IR laser photothermal trace gas detection applied to environmental and biological problems

Abstract

Please be advised that this information was generated on 2018-05-09 and may be subject to change. Per a.o.Inl 'rared Phys. Technol. Vol. 36, No. I, pp. 483488, 1995 Copyright '7 1995 Elsevier Science Ltd 1350 A~tract--The photothermal effect is utilized for the detection of molecular trace gases in the open air, with high spatial and temporal resolution. An IR CO 2 laser is used to excite specific molecules to higher vibrational levels. The subsequent change in temperature and refractive index is monitored by the deflection of a HeNe laser beam which crosses the CO 2 laser beam. To improve the detection limit of the system the CO, laser has an intracavity focus near to which the HeNe laser passes 31 times in a multipass mirror setup. A detection sensitivity of I ppbv (C2H4) and 0.5 ppbv (NH3) is achieved under practical conditions.

A Photothermal Interferometer for Gas-Phase Ammonia Detection

“…There is a practical limit to the speed at which the laser frequency can be switched. de Vries et al , reported this limit as 15 s. This does not solve the problem of water vapor fluctuations during ammonia measurement and is not significantly faster than the chemical zero.…”

“…Rooth et al, Sauren et al, , Moeckli et al, and Thony and Sigrist detected pressure waves (sound) from modulated infrared absorption by ammonia using microphones in photothermal acoustic cells (PTA). de Vries et al ,, and Zimerman and Boccara , detected the photothermal deflection (PTD) of a He−Ne probe beam by a modulated CO 2 laser beam. An advantage of all three infrared photothermal techniques, PTI, PTA, and PTD, is that carbon dioxide lasers available to generate infrared light are relatively inexpensive and maintenance-free and have low power requirements.…”

“…The PTD groups listed above have measured the sensitivity to ethylene at high concentrations and have extrapolated these measurements to estimate a detection limit for ammonia of 500 ppt in 1 s. − ,, Ammonia measurements by PTD are shown to compare well with a continuous-flow denuder technique above 10 ppb. , Open-path detection using PTD has the advantage of avoiding sample line hysteresis effects or surface-based particle decomposition, both tricky problems for all ammonia detection techniques that require sample flow. However, measured sensitivity to sub-ppb levels of ammonia has not been reported for PTD, and the open-path configuration makes calibration at subambient NH 3 concentrations difficult.…”