Sensitive Detection of Atmospheric Trace Gases by Laser Photoacoustic Spectroscopy

Abstract

The sensitive and selective detection of trace gases is of great current interest. Various detection schemes have been proposed including spectroscopic techniques [1]. Recently, diode lasers in the 1.53 µm and 1.65 µm wavelength region have been used for monitoring specific species such as ammonia (NH3) [2,3]. A detection sensitivity of 8 ppb has been reported for laboratory measurements at atmospheric pressure by applying photoacoustic (PA) detection [2] and of 55 ppm-m at a total pressure of 76 Torr by using derivative absorption [3]. However, in order to apply a detection scheme to environmental conditions, a sufficient detection selectivity is often more difficult to achieve than the sensitivity. The stringent requirements are best met by using the fundamental or mid IR region between 3 and 20 µm. Hence, the combination of widely tunable lasers and photoacoustic (PA) detection appears to provide a powerful tool for atmospheric sensing particularly with respect to multicomponent capability, detection sensitivity and selectivity. During the last years we have developed a CO laser-, a mobile CO2 laser- and a continuously tunable, high-pressure CO2 laser-PA system [4]. This paper discusses the main characteristics and most recent results obtained with the mobile system as well as current efforts on novel developments of tunable IR lasers with wide tuning range and narrow bandwidth based on nonlinear optical effects.