Hybrid Fluorometric Flow Analyzer for Ammonia

We describe a robust, highly sensitive instrument for the determination of ambient ammonia. The instrument uses two syringe pumps to handle three liquids. The flow configuration is a hybrid between traditional flow injection (FI) and sequential injection (SI) schemes. This hybrid flow analyzer spends 87% of its time in the continuous flow FI mode, providing the traditional FI advantages of high baseline stability and sensitivity. The SI fluid handling operation in the remaining time makes for flexibility and robustness. Atmospheric ammonia is collected in deionized water by a porous membrane diffusion scrubber at 0.2 L/min with quantitative collection efficiency, derivatized on-line to 1-sulfonatoisoindole, and measured by fluorometry. In the typical range for ambient ammonia (0−20 ppbv), response is linear (r² = 0.9990) with a S/N = 3 limit of detection of 135 pptv (15 nM for 500 μL of injected NH₄⁺_(aq)) with an inexpensive light emitting diode photodiode-based detector. Automated operation in continuously repeated, 8-min cycles over 9 days shows excellent overall precision (n = 1544 p_NH3 = 5 ppbv, RSD = 3%). Precision for liquid-phase injections is even better (n = 1520, [NH₄⁺_(aq)] = 2.5 μM, RSD = 2%). The response decreases by 3.6% from 20 to 80% relative humidity.