No description available.

The formation of base pair radical anions and cations is closely related to many fascinating research fields in biology and chemistry such as genetic mutation, radiation-induced DNA damage and dynamics of charge transfer in DNA. However, the relevant knowledge so far mainly comes from studies on isolated base pair radical anions and cations, and their behavior in the DNA environment is less understood. In this study, we focus on how the nucleobase sequence affects the properties of the guanine¡Vcytosine (G:C) base pair radical anion and cation. The energetic barrier and reaction energy for the proton transfer along the N1(G)¡VH¡E¡E¡EN3(C) hydrogen bond and the stability of (G:C)¡E (i.e., electron affinity and ionization potential of G:C) embedded in different sequences of base-pair trimer were evaluated using density functional theory and two-layer ONIOM method. The computational results demonstrated that the presence of neighboring base pairs has an important influence on the behavior of (G:C)¡E in the gas phase. The excess electron and positive hole were found to be localized on the embedded G:C and the charge leakage to neighboring base pairs was very minor in all of the investigated sequences. Accordingly, the sequence behavior of the proton transfer reaction and the stability of (G:C)¡E is chiefly governed by electrostatic interactions with adjacent base pairs. However, the effect of base stacking, due to its electrostatic nature, is severely screened upon hydration, and thus, the sequence dependence of the properties of (G:C)¡E in aqueous environment becomes relatively weak and less than that observed in the gas phase. The effect of geometry relaxation associated with neighboring base pairs as well as the possibility of proton transfer along the N2(G)¡VH¡E¡E¡EO2(C) channel have also been investigated. The implications of the present findings to the electron transport and radiation damage of DNA are discussed.

Oxygenated volatile organic compounds ((O)VOCs) contribute to ozone formation, affect the oxidising capacity of the troposphere and are sources of growth, and in some cases formation, of aerosols. It is therefore important to identify and quantify sources of (O)VOCs in the troposphere. In the late 1990s a unique technique for quantification of organic trace gas species, proton transfer reaction mass spectrometry (PTR-MS) was developed. PTR-MS potentially offers rapid response and high sensitivity without the need for sample pre-concentration. Concentrations can be derived from the PTR-MS either by calibration or can be calculated from measured ion count rates and kinetic considerations. In this work, the methodology of PTR-MS application is critically assessed. The uncertainties and inaccuracies associated with each parameter employed in the calculation of concentrations are reviewed. This includes a critical appraisal of models for the calculation of the collisional rate constant currently applied in the field of PTR-MS. The use of a model to account for the effects of the electric field, available in the literature but not previously applied to the PTR-MS, is advocated. Collisional rate constants employing each of the models discussed have been calculated for the reactions of H3O+ with over 400 molecules for PTR-MS. In PTR-MS it cannot be assumed that the product ion occurs only at the protonated non-dissociated mass. Few product distributions obtained from PTR-MS are cited in the literature, and even then the reaction chamber conditions (pressure, temperature and electric field strength) are not always specified. A large volume of product distributions for trace gases with H3O+ in select ion flow tube mass spectrometry (SIFT) exists in the literature and is reviewed. In SIFT, no electric field is applied to the reaction chamber and the extent and even nature of fragmentation can differ in PTR-MS. In addition to the application of an electric field, the energy in the reaction chamber can be increased by increasing the temperature or by variation of the reagent ion. In this work, the increase in energy via the three methods is approximated to enable a comparison of product distributions. The review of product distributions in PTR-MS, select ion flow drift tube mass spectrometry (SIFDT), variable temperature select ion flow tube mass spectrometry (VT-SIFT), SIFT, proton transfer reaction time of flight mass spectrometry (PTR-TOF-MS), proton transfer reaction ion trap mass spectrometry (PTR-ITMS) and electron ionisation mass spectrometry (EI-MS) is used alongside thermodynamic considerations to collate a list of potential contributors to a range of mass to charge ratios (m/z) in the PTR-MS. The need for further measurements of product distributions as a function of temperature, pressure and electric field strength for a wider range of (O)VOCs is highlighted. This enables dissociation to be better used as a tool for compound identification rather than being considered a hindrance. The collation of likely product distributions is applied to identify possible contributors to m/z observed during PTR-MS measurements of emission from cattle slurry. Field measurements were made during fertilisation of a grassland site south of Edinburgh in 2004 and 2005 and in laboratory-based measurements in 2006. Contextual reasoning, previous measurements and isotope ratios are used to narrow the list of possible contributors. Large concentrations of m/z cautiously identified as alcohols followed by a latter peak in carboxylic acids were observed during laboratory measurements. Increases in the corresponding m/z were also observed during the fertilisations. Other tentatively identified compounds emitted included phenol, methyl phenol, trimethylamine, and various sulphur containing compounds.

Electrospray ionization mass spectrometry (ESIMS) is widely used to characterize neutral and ionic species in solvents. Typically, electrical, thermal, and pneumatic potentials are applied to create electrosprays from which charged ionic species are ejected for downstream analysis by mass spectrometry. Most recently, ESIMS has been exploited to investigate ambient proton transfer reactions at air-water interfaces in real time. We assessed the validity of these experiments via complementary laboratory experiments. Specifically, we characterized the products of two reaction scenarios via ESIMS and proton nuclear magnetic resonance (1H-NMR): (i) emulsions of pH-adjusted water and isoprene (C5H8) that were mechanically agitated, and (ii) electrosprays of pH-adjusted water that were collided with gas-phase isoprene. Our experiments unambiguously demonstrate that, while isoprene does not oligomerize in emulsions, it does undergo protonation and oligomerization in electrosprays, both with and without pH-adjusted water, confirming that C-C bonds form along myriad high-energy pathways during electrospray ionization. We also compared our experimental results with some quantum mechanics simulations of isoprene molecules interacting with hydronium at different hydration levels (gas versus liquid phase). In agreement with our experiments, the kinetic barriers to protonation and oligomerization of isoprene were inaccessible under ambient conditions. Rather, the gas-phase chemistries during electrospray ionization drove the oligomerization of isoprene. Therefore, we consider that ESIMS could induce artifacts in interfacial reactions. These findings warrant a reassessment of previous reports on tracking chemistries under ambient conditions at liquid-vapor interfaces via ESIMS. Further, we took some high-speed images of electrosprays where it was possible to observe the main characteristics of the phenomena, i.e. Taylor cone, charge separation, and Coulomb fission. Finally, we took the freedom to speculate on possible mechanisms that take place during electrospray ionization that affected our system and possibly may influence other common analytical techniques on ESIMS.

No description available.

<p dir="ltr">While electrospray ionization facilitates the mass determination of smaller analytes, ESI of macromolecular native protein complexes is complicated by narrow charge state distributions and overlapping charge states, hindering mass analysis. This problem is further exacerbated with heterogeneous protein mixtures that yield ions of similar <i>m/z</i> values. Charge-reduction of a selected precursor population via ion/ion reaction provides further <i>m/z</i> separation and utilizes an extended mass range, allowing for mass determination. All experiments were performed on a TripleTOF 5600 quadrupole TOF mass spectrometer (SCIEX), modified for ion/ion reactions. Alternatively pulsed nano-electrospray ionization allowed for sequential injection of reagent and analyte ions. Selected cations were reacted with different anions, and charge-reduced product ions were mass analyzed on a TOF with modifications and tuning for an extended <i>m/z</i> range of 200,000+. Charge reduction via proton transfer reaction (PTR) involves a perfluorinated anion reacting with a multiply charged cation and results in the loss of one proton removal at a time. Through multiple iterations of PTR, the overlapping charge states of protein ions (from an unstained protein standard mixture consisting of 12 recombinant proteins of masses10kDa-200kDa) within the <i>m/z</i> 6500-8500 and <i>m/z</i> 4000-5000 ranges, generated under native conditions by ESI, transforms to a product spectrum with single-digit charge states, thereby deconvoluting the precursor “blob”.</p>

No description available.

This thesis describes the on-road analysis of criteria air pollutants (CAPs) and volatile organic compounds (VOCs) from a moving vehicle. CAPs and VOCs have numerous direct and indirect effects on the environment and public health and are generated from a variety of point and diffuse sources. The concentration of these pollutants can vary on the scale of metres and seconds due to variable emission rates of sources, meteorology, and the topography of an area. CAPs are conventionally measured on a spatial scale of tens of kilometres and one hour or longer time resolution, which limits the understanding of their impact and leaving many communities lacking information regarding their air quality. VOCs are not measured as frequently as CAPs, owing to the difficulty, challenges, and cost associated with sampling. The Mobile Mass Spectrometry Lab (MMSL) was developed to collect high geospatial (15 – 1,500 m) and temporal (1 – 10 s) resolution measurements of CAPs (O3, NOx, PM2.5), CO2, CH4, and VOCs. CAPs and greenhouse gases were monitored using standard analyzers, while VOCs were measured using a proton-transfer reaction time-of-flight mass spectrometer (PTR-MS). PTR-MS is a real-time, direct, in situ technique that can monitor VOCs in the ambient atmosphere without sample collection. The PTR-MS monitored up to mass-to-charge 330 with a sample integration time of 1 or 10 seconds and had detection limits into the low- to mid-ppt. PTR-MS is a soft ionization technique that is selective to all compounds with a proton affinity less than water, which excludes the atmospheric matrix and includes most VOCs. The measurements provided by the PTR-MS provided a rich dataset for which to develop workflow and processing methods alongside sampling strategies for the collection of high geospatial and temporal VOC data. The first on-road deployment of the MMSL was performed across the Regional District of Nanaimo and the Alberni-Clayoquot Regional District in British Columbia, Canada, from July iv 2018 – April 2019 to monitor the geospatial and temporal variation in the concentration of CAPs and VOCs. VOCs detected in the areas include hydrocarbons like toluene, C2-benzenes, and terpenes, organic acids like acetic acid, oxygenated compounds like acetone and acetaldehyde, and reduced sulfur compounds like methanethiol and dimethyl sulfide. While observed concentrations of VOCs were mostly below detection limits, concentration excursions upwards of 2,200 ppb for C2-benzenes (reported as ethylbenzene) for instance, were observed across the various communities and industries that comprise central Vancouver Island. VOCs like monoterpenes, were observed near the wood industries up to 229 ppb. Combustion related VOCs, like toluene and C2-benzenes, were often observed on major transportation corridors and was found to vary significantly between seasons, with winter measurements often exceeding those made in the summer. Reduced sulfur compounds, common components of nuisance odours, were measured around a few industries like waste management and wood industries. The second on-road deployment of the MMSL focused on the analysis of VOCs in the community around a wastewater treatment plant (WWTP) to identify the source of odours in the area. VOCs were also monitored in the odour control process of the WWTP to identify the VOCs being emitted, how much were emitted, and where potential deficiencies were in the process in a unique study. Median emission rates at the facility for methanethiol, dimethyl sulfide, and dimethyl disulfide were determined to be 100, 19, and 21 kg yr-1, respectively. VOC monitoring in the community encompassed the WWTP and the other major industries in the area, including agricultural land, a composting facility, and a marina. The highest measurements of odorous reduced sulfur compounds were observed around the WWTP, upwards of 36 ppb for methanethiol. Unsupervised multivariate analysis was performed to identify groups of VOCs present and their potential sources. Three groups were identified, one of which was related to reduced sulfur compounds. This group was observed around the WWTP, indicating that the WWTP was the likely source of malodours in the community. / Graduate

The aim of this work is the study of plasma processes and the synthesis of organic compounds due to electric discharge generated in gas mixture corresponding to the composition of the atmosphere of Saturn's largest moon Titan. This study focuses on the mimic of Titan's atmosphere at atmospheric pressure and ambient laboratory temperature. The chemical composition of Titan's atmosphere is very similar to atmosphere of prehistoric Earth. Many articles have been published with theoretical model-research, and laboratory experiments are the pursuit of their interconnection. The main aim of thesis is the identification of synthesized gaseous organic, amino, imino and cyano compounds by use to various analytical methods such as the PTR-MS, FTIR and GC-MS. The OES and electric measurements were applied to the determination of selected electric discharge parameters. The gaseous products and radicals formed in an atmospheric discharge fed by different mixtures of N2:CH4 (0,5 up to 5 % of CH4) operated in a flowing regime at the total gas mixture flows from 50 to 200 sccm at different discharge currents from 15 up to 40 mA were determined. A part of experiments was carried out with admixtures of CO2 and hydrogen. This first part of results has been obtained using OES in dependence on the gas mixture composition and supplied power. The bands of the nitrogen second positive and the first negative systems, CN violet system and Swan system of C2 were recorded. Besides them, atomic lines H, H, and C (in the second order) were also observed. These spectra allowed calculation of rotational and vibrational temperatures. FTIR in situ analysis of the gaseous products showed presence of various nitrile compounds and hydrocarbons in all experiments. The HCN, C2H2, NH3 were the main products generated in our system. The dependences of their concentrations on various experimental parameters were measured. The other part of this work was devoted to estimate the influence of CO2 traces addition on the reactivity in the gaseous mixtures mentioned above. Besides the main products mentioned above, CO2 and CO were detected and also some more complicated oxygen molecules has been confirmed but not estimated because of FTIR spectra complexity. In the case of hydrogen traces addition into the reaction gas mixture, no other compounds were determined. Impurities of CO2 as well as hydrogen have a great positive influence on the production efficiency of the major generated compounds at all conditions. The more detailed gaseous products analyses were carried out using the in situ PTR-MS. A huge number of different molecular structures containing nitrile groups (–CN), amino groups (–NH2, –NH–, –N CH3CN > C2H5CN. Besides them, many other hydrocarbons and nitriles were detected. Presence of all compounds was studi

Anthropogenic emissions into the troposphere can impact air quality, leading to poorer health outcomes in the affected areas. Volatile organic compounds (VOCs) are a group of chemical compounds, including some which are toxic, that are precursors in the formation of ground-level ozone and secondary organic aerosols. VOCs have a variety of sources, and the distribution of atmospheric VOCs differs significantly over time and space. Historically, the large number of chemical species present at low concentrations (parts-per-trillion to parts-per-billion by volume) have made VOCs difficult to measure in ambient air. However, with improvements in analytical instrumentation, these measurements are becoming more common place. Direct mass spectrometry (MS), such as membrane introduction mass spectrometry (MIMS) and proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) facilitate real-time, continuous measurements of VOCs in air, with full scan mass spectral data capturing changes in chemical composition with high temporal resolution. Operated on-road, mobilized direct MS has been used for quantitative mapping of VOCs at the neighborhood scale, but identifying VOC sources based on the observed mixture of molecules in the full scan MS dataset has yet to be explored. This dissertation describes the use of chemometric techniques to interrogate full scan MS data, and the progression from discriminating VOC samples of known chemical composition based on full scan MIMS data through to the apportionment of VOC sources measured continuously with a PTR-ToF-MS system operating in a moving vehicle. Lab‐constructed VOC samples of known chemical composition and concentration demonstrated the use of principal component analysis (PCA) to discriminate, and k-nearest neighbours to classify, samples based on normalized full scan MIMS data. Furthermore, multivariate curve resolution-alternating least squares (MCR-ALS) was used to resolve mixtures into molecular component contributions. PCA was also used to discriminate ‘real-world’ VOC mixtures (e.g., woodsmoke VOCs, headspace above aqueous hydrocarbon samples) of unknown chemical composition measured by MIMS. Using vehicle mounted MIMS and PTR-ToF-MS systems, full scan MS data of ambient atmospheric VOCs were collected and PCA was applied to the normalized full scan MS data. A supervised analysis performed PCA on samples collected near known VOC sources, while an unsupervised analysis using PCA followed by cluster analysis was used to identify groups in a continuous, time series PTR-ToF-MS dataset measured between Nanaimo and Crofton, British Columbia (BC). In both the supervised and unsupervised analysis, samples impacted by emissions from different sources (e.g., internal combustion engines, sawmills, composting facilities, pulp mills) were discriminated. With PCA, samples were discriminated based on differences in the observed full scan MS data, however real-world samples are often impacted by multiple VOC sources. MCR-weighted ALS (MCR-WALS) was applied to the continuous, time series PTR-ToF-MS data from three field campaigns on Vancouver Island, BC for source apportionment. Variable selection based on signal-to-noise ratios was used to reduce the mass list while retaining the observed m/z that capture changes in the mixture of VOCs measured, improving model results, and reducing computation time. Both point (e.g., anthropogenic hydrocarbon emissions, pulp mill emissions) and diffuse (e.g., VOCs from forest fire smoke) VOC sources were identified in the data, and were apportioned to determine their contributions to the measured samples. The data analyzed captured fine scale changes in the ambient VOCs present in the air, and geospatial maps of each individual source, and of the source apportionment were used to visualize the distribution of VOC sources across the sampling area. This work represents the first use of MCR-WALS to identify and apportion ambient VOC sources based on continuous PTR-ToF-MS data measured from a moving vehicle. The methods described can be applied to larger scale field campaigns for the source apportionment of VOCs across multiple days to capture diurnal and seasonal variations. Identifying spatial and temporal trends in the sources of VOCs at the regional scale can help to identify pollution ‘hot spots’ and inform evidence-based public policy for improving air quality. / Graduate / 2022-08-17