Date of Award
Doctor of Philosophy
The use of tunable diode lasers for the spectroscopic analysis of gaseous air pollutants was studied. The diode laser was first utilized to measure the infrared line positions and linewidths, which are important parameters of pollution monitoring schemes based on infrared absorption. It was then used in conjunction with photoacoustic detection technique to measure the concentration of ammonia;An unique internal calibration method for the accurate determination of spectral locations to the order of the monochromaticity of the laser was developed. This is based on low-pressure absorption measurements and subsequent fitting of a Voigt profile and determining the Doppler component. With this technique, the self- and the nitrogen-broadening coefficients of the NH(,3) line at 852.7 cm('-1) were determined to be 26.2 (+OR-) 0.4 MHz torr('-1) and 5.2 (+OR-) 0.1 MHz torr('-1) respectively. Also, the spectral locations for several SO(,2) lines in the 1180.1 cm('-1) region were accurately determined;Wavelength modulation using diode lasers as excitation sources^was applied to the photoacoustic detection of NH(,3). The reduction^of background signal due to window heating by the wavelength^modulation technique made possible the improvement of signal-to-^noise ratio in photoacoustic detection. A nonresonant cell with a^cell volume about 2 cm('3) was constructed and spectrophone setups^were designed. The noise as a function of modulation frequency^was studied in this work, and the variation of signal with modulation^frequency was predicted based on existing theory. These functions^were compared to select an optimum modulation frequency so thatan improvement in signal-to-noise ratio could be achieved. Inaddition to the modulation frequency, it was observed that thewavelength-modulated photoacoustic signal was also affected bymodulation amplitude, total pressure in the gas cell and timeconstant on the lock-in amplifier. The experimental conditions forobtaining the maximum signal were determined and applied to theevaluation of system sensitivity. The detection limit of thiswavelength-modulated photoacoustic detection system for NH(,3) in;nitrogen was found to be 25 ppm;('1)DOE Report IS-J-556. This work was performed under contractW-7405-eng-82 with the U.S. Department of Energy.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Chang, Tsu-Yen, "Spectroscopic analysis of gaseous air pollutants with tunable diode lasers " (1981). Retrospective Theses and Dissertations. 7405.