially-produced group of lignin/tannin-derivatives and a microbially-produced group of carboxyl-rich alicyclic molecules. While the first pool has previously been shown to contribute substantially to the absorption/emission of chromophoric DOM, the second pool most likely would not. The mass labeling method developed here revealed compositional features that are not observable by common ESI mass spectrometric analyses and may serve as a useful way to link the physicochemical properties of DOM to its structure and composition.