Colorimetric derivatization of ambient ammonia (NH ₃ ) for detection by long-path absorption photometry

In the last few decades, various techniques, including spectroscopic, mass spectrometric, chemiluminescence and wet chemical methods, have been developed and applied for the detection of gaseous ammonia (NH3). We developed an online NH3 monitoring system - salicylic acid derivatization reaction and long-path absorption photometer (SAC-LOPAP) - based on a selective colorimetric reaction to form a highly absorbing reaction product and a LOPAP, which could run stably for a long time and be applied to the continuous online measurement of low concentrations of ambient NH3 by optimizing the reaction conditions, adding a constant-temperature module and liquid flow controller. The detection limit reached with this instrument was 40.5 parts per trillion (ppt) with a stripping liquid flow rate of 0.49 mL min(-1) and a gas sample flow rate of 0.70 L min(-1). An inter-comparison of our system with a commercial Picarro G2103 analyzer (Picarro, USA) in Beijing was presented, and the results showed that the two instruments had a good correlation with a slope of 1.00 and an R-2 of 0.96, indicating that the SAC-LOPAP instrument involved in this study could be used for the accurate measurement of NH3.