Ultra-sensitive ambient ammonia detection using CO2-laser-based photoacoustic spectroscopy

An ambient trace-ammonia sensor that uses resonant photoacoustic spectroscopy and a line-tunable CO2 laser has been developed. This system achieves a 1sigma 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 recision corresponds to a minimum detectable fractional absorbance of 8.8 x 10(-9) minimum detectable absorption coefficient of 9.6 x 10(-10) cm(-1), and a minimum path-length-normalized detectivity of 1.1 x 10(-8) W cm(-1)/root(Hz) over bar. Background interference from CO2, H2O, and cell-window absorption were subtracted by switching to a neighboring off-resonance laser wavelength.