Structure-diagnostic ion molecule reactions with environmental pollutants studied by theory and experiment

Amiri, Roshanak (2023) Structure-diagnostic ion molecule reactions with environmental pollutants studied by theory and experiment. Masters thesis, Memorial University of Newfoundland.

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Abstract

Atmospheric pressure chemical ionization (APCI) is widely used as a soft ionization technique for mass spectrometry. The APCI source acts not only as an ionization source, but also a reaction chamber for ion-molecule reactions. In this study, we investigated ion-molecule reactions between oxygen and two groups of environmental pollutants that are selective towards toxic isomers. The compounds of interest were tricresyl phosphates (TCPs) and tetrachlorodibenzo-p-dioxins (TCDDs). We propose the mechanisms of the ion molecule reactions of TCPs and TCDDs and provide support through computational and experimental analysis using density functional theory (DFT) and cyclic ion mobility-mass spectrometry (cIM-MS), respectively. Ortho-substituted isomers of TCPs and their toxic metabolites (e.g., CBDP: cresyl saligenin phosphate) can cause neurotoxic effects in humans. When TCP is introduced to an atmospheric pressure chemical ionization (APCI) source using gas chromatography (GC), abundant radical cations M˙⁺ are formed by charge exchange. The mass spectrum of an ortho-substituted isomer displays two intense peaks that are absent in the spectra of non-ortho-substituted isomers, leading us to propose a structure-diagnostic ion-molecule reaction between ions M˙⁺ and ozone. The mechanism proposed in this thesis consists of a multi-step reaction starting with the rearrangement of the molecular ion to a distonic isomer followed by an oxidation step and then, decomposition into [CBDP-H]⁺. This proposal is consistent with the results obtained from a series of isotopically-labelled analogues. Cyclic ion mobility experiments with a triorthocresyl phosphate standard, reveals the presence of at least two hydrogen shift isomers of the product ion [CBDP-H]⁺ that are connected by a low-lying energy barrier. The selectivity of the ion-molecule reaction towards the ortho-substituted cresyl groups in TCP structures provides us with an identification tool that differentiates potentially toxic and non-toxic tri-aryl phosphate esters present in complex mixtures of isomers that are produced in large volume by industry. TCDDs are infamous for their toxicity and persistence in the environment after being generated from the combustion of polychlorinated compounds. The analysis of (mixed) halogenated dibenzo-p-dioxin isomers is challenging due to the limitations of traditional separation techniques and the paucity of authentic standards. Hard ionization techniques, including EI, are not informative due to a lack of isomer selective fragmentation. However, when APCI is used in negative mode, TCDD isomers undergo isomer selective bond cleavages. Previous experimental studies have reported on two selective ion-molecule reactions between TCDDs and O₂. First, the oxidation reaction resulting in [M-Cl+O]⁻ ions, and second, the ether cleavage reaction resulting in a radical anion and a neutral product. In this thesis, mechanisms are proposed for both oxidation and ether cleavage reactions using Density Functional Theory (DFT) calculations. We also calculated theoretical collision cross section (CCS) values for the ether cleavage products using MobCal-MPI to support further studies on separating their isomeric structures using cyclic-ion mobility. These mechanisms will guide the eventual development of experimental methods that can differentiate between (potentially) toxic and non-toxic mixed halogenated dibenzo-p-dioxins.

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