Spectroscopic analysis of gaseous air pollutants with tunable diode lasers
Thesis/Dissertation
·
OSTI ID:5611417
The use of tunable diode lasers for the spectroscopy analysis of gaseous air pollutants was studied. The diode laser was first utilized to measure the infrared line positions and line widths, 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. With this technique, the self- and the nitrogen-broadening coefficients of the NH/sub 3/ line at 852.7 cm/sup -1/ were determined, and the spectral locations for several SO/sub 2/ lines in the 1180.1 cm/sup -1/ region were accurately determined. Wavelength modulation using diode lasers as excitation sources was applied to the photoacoustic detection of NH/sub 3/. 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 selected optimum modulation frequency so that improvement in signal-to-noise ratio could be achieved. In addition to the modulation frequency, it was observed that the wavelength-modulated photoacoustic signal was also affected by modulation amplitude, total pressure in the gas cell and time constant on the lock-in amplifier. The experimental conditions for obtaining the maximum signal were determined and applied to the evaluation of system sensitivity. The detection limit of this wavelength-modulated photoacoustic detection system for NH/sub 3/ in nitrogen was found to be 25 ppM.
- DOE Contract Number:
- W-7405-ENG-82
- OSTI ID:
- 5611417
- Country of Publication:
- United States
- Language:
- English
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