Colorimetric derivatization of ambient ammonia (NH₃) for detection by long-path absorption photometry

<p>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 (NH<span class="inline-formula">₃</span>). We developed an online NH<span class="inline-formula">₃</span> 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 NH<span class="inline-formula">₃</span> 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<span class="inline-formula">⁻¹</span> and a gas sample flow rate of 0.70 L min<span class="inline-formula">⁻¹</span>. 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 <span class="inline-formula"><i>R</i>²</span> of 0.96, indicating that the SAC-LOPAP instrument involved in this study could be used for the accurate measurement of NH<span class="inline-formula">₃</span>.</p>