Development of a photoacoustic gas sensor for in situ and on-line measurement of gaseous water and toluene†

Abstract

Developments towards a portable sensor system for the detection of benzene, toluene and xylene for screening are described. The monitoring system is based on the resonant photoacoustic technology. As light sources, NIR laser diodes at wavelengths of overtone vibrations of the alkyl CH bonds are used. The resonator is of cylindrical shape with diameter 2R = 5 cm and total length L = 10 cm and is operated in its first azimuthal mode. The acoustic wave is detected by an electret microphone. Test measurements of water and toluene vapor are presented. For the detection of water vapor a 1.31 µm NIR laser diode was used. Water concentrations down to 0.5 mg l^–1 in air could be measured. For the detection of toluene vapor a wavelength of 910 nm exciting the third overtone vibration of the alkyl CH bonds was used. The sensitivity of 3.2 mg l^–1 in air may be increased by several orders of magnitude as soon as suitable laser diodes with wavelengths fitting the absorption lines of the first overtone become available.

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