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The Development of Laser-Based Analytical Tools for Molecular Composition Characterization Across Combustion and Biological Systems Open Access

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In this dissertation, several laser-based analytical tools are presented along with accompanying scientific problems for which they provide insight. The demonstrated applications of these instruments are in the field of combustion and biological metabolism, however they are likely useful for other areas where similar types of molecules are of interest. Soot, or black carbon, is the second greatest climate forcer in our atmosphere after CO2. Soot is generated when hydrocarbon fuel is incompletely oxidized to CO2 and H2O. While greenhouse gases have atmospheric lifetimes of tens to thousands of years, the atmospheric lifetime of soot is only ~7 days. Most accept that polynuclear aromatic hydrocarbon (PAH) molecules are the fundamental building blocks of soot particulate, however there is a significant gap in experimental evidence for the chemical composition and molecular structure of soot particulate. Ångstrom Exponent (AE) has long been a metric for describing the relationship between wavelength and absorbance to enable climate modelers to predict the environmental impact of atmospheric particulate. More recently, AE values have also been used to categorize “black” and “brown” carbon. In a 60% ethylene, 40% nitrogen coflow diffusion flame, values for AE were calculated from spatially resolved, in situ hyperspectral optical extinction measurements (500-3300 nm). In areas of a flame known to contain mature soot, AE values are analogous to atmospheric black carbon (0.8-1.2) over the entire spectral range. In the absence of internally or externally mixed organic components (e.g. sulfates), as would be common to atmospheric brown carbon, the liquid-like particles in the flame behave somewhat differently to atmospheric brown carbon. Brown carbon, which typically only absorbs wavelengths <700 nm, has AE values >2. The liquid-like particles yielded AE values similar to brown carbon (2-5), however this trend was observed into the near infrared (1020 nm). The values of AE can also be related to molecular size. Previous in situ and ex situ work by our research group has suggested sizes of the PAH formed by this flame system are the size of circumpyrene. By combining thermophoretic sampling and mass spectrometry imaging, the distribution of sizes and identities of these PAH can be revealed. Using these techniques, we obtained spatially resolved mass spectra for an array of flame positions, spanning flame areas with nascent and mature soot particulate. This data confirms that the average PAH molecular size is 500-600 Da, in good agreement with previous in situ techniques, however PAH from 239 838 Da were observed providing the complete inventory of PAH chemical formulae of molecules comprising BC particulate. For biological samples, an ion source capable of making highly spatially resolved mass spectrometric measurements is also described. This system is an adapted form of laser ablation electrospray ionization (LAESI)-MS, where the optical train was improved to enable simultaneous observation and analysis of thin biological samples. By combining these aspects, small populations of adherent cells were metabolically interrogated with simultaneous observation. As few as five mammalian cells at a time were sampled directly, eliminating the need for preparation steps that have been shown to cause cellular perturbations. This technology enables real time targeting of samples for analysis based on any visual cues observed under microscope.

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