Abstract
We report on a quartz-enhanced photoacoustic (QEPAS) gas sensing system for hydrogen sulphide (H₂S) detection. The system architecture is based on a custom quartz tuning fork (QTF) optoacoustic transducer with a novel geometry and a quantum cascade laser (QCL) emitting 1.1 mW at a frequency of 2.913 THz. The QTF operated on the first flexion resonance frequency of 2871 Hz, with a quality factor Q = 17,900 at 20 Torr. The tuning range of the available QCL allowed the excitation of a H₂S rotational absorption line with a line-strength as small as S = 1.13·10⁻²² cm/mol. The measured detection sensitivity is 30 ppm in 3 seconds and 13 ppm for a 30 seconds integration time, which corresponds to a minimum detectable absorption coefficient α(min) = 2.3·10⁻⁷ cm⁻¹ and a normalized noise-equivalent absorption NNEA = 4.4·10⁻¹⁰ W·cm⁻¹·Hz(-1/2), several times lower than the values previously reported for near-IR and mid-IR H₂S QEPAS sensors.
Highlights
Recent breakthroughs in photonics and nanotechnology are enabling terahertz (THz) frequency research to be applied in a widespread range of sensing applications, such as medical diagnostics, global environmental monitoring, homeland security, petrochemical, chemical, industrial process monitoring, as well as noninvasive industrial and food industry inspection
Gas sensors based on quantum cascade laser (QCL) wavelength modulation absorption spectroscopy [5] or saturable absorption spectroscopy [6] for methanol vapors detection have been realized, providing a minimum detectable absorption coefficient normalized to the detection bandwidth of 5.5·10−6 cm−1 Hz−1/2 and saturation intensity of 25 μW/mm2, respectively
The THz laser source employed in this work is a single-mode bound-to-continuum QCL emitting at 2.91 THz (~97.11 cm−1), operated in a continuous wave (CW) mode and mounted on the cold finger of a continuous-flow cryostat equipped with polymethylpentene (TPX) windows
Summary
Recent breakthroughs in photonics and nanotechnology are enabling terahertz (THz) frequency research to be applied in a widespread range of sensing applications, such as medical diagnostics, global environmental monitoring, homeland security, petrochemical, chemical, industrial process monitoring, as well as noninvasive industrial and food industry inspection. The THz region offers advantages in terms of selectivity, because gas molecules have clear spectral “fingerprints” absorption spectra, arising from rotational quantum transitions These spectra allow unambiguous, more efficient and accurate detection compared to the characteristic ro-vibrational complex structures in the mid-IR. The destructive effects of H2S on equipment and pipelines along with the constant threat to personnel safety justify major investment in H2S sensing technology Such measurements cannot be performed using near or mid-infrared optical sensors, due to the ambiguity related to the dense absorption spectra of the buffer natural gas mixture (ethane, methane, propane and other hydrocarbons), while the THz range offers a spectral region of simplified absorption features and leads to a potential solution for sensitive H2S detection in natural gas
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