Photoacoustic Spectroscopy Using a Quantum Cascade Laser for Analysis of Ammonia in Water Solutions.

Ammonia (NH) 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 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.

Widely-Tunable Quantum Cascade-Based Sources for the Development of Optical Gas Sensors.

Spectroscopic techniques based on Distributed FeedBack (DFB) Quantum Cascade Lasers (QCL) provide good results for gas detection in the mid-infrared region in terms of sensibility and selectivity. The main limitation is the QCL relatively low tuning range (~10 cm) that prevents from monitoring complex species with broad absorption spectra in the infrared region or performing multi-gas sensing. To obtain a wider tuning range, the first solution presented in this paper consists of the use of a DFB QCL array.

New Signal Processing for Fast and Precise QEPAS Measurements.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) gas sensors have been widely developed over the last decade. This technique takes the advantage of a high quality factor tuning fork to enable high-sensitivity and high-selectivity miniature gas sensors. Lock-in detection is classically used to measure the resonator amplitude, which is proportional to the gas concentration, but this technique is slow and leads to measurement drifts as it does not follow the resonator frequency drifts over temperature and pressure.

Quartz Enhanced Photoacoustic Spectroscopy Based on a Custom Quartz Tuning Fork.

We have designed and fabricated a custom quartz tuning fork (QTF) with a reduced fundamental frequency; a larger gap between the prongs; and the best quality factor in air at atmospheric conditions ever reported, to our knowledge. Acoustic microresonators have been added to the QTF in order to enhance the sensor sensitivity. We demonstrate a normalized noise equivalent absorption () of 3.