Rapid ppb-Level Methane Detection Based on Quartz-Enhanced Photoacoustic Spectroscopy

In the paper, quartz-enhanced photoacoustic spectroscopy (QEPAS) and heterodyne quartz-enhanced photoacoustic spectroscopy (H-QEPAS)-based ppb-level methane (CH₄) detection using a self-designed low-frequency round-head quartz tuning fork (QTF) and power-amplified diode laser is reported for the first time. Compared to the standard 32.768 kHz QTF, the novel round-head QTF, with a resonance frequency (f₀) of 9.7 kHz, is utilized as the acoustic wave transducer, benefiting from a longer energy accumulation time and reduced optical noise. A Raman fiber amplifier (RFA) is adopted to amplify the optical power of the continuous wavelength distributed feedback (CW-DFB) diode laser to 300 mW. Acoustic microresonators (AmRs) at specific sizes are on both sides of the QTF for enhancement of acoustic waves. It is observed that, after the installation of AmRs, the signal level is enhanced by a factor of 107.029 compared to the bare QTF. Both CH₄-QEPAS and CH₄-H-QEPAS sensors show excellent linearity in response to optical power and CH₄ concentration, with R-squared values exceeding 0.99 for each. The minimum detection limit (MDL) is determined to be 1.321 and 2.126 ppb for CH₄-QEPAS and CH₄-H-QEPAS, respectively, when the integration time of the sensor systems is extended to 1000 s. Compared to the 50 s measurement period of the CH₄-QEPAS sensor, CH₄-H-QEPAS can identify the f₀ of QTF and finish the measurement in 3 s, demonstrating its rapid measurement capability. Furthermore, H-QEPAS technology allows for the acquisition of the f₀ without interrupting the measurement, enabling real-time calibration of the f₀. Finally, the sensor is utilized for continuous monitoring of CH₄ concentrations in air and human-exhaled gases, demonstrating its practical measurement capabilities.