# Near-Infrared Quartz-Enhanced Photoacoustic Sensor for H\({}_{2}\)S Detection in Biogas  
[PERSON]  
1 College of Physics and Electronic Engineering, Shanxi University, Taiyuan 030006, China; [EMAIL_ADDRESS] (F.Z. ); [EMAIL_ADDRESS] (Y.G. ); [EMAIL_ADDRESS] (L.Y. ); [EMAIL_ADDRESS] (Y.Y. ); [EMAIL_ADDRESS] (L.J. ); [EMAIL_ADDRESS] (J.R.)  
[PERSON]  
1 College of Physics and Electronic Engineering, Shanxi University, Taiyuan 030006, China; [EMAIL_ADDRESS] (F.Z. ); [EMAIL_ADDRESS] (Y.G. ); [EMAIL_ADDRESS] (L.Y. ); [EMAIL_ADDRESS] (Y.Y. ); [EMAIL_ADDRESS] (L.J. ); [EMAIL_ADDRESS] (J.R.)  
ZolySense Lab, Physics Department, Politecnico di Bari, I-70100 Bari, Italy; [EMAIL_ADDRESS] (S.d.R. ); [EMAIL_ADDRESS] (A.Z. ); [EMAIL_ADDRESS] (P.P.) Zugli Tajyuan  
3 State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi University, Taiyuan 030006, China  
blue colorimetric method [7], and potentiometric titration with a sulfide ion-selective electrode [8]. All these methods suffer from slow time response and short lifespan. In addition, gas chromatography tends to be costly, invasive, and occupy a large spatial footprint, thus cannot be used for real-time and in-situ applications. Optical gas sensors based on laser absorption spectroscopy (LAS) can represent a suitable solution for in-situ and fast trace gas detection, since they exploit the laser's narrow spectral resolution, allowing high sensitivity and selectivity [9; 10; 11; 12; 13; 14; 15]. In LAS, the gas concentration information is obtained by detecting the optical intensity via a photodetector. Among all LAS-based techniques, the optical sensors for trace gas detection can be divided in three major groups: (i) Direct absorption-based sensors, in which the laser beam passes through a single- or multi-pass absorption cell [16], (ii) cavity-enhanced based sensors, in which the gas cell is composed of a Fabry-Perot optical resonator [17], (iii) photoacoustic spectroscopy (PAS) sensors, where an acoustic cell is used to amplify sound waves generated by the gas itself when it absorbs intensity-modulated light [18]. Both multi-pass cells and optical cavities must be combined with optical detectors. The main strength of PAS is that no optical detectors are needed, and wavelength-insensitive microphones are used to detect the weak sound waves within the absorbing gas. Quartz-enhanced photoacoustic spectroscopy (QEPAS) is an excellent alternative to the conventional PAS, which eliminates constraints due to the acoustic cell size because the weak photoacoustic signal is detected via a quartz tuning fork (QTF) instead of a microphone [19; 20; 21]. A laser beam is focused between QTF prongs and sound waves produced by the modulated absorption of the gas put the QTF prongs in vibration. When the laser modulation frequency is resonant with one of the in-plane anti-symmetrical flexural modes of the QTF, the QTF converts the vibration energy into electrical energy through the piezoelectric effect occurring in the quartz crystal [22; 23]. Because of the quartz piezoelectric property, the strain induced by the acoustic wave hitting the QTF prongs generates electrical charges. QTF prongs are provided with golden patterns for collecting the electric charges and generating, in turn, an electrical signal. The electrical signal \(s\) will be proportional to the intensity of the sound wave by the relation: \(s=C\)-\(P\)-\(c_{gas}\)-\(\sigma\), where \(c_{gas}\) is the gas concentration, \(P\) is the optical power, \(N\) the number of the molecule per unit volume and \(\sigma\) the optical transition cross-section [19]. To probe and amplify the sound wave generated between QTF prongs, an acoustic resonator system is usually coupled with the QTF. The acoustic system composed of a QTF and an acoustic resonator is referred to as a QEPAS spectrophone [24].