Abstract
A quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for H2S detection operating in near-infrared spectral range is reported. The optical source is an erbium-doped fiber amplified laser with watt-level optical power. The QEPAS spectrophone is composed of a quartz tuning fork with a resonance frequency of 7.2 kHz, a quality factor of 8500, and a distance between prongs of 800 µm, and two tubes with a radius of 1.3 mm and a length of 23 mm acting as an organ pipe resonator. With this spectrophone geometry, the photothermal noise contribution of the spectrophone was removed and the theoretical thermal noise level was achieved. The position of both tubes with respect to custom quartz tuning fork has been investigated as a function of signal amplitude, Q-factor, and noise of the QEPAS sensor when a high-power laser was used. Benefit from the linearity of the QEPAS signal to the excitation laser power, a detection sensitivity of 330 ppb for H2S detection was achieved at atmospheric pressure and room temperature, when the laser power was 1.6 W and the signal integration time was set to 300 ms, corresponding to a normalized noise equivalent absorption of 3.15 × 10−9 W cm−1/(Hz)1/2. The QEPAS sensor was then validated by measuring H2S in a biogas sample.
Highlights
Hydrogen sulfide (H2 S), a colorless, extremely toxic, acidic gas, is an important chemical raw material [1]
Among all laser absorption spectroscopy (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]
A sensitive H2 S trace gas sensor based on the quartz-enhanced photoacoustic spectroscopy (QEPAS) technique was realized
Summary
Hydrogen sulfide (H2 S), a colorless, extremely toxic, acidic gas, is an important chemical raw material [1]. 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]. Spagnolo et al [31] reported an H2 S QEPAS sensor exploiting a custom QTF with a prong spacing as large as 700 μm and a THz QCL emitting at 2.91 THz. Detection sensitivity of 13 ppm with 30 s integral time has been reached. The sensor was calibrated starting from a 50 ppm H2 S:N2 certified gas mixture
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