Quasi-Simultaneous Detection of Ammonia and Nitrous Oxide by Photoacoustic Phase-Resolved Method: A Proof-of-Concept

This work presents a method for the quasi-simultaneous detection of trace gas molecules using photoacoustic spectroscopy with phase-resolved method. By utilizing two lasers to excite ammonia (NH₃) and nitrous oxide (N₂O) in a mixture and employing phase-sensitive detection, the individual contributions of these gases were separated. The separation was achieved by adjusting the phase modulation of the N₂O exciting laser until a 90-degree phase difference was established between its signal and that of NH₃. The results demonstrate the absence of spectral overlap between the two compounds, enabling independent and quasi-simultaneous detection. Calibration curves highlighted the behavior and cross-interference between NH₃ and N₂O. For NH₃, linear profiles with regression coefficients R = 0.998 and R = 0.999 were observed, when N₂ or N₂ + N₂O were used as diluent gases, respectively. The corresponding sensitivities were (3.0 ± 0.1) μV·ppmv^-1and (3.10 ± 0.08) μV·ppmv^-1. The lower detection limits (LDLs) were 51 ppbv and 67 ppbv, respectively. For N₂O, calibration data were similarly robust with LDLs of 262 ppbv and 255 ppbv when diluted with N₂ and N₂ + NH₃, respectively. Minimal cross-interference effects were found, confirming the selectivity and reliability of this method for quantifying NH₃ and N₂O in gas mixtures. The technique offers advantages over other methods and is applicable to studying emissions related to agriculture with nitrogen fertilizers.