Photo-acoustic spectroscopy using a quantum cascade laser (QCL) for analysis of ammonia in water solutions

Ammonia (NH$_3$) toxicity, stemming from nitrification, can adversely affect aquatic life and influence the taste and odor of drinking water. This underscores the necessity for highly responsive and accurate sensors to continuously monitor NH$_3$ levels in water, especially in complex environments where reliable sensors have been lacking until this point. Herein, we detail the development of a sensor comprising a compact and selective analyzer with low gas consumption and a timely response, based on photoacoustic spectroscopy. This, combined with an automated liquid sampling system, enables the precise detection of ammonia traces in water. The sensor system incorporates a state-of-the art quantum cascade laser as the excitation source emitting at 9 \textmu m in resonance with the absorption line of NH$_3$ located at 1103.46 cm$^{-1}$. Our instrument demonstrated detection sensitivity at low ppm level for total ammonia nitrogen with response times less than 60 seconds. For the sampling system, an ammonia stripping solution was designed resulting in a prompt full measurement cycle (6.35 mins). A further evaluation of the sensor within a pilot study showed good reliability and agreement with the reference method for real water samples, confirming the potential of our NH$_3$ analyzer for water-quality monitoring applications.