Photoacoustic sensors for trace gas detection
Trace gas detectors based on optical systems represent the best option for measuring tiny quantities of trace gases, especially when fast on-line detection and high selectivity are needed. Since the advent of lasers, several spectroscopic methods have been developed for in situ trace-gas detection, leading in many cases to compact, reliable and cost-effective sensors.
Three main criteria drive the development of high-sensitivity optical sensors: i) selection of optimal molecular transition in terms of absorption strength and absence of possible interfering gases; ii) long optical absorption length and/or use of buildup optical cavity; iii) efficient spectroscopic detection schemes.
Amid small-size and efficient spectroscopic sensors, PhotoAcoustic Spectroscopy (PAS) technique represents one of the best choices. PAS is characterized by a compact, cost-effective and robust architecture, which makes this technique ideal for in-situ gas sensing. In recent years, our Quartz-Enhanced PhotoAcoustic Spectroscopy (QEPAS) combined with quantum cascade lasers (QCLs) have demonstrated sensitivities up to 50 ppt concentration levels (1 second integration time, SF<sub>6</sub>molecule) with compact and robust apparatuses. In the QEPAS technique the acoustic energy coming from relaxation of excited molecules is accumulated in a sharply resonant quartz tuning fork (QTF) with a very high quality factor, which acts as piezoelectric acoustic transducer. QEPAS is characterized by a direct proportionality between the signal amplitude and the laser power available for gas excitation, so the higher the optical power focused between the QTF prongs, the lower the sensor detection limit.
Till now, the best performances have been obtained with mid-infrared (mid-IR) QCLs resonant with strong ro-vibrational molecular transitions. Very recently we demontrated for the first time high-sensitivity QEPAS gas detection by a THz QCL. Recent activities include the development of QEPAS sensors for environmental monitoring of dangerous gases (e.g. H<sub>2</sub>S) as well as the implementation of novel PAS-based techniques for increasing the sensitivity detection. In particular, in order to take advantage of the proportionality between QEPAS signal and laser power, we combined optical build-up cavities and PAS detection in a compact sensor. With this technique, named Intracavity-QEPAS, we demonstrated the possibility ot improve the QEPAS detection sensitivity by a factor equal to the cavity power enhancement factor.
We are currently working on the development of new Intracavity-QEPAS sensors for detection of toxic gases and unstable molecules. We are also working on the realization of cantilever-enhanced PAS sensors, which should furtherly pull down the minimum detection sensitivity to values close to more sophisitcated techniques like cavity ring-down spectroscopy.
This activity is carried on in collaboration with Università e Politecnico di Bari, CNR-IFN Bari and CNR-INO Napoli.