Pulse-Laser Infrared Photothermal Spectrometry of Condensed-Phase Aerosols Based on Photothermal Deflection Spectroscopy

• Main Author: Dada, Oluwatosin Olubunmi
• Format: Others
• Published: DigitalCommons@USU 2008
• Subjects: Analytical Chemistry
• Online Access: https://digitalcommons.usu.edu/etd/52; https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1051&context=etd

The research reported in this dissertation is aimed at development of photothermal deflection detector that is consistent with constraints imposed by aerosols sampling and analysis. The main objectives of this project are: 1) the design and evaluation of initial apparatus prototypes and 2) a test of the prototype apparatus with surrogate substances using conventional mid-infrared gas lasers. Experimental evaluation of reduced size photothermal apparatus is first performed with gaseous samples. Analysis of trichlorofluoromethane (CFC-11) and ethanol vapors is performed with both conventional large apparatus and the prototype small apparatus. Comparison studies in terms of photothermal signal from both apparatuses are carried out to examine how apparatus downscaling affects photothermal signal. The main result indicates that the small apparatus performs about the same as the conventional laboratory-size apparatus. For better understanding of heat transport from aerosols collect on a flat substrate and its surrounding media, a numerical methodology based on finite element analysis is used to investigate the heat transfer between aerosol, substrate, and air. The numerical modeling is extended to prediction of photothermal signals from laser excited optical beam deflection due to temperature gradient inside the air above aerosols. The modeling results help to understand how aerosol properties such as size, shape, thermo-optical properties, and particles arrangement affect photothermal signal. The model is also used to investigate the influence of substrate properties on photothermal signal. A photothermal deflection apparatus is then used to examine the potential of mid-infrared photothermal spectroscopy for aerosol analysis. Laboratory-generated ammonium nitrate aerosols are deposited on germanium substrate by using Micro-Orifice Uniform Deposits Impactor (MOUDI). Photothermal spectroscopy, based on optical beam deflection (mirage effect), is used to analyze the collected aerosols. The measurements are carried out in terms of aerosol number concentration and aerosol mass concentration. Based on standard 30 minutes sampling time and 30 L/min flow rate, the limit of detection obtained for our instrument are 18 particles, equivalence of 0.2 μg m-3 mass concentrations for 3 μm particles diameter) for aerosol number concentration measurement and 0.65 μg m-3 for aerosol mass concentration measurement. Out of curiosity on whether standard materials could be used to calibrate a photothermal lens apparatus without having to make up liquid samples, it was thought that colored glass filters would serve as such standards. Photothermal lens measurements and finite element modeling are used to examine the physical changes taking place in optical filter glasses. Colored-glass and neutral density filters are found to have a strong positive temperature-dependent refractive index change.