Real-time and Non-intrusive Detection of Ambient Ammonia using the Photothermal Deflection Technique
The recent development of a non-intrusive photothermal deflection (PD) instrument allows the quantitative detection of local ammonia concentrations in air to be measured continuously, sensitively and fast. Ammonia is vibrationally excited by an infrared CO2 laser in an intracavity configuration. A HeNe beam passes over the CO2 laser beam in a multipass arrangement and is deflected by a thermal, CO2 laser induced, refractive index gradient. This deflection, due to the mirage effect, is a measure for the local ammonia concentration. In ambient air the detection limit is 0.5 ppb and the spatial resolution is 2.5 cm times π (0.282 mm2). The time resolution is 0.1 s for single line operation and 15 s for multi-line operation. The system is fully automated and suited for measuring periods of at least one week. Results are compared with those of a continuous-flow denuder system.
KeywordsDioxide Ozone Polyethylene HeNe
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