A REAL-TIME, NONINTRUSIVE TRACE GAS DETECTOR BASED ON LASER PHOTOTHERMAL DEFLECTION

We present the layout and technical details of a trace gas monitor bas ed on photothermal deflection. The operating principle of this instrum ent, i.e., the deflection of a (weak) probe laser beam by the thermal refractive index gradient induced by trace gas absorption of an intens e pump laser beam, allows nonintrusive measurements with good space an d time resolution. An intra-cavity CO2 laser is used as the pump beam and a red HeNe laser as the probe. The latter runs perpendicular to th e pump beam to optimize spatial resolution. To increase sensitivity, t he probe laser is incorporated in a multipass setup. The instrument is demonstrated by the localization of ethylene emission sites on a cher ry tomato and by monitoring ammonia production due to nitrogen fixatio n by cyanobacteria. Both C2H4 and NH3 can be detected at the 1-3 ppb l evel, at a spatial resolution of 2 mm (along the pump laser)X0.6 mm (p erpendicular to it), and a response time of 0.1 s (without background correction) or 15 s (including background correction). Sensitivity can be increased at the expense of spatial resolution, and vice versa. In principle, this instrument is applicable to all those gases possessin g a characteristic (''fingerprint'') spectrum in the CO2 laser range. The great advantage of the photothermal deflection technique with resp ect to other trace gas detection schemes lies in the nonintrusive char acter of the measurements. There is no need to enclose the sample in a vessel or to suck large volumes of air into the detector; measurement s can be performed in open air and in real time. This should prove esp ecially useful where sticky (polar) gases, like H2O, NH3, CH3OH, etc., are to be detected quantitatively. Main applications include air qual ity monitoring, especially concerning dry deposition rate measurement' s using the eddy correlation technique, and the study of volatile meta bolite emission of biological samples. (C) 1995 American Institute of Physics.