- Polycyclic aromatic hydrocarbons (PAHs) are a group of environmental contaminants consisting of fused benzene rings. Parent-PAHs, methylated-PAHs (MPAHs), and PAHs with molecular weight of 302 a.m.u (MW302-PAHs) are considered as unsubstituted-PAHs. These unsubstituted-PAHs undergo transformation reactions resulting in the formation of PAH-transformation products (PAH-TPs), or substituted-PAHs, including nitrated-, oxygenated-, and hydroxylated PAHs (NPAHs, OPAHs, and OHPAHs, respectively). Heterocylic PAHs (Hetero PAHs) are a group of PAHs with carbons substituted by atoms such as, nitrogen, oxygen, or sulfur, and are studied with PAH-TPs. Some PAHs and PAH TPs are of concern due to their persistence in the environment. Furthermore, some PAHs and PAH-TPs are known to be toxic, mutagenic, and/or carcinogenic, signifying the importance of studying PAHs and PAH-TPs in the environment.
PAHs and PAH-TPs exist as environmental mixtures. The chemical analysis of environmental mixtures are broadly defined to fall into two categories: targeted and non-targeted analysis (NTA). In targeted analysis, environmental mixtures are characterized by the presence of a specific list of analytes. In NTA, the list of analytes are expanded to include other unknown compounds. Suspect screening analysis (SSA) is considered to be a subset of NTA because, although the list of analytes are expanded beyond specific compounds, there is a priori hypothesis of the identities of the unknown compounds. Targeted analysis, NTA, and SSA can further be combined with health assessments, and toxicity and mutagenicity tests to evaluate the potential human and environmental health implications of the mixtures.
This dissertation addresses the identification and prediction of PAH-TP formations in the environment. The first chapter discusses the background information on PAHs and PAH-TPs, as well as the various chemical and toxicological techniques used to characterize environmental mixtures. An integrated framework to identify TPs in the environment, which serves as the guideline of the analytical, toxicological, and computational works in this dissertation, is outlined in the first chapter. The second chapter discusses the research project to determine the presence of PAHs and PAH TPs in pavement sealcoat products—a complex environmental mixture—using gas chromatography with mass spectrometry (GC/MS). The study also evaluated the potential health implications of these products using the benzo[a]pyrene-equivalent concentration assessment, Ames mutagenicity assay (Salmonella typhimurium), and embryonic zebrafish (Danio rerio) developmental toxicity test. This work demonstrated an example of targeted chemical analysis, in combination with the toxicological assessments.
While GC/MS is an important tool for targeted analysis, NTA and SSA requires higher sensitivity and multidimensional instrumentation, such as two dimensional gas chromatography coupled with time of flight mass spectrometry (GC×GC/ToF-MS). The third chapter addresses the research bottleneck from use of GC×GC/ToF-MS in comparative samples analysis, including soil extracts pre- and post-bioremediation. Due to increased chromatographic separation and higher resolution MS, GC×GC/ToF-MS analysis results in large amount of data output. A Python™ script, OCTpy, was developed to automate the data analysis of GC×GC/ToF-MS output from comparative samples. The results from the automated analysis was compared with the results from manual analysis to benchmark OCTpy’s effectiveness in data analysis time savings and accuracy.
The fourth chapter addresses the formation prediction of atmospheric PAH TPs as a result of reactions between gas- and particle-phases parent-PAHs and atmospheric oxidants, particularly, hydroxyl radicals (OH•) and nitrogen oxides (NOx), including nitrogen dioxide, nitrate, and dinitrogen pentoxide (NO2, NO3, and N2O5, respectively), and ozone (O3). To determine the formation of these PAH-TPs, a predictive approach, combining computational chemistry (thermodynamic stability, electron density, and average local ionization energy (ALIE) calculations) and Clar's resonance structures, was used to evaluate the location of reactive sites on PAHs (i.e., the carbon where atmospheric radicals attack). To validate the PAH-TP predictions, the structures of NPAHs, OPAHs, and OHPAHs in the predictions were compared to those reported in published laboratory experiments in both gas- and particle-phases. ALIE was determined to provide robust results to predict the formation of atmospheric PAH-TPs based on the reactive sites of parent-PAHs. The ALIE approach could differentiate the reactive sites of a parent-PAH to be on specific carbon site or on a carbon-carbon bond. This predictive capability plays an important role in SSA of TPs, particularly to identify PAH-TPs, which had not been previously studied.
In the final chapter, a summary of the conclusions from the previous chapters are discussed. This section also provides recommendations for future research steps for each of the research project. Future suggestions for the identification of TPs in the environment are also presented.