# Near-Infrared Quartz-Enhanced Photoacoustic Sensor for H\({}_{2}\)S Detection in Biogas
## 2 Description of Experimental System  
The main requirement for a QEPAS sensor is to avoid spectral interference effects arising from other gas species, typically present at high concentration levels in ambient air (mainly H\({}_{2}\)O in 1%-2% range and CO\({}_{2}\) in 300-400 ppm range) which can negatively affect the sensitivity and reproducibility performance of a QEPAS sensor. According to HITRAN database, H\({}_{2}\)S gas absorption line located at 6320.6 cm\({}^{-1}\) with a line strength of 1.06 \(\times\) 10\({}^{-22}\) cm/molecule can be selected as the target detection line because the strongest water and CO\({}_{2}\) lines nearby are located at 6319.3 cm\({}^{-1}\) and 6319.2 cm\({}^{-1}\) with line-strengths three and two orders of magnitude lower, respectively [32].  
A schematic of the QEPAS sensor based on a near-infrared distributed feedback (DFB) laser and an EDFA for H\({}_{2}\)S detection is depicted in Figure 1. A DFB laser (model FITEL Inc. FRL15 DCWD-A82) with a wavelength emission at 1.58 \(\upmu\)m was used as the excitation laser. The laser was driven by a control electronics unit (CEU) connected to a computer via an RS232 serial port. The wavelength of the laser can be finely adjusted by setting the temperature and the DC current via the CEU. To scan across the selected absorption line, a low-frequency (0.2 Hz) ramp was provided to the laser current. A _2f_ wavelength modulation spectroscopy (_2f_-WMS) approach was adopted by applying a sinusoidal dither at half of the QTF resonance frequency to the laser driver current and by demodulating the QTF signal at the QTF resonance frequency by a lock-in amplifier, after being converted in a voltage signal by means of a transimpedance amplifier. For all measurements reported in this work, the lock-in detection bandwidth was set to 0.833 Hz (corresponding to a time constant of 300 ms). The laser beam was transmitted to the EDFA via an optical fiber to obtain an adjustable output power. Then, the laser beam was focused in the acoustic detection module (ADM), where the QEPAS spectrophone is located. The gas flow was set to 150 sccm while the pressure was stabilized to 760 Torr within the ADM. The laser beam passes through the ADM via two CaF\({}_{2}\) windows located on both sides of the gas enclosure.  
A custom QTF with a prong spacing of 800 \(\upmu\)m was used to assemble the QEPAS spectrophone, as shown in Figure 2a,b [35]. The geometrical parameters (prong length and width and the prong spacing), and electrical parameters (resonance frequency, quality factor, and resistance) of the QTF, are listed in Table 1. As a comparison, the same parameters of a standard 32.7 kHz-QTF are also reported.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Prong Parameters**} & \multicolumn{4}{c}{**Electrical Parameters**} \\ \hline  & **Spacing s** & **Length 1** & **Width w** & **Frequency** & **Q Factor** & **Resistance** \\  & **(mm)** & **(mm)** & **(mm)** & **(Hz)** & **Q Factor** & **(KJ)** \\ \hline Standard & 0.3 & 3.8 & 0.6 & 32.768 & 12.000 & 120 \\ QTF & 0.8 & 10 & 0.9 & 7205 & 8500 & 290 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Geometrical (prong length, prong width, and prong spacing) and electrical parameters (resonance frequency, quality factor, and electrical resistance) of the custom quartz tuning fork (QTF) and a standard 32.7 kHz-QTF.  
The prong spacing of custom QTF is 2.6 times larger than the standard 32.7 kHz-QTF (300 \(\upmu\)m). This can be very useful when high-power lasers are used because a large prong spacing can reduce the portion of light hitting the internal surface of QTF prongs. When light touches the prong surface, a photothermal noise contribution is generated that can overwhelm the ultimate noise level, namely the thermal noise of the QTF, limiting the ultimate detection limit of the QEPAS sensor [36].