Molecular Characterization of Marine and Terrestrial Dissolved Organic Matter Using Ultrahigh Resolution Mass Spectrometry

Various analytical techniques have been employed to probe the chemical identity and characteristics of complex Dissolved Organic Matter (DOM) mixtures. With continuing advances in readily available highly developed mass spectrometers, the amount of information generated for analysis is steadily rising. Currently, Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) at high magnetic field (> 9 Tesla), is the only advanced analytical technique capable of ultrahigh resolution and mass accuracy that can distinguish upwards of 10,000 spectral peaks. This technique facilitates the identification of thousands of unambiguous molecular formulae for complex DOM. The most commonly used ionization method, Electrospray Ionization (ESI), has proven to be an excellent source for DOM ionization prior to MS analysis. ESI coupled to FT-ICR-MS provides an ideal combination necessary for investigating and characterizing DOM from unique natural environments. 9 Tesla), is the only advanced analytical technique capable of ultrahigh resolution and mass accuracy that can distinguish upwards of 10,000 spectral peaks. This technique facilitates the identification of thousands of unambiguous molecular formulae for complex DOM. The most commonly used ionization method, Electrospray Ionization (ESI), has proven to be an excellent source for DOM ionization prior to MS analysis. ESI coupled to FT-ICR-MS provides an ideal combination necessary for investigating and characterizing DOM from unique natural environments. DOM represents the largest reservoir of organic carbon stored in the oceans. Its source has been proposed to originate from marine primary and bacterial production, with limited land-derived contributors. Marine DOM is a complex mixture of biomolecules that either exist naturally or have been transformed from living and decaying organisms in the ocean. Previous research has identified only a small portion of deep sea DOM, due to the complexity of the mixture and lack of advanced techniques available; however, ultrahigh resolution mass spectrometry has succeeded for DOM characterization where other techniques have failed. In Chapter 3, we compare ultrahigh resolution mass spectra of marine DOM isolated from two sites in the Weddell Sea (Antarctica) using ESI and Atmospheric Pressure Photoionization (APPI). These spectra, obtained on a 9.4 Tesla FT-ICR-MS, indicate the two ionization techniques are complementary. Ions produced by APPI extend to higher carbon undersaturation compared to ESI, indicated by higher double-bond equivalence minus oxygen (DBE-O) values, while ions in the ESI spectra are more oxygenated. Moreover, many sulfur-containing compounds were efficiently ionized by ESI but not detected by APPI. These results show that the differences in mass spectra obtained by ESI and APPI FT-ICR-MS are significant and that both are necessary to obtain a complete description of the molecular composition of marine DOM. Peatlands are extraordinary carbon reservoirs due to their sequestration and emission of greenhouse gases. Chapter 4 depicts the importance of investigating the molecular characterization of terrestrial DOM from the Glacial Lake Agassiz Peatlands (GLAP) of northern Minnesota, to reveal the potentially pivotal role it plays in global carbon cycling. ESI-FT-ICR-MS was used to identify the qualitative differences between DOM in fen and bog porewaters of the Red Lake II system in the GLAP. Approximately 80% of molecular composition observed in surface porewater was maintained throughout the bog profile (0.17 to 2.50m). The qualitative stability of the molecular composition of DOM was accompanied by a quantitative increase in Dissolved Organic Carbon (DOC) with depth. The composition of DOM in the fen was significantly different at depth with slightly varying DOC levels. Using Aromaticity Index (AI) values we identified condensed aromatic phenol-type compounds in the porewaters of both peatlands. Surface bog and deep fen DOM had surprising similar molecular compositions. We suggest that enzymatic degradation via phenol oxidase and slower hydrologic transport down the bog vertical profile are responsible for the observed variations in DOM composition. In Chapter 5, molecular composition and optical properties were correlated for two samples of DOM from different peat formations in the GLAP. Fen and bog DOM were analyzed using 9.4 T FT-ICR-MS to determine the aromatic content as a function of depth. UV/Vis absorbance and Excitation Emission Matrix Fluorescence Spectroscopy (EEMS) were used to identify changes in the optical properties associated with the chromophoric fractions of DOM (CDOM). Higher specific UV absorbance (SUVA) at 254 nm indicated more abundant aromatic content for surface bog and deep fen DOM. EEMS results were also found to be in agreement with the absorption spectra and molecular characterization as determined by FT-ICR-MS. The strong correlations we have observed suggest that optical spectroscopy techniques represent an effective surrogate approach to characterizing DOM provided some detailed molecular information is available for calibrating the observed correlations. Finally, comparative analysis of sample preparatory methods for ESI-FT-ICR-MS is presented for terrestrial DOM in Chapter 6. Freeze drying and solid phase extraction using a modified styrene divinyl benzene polymer sorbent (Varian PPL) were considered. Molecular composition was determined using a 9.4Tesla ESI-FT-ICR-MS for Red Lake II fen and bog DOM of the GLAP. 78% of the DOM composition was found to be common to both freeze dried and SPE Bog 0.17m. The unique SPE Bog 0.17m molecular formulas were characterized by higher aromaticity index values and higher DBE-O values which correspond to more aromatic and condensed structures. Similar results were observed for Fen 2.50m DOM. Mass spectral comparisons of SPE DOM and salt water treated SPE DOM produced 93% and 94% common molecular formulas for Fen 2.50m and Bog 0.17m respectively. No outstanding signature originating from the SPE cartridge or by saltwater contributions was identified. The results emphasize the ability of SPE to elute a more representative and effectively prepared DOM sample for high resolution ESI-FT-ICR-MS.