Ultra-sensitive ambient ammonia detection using CO2-laser-based photoacoustic spectroscopy
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An ambient trace-ammonia sensor that uses resonant photoacoustic spectroscopy and a line-tunable CO2 laser has been developed. This system achieves a 1σ replicate precision of 32 parts-per-trillion (ppt) with an averaging time of 5 s and a total measurement time of 40 s. This 32-ppt precision corresponds to a minimum detectable fractional absorbance of 8.8×10-9, a minimum detectable absorption coefficient of 9.6×10-10 cm-1, and a minimum path-length-normalized detectivity of 1.1×10-8 W cm-1/Hz1/2. Background interference from CO2, H2O, and cell-window absorption were subtracted by switching to a neighboring off-resonance laser wavelength .
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