Sensitive and selective monitoring of trace gases by laser photoacoustic spectroscopy

Important features of laser photoacoustic (PA) spectrometers employed in trace gas monitoring and multicomponent gas analysis are discussed. Narrowband laser sources with wide tunability in the mid-infrared range are employed. We have developed and applied different laser-based PA spectrometers. These arrangements are briefly presented and the excellent detection performance in terms of sensitivity and specificity is illustrated by various examples. A high-pressure CO2 laser (continuous total tuning range 76 cm(-1), linewidth 500 MHz), an optical parametric oscillator (OPO)-based difference frequency generation (DFG) system (tuning range 2.5-4.5 mu m, linewidth 0.25 cm) and a line-tunable CO2 laser, implemented in a trailer for in situ measurements, are employed as laser sources. Novel PA cells and microphone arrays with up to 80 individual microphones have been developed. A detection limit of 10(-9) cm(-1) atm(-1) and a large dynamic range of seven orders of mangnitude have been achieved with an extracavity multipass resonant PA cell. Examples to be presented include in situ multicomponent measurements on dynamically controlled atmospheres in fruit storage chambers or analyses of benzene-toluene-p-xylene (BTX) mixtures.