TRACE GAS MONITORING BY LASER-PHOTOACOUSTIC SPECTROSCOPY

Photoacoustic spectroscopy (PAS) with tunable IR lasers represents a promising tool for trace gas monitoring. The principles of PAS applied to multicomponent gas mixtures are briefly outlined. We present three photoacoustic (PA) systems and discuss typical results. A CO laser PA system has been applied to the selective analysis of volatile organic compounds (VOCs) in motor vehicle exhausts and to the study of the dimerization of fatty acid vapors. As a second instrument, an automated mobile CO2 laser PA system has been employed for in situ air monitoring in industrial, urban and rural environments. A detection limit in the ppb-range can be achieved depending on molecular absorption cross sections and absorption interferences. Despite some cases which demonstrate the feasibility to differentiate even between isomers, the detection selectivity could often be substantially improved by employing a continuously rather than a line-tunable laser source. This is demonstrated by our high-pressure CO2 laser which offers continuous tunability in the common CO2 laser emission branches with a linewidth of only 0.017 cm(-1). Other approaches that we presently pursue include the use of CO2 laser isotopes, measurements with a PA Stark cell for monitoring ammonia and, finally, spectrometers with tunable MIR lasers based on optical parametric oscillation (OPO) or difference frequency generation in nonlinear optical crystals as sources.