High-resolution molecular characterization of complex environmental mixtures: Aquatic dissolved organic matter and wildfire-influenced aerosol
Date of Award
Open Access Dissertation
Doctor of Philosophy in Chemistry (PhD)
Administrative Home Department
Department of Chemistry
Committee Member 1
Committee Member 2
Committee Member 3
The natural environment is replete with organic matter of varying complexities. Whether it is particulate material in the atmosphere, decades old organic matter trapped within glaciers, or biological debris flowing with rivers and streams, natural organic matter (NOM) is exquisitely complex. High-resolution mass spectrometry allows us to have a glimpse of the molecular composition of NOM and delineate the elemental compositions of thousands of chemical species that form it. In this dissertation, the overarching aim was to explore the molecular diversity of complex mixtures from two sources: Surface water and atmospheric organic aerosol. The first objective of this dissertation was to demonstrate ionization selectivity of three popular ionization methods so that the necessity of using more than one technique for untargeted qualitative analysis of complex mixtures could be validated. Electrospray ionization (ESI), atmospheric pressure photoionization (APPI), and atmospheric pressure chemical ionization (APCI) were tested on commercial humic substances in combination with the Fourier Transform - Orbitrap Elite Mass Spectrometer. Our findings provide evidence for the tendency of ESI to access polar, more oxygenated compounds that constitute a majority of humic substances. A minor fraction comprising relatively less polar, aromatic compounds, could be accessed with either APPI or APCI, highlighting the importance of employing complementary ionization methods to obtain representative molecular compositions of complex mixtures. The second objective of this dissertation was to demonstrate the extreme molecular complexity of organic aerosol collected downwind of wildfires in the Pacific Northwest of the United States. The focus was particularly on the fraction of organic aerosol that had aged to develop an abundance of tar balls (TB) that are carbonaceous spherules of extremely variable optical properties and whose detailed molecular composition is yet to be elucidated. We attempted to find a preliminary TB-specific molecular signature by comparing several TB-rich and non-TB aerosol mixtures. Using Fourier Transform - Ion Cyclotron Resonance Mass Spectrometers and complementary ionization techniques, ESI and laser desorption ionization, we present detailed molecular composition of TB, which indicates them to be a mixture of low-oxygen organic constituents enclosed in a more oxidatively aged shell.
Ijaz, Amna, "High-resolution molecular characterization of complex environmental mixtures: Aquatic dissolved organic matter and wildfire-influenced aerosol", Open Access Dissertation, Michigan Technological University, 2021.