Laser photoacoustic spectrometry for trace gas monitoring

Abstract

Spectroscopic methods offer important features with respect to atmospheric trace gas monitoring. Some relevant techniques are briefly discussed. Emphasis is put on various aspects of photoacoustic spectrometry. Its basic principles and important experimental characteristics are outlined. Depending on molecular absorption cross-sections, a detection limit in the low ppbv concentration range is typically achieved without the use of any preconcentration device. In multi-component mixtures this limit may be increased owing to absorption interferences. Two photoacoustic (PA) systems have been developed and some specific applications are presented. A CO-laser PA system operating in the 5–6.5 µm wavelength range was applied to the analysis of motor vehicle exhausts. Apart from nitrogen oxides, CO₂ and water vapour, ten different volatile organic compounds (VOCs) including o-, m- and p-xylene were selectively detected at parts per million by volume (ppmv) concentrations. As a second instrument, an automated CO₂ laser PA system was implemented in a trailer for in situ measurements. It was applied to the detection of stack gases at a pharmaceutical plant and to air monitoring in urban and rural environments. As an example, ethene, ammonia and ozone were detected at ppbv concentrations in rural air. Finally, new aspects are discussed, in particular the introduction of CO₂ isotopes as the laser medium, the design of PA Stark cells and the development of a continuously tunable high-pressure CO₂ laser and of a widely tunable mid-infrared laser based on frequency mixing in a non-linear crystal. Although PA spectrometry already represents a valuable alternative to more established spectroscopic and non-spectroscopic schemes, these novel developments should further enhance its potential and versatility.