Ammonia trace measurements at ppb level based on near-IR photoacoustic spectroscopy

摘要

A photoacoustic sensor using a laser diode emitting near 1532 nm in combination with an erbium-doped fibre amplifier has been developed for ammonia trace gas analysis at atmospheric pressure. NH_3 concentration measurements down to 6 ppb and a noise-equivalent detection limit below 3 ppb in dry air are demonstrated. Two wavelength-modulation schemes with 1f and 2f detection using a lock-in amplifier were investigated and compared to maximise the signal-to-noise ratio. A quantitative analysis of CO_2 and H_2O interference with NH_3 is presented. Typical concentrations present in ambient air of 400 ppm CO_2 and 1.15% H_2O (50% relative humidity at 20℃) result in a NH_3 equivalent concentration of 36 ppb and 100 ppb, respectively.