Acetylene-enhanced methane-QEPAS sensing.

Methane (CH₄), as a critical greenhouse gas and explosive hazard, demands highly sensitive detection for environmental monitoring and industrial safety. To address the limitation of its slow relaxation in quartz-enhanced photoacoustic spectroscopy (QEPAS), this paper reported a novel acetylene (C₂H₂)-enhanced QEPAS technique for CH₄ sensing for the first time to our knowledge. Unlike the commonly used catalyst of water vapor (H₂O), whose concentration in the air frequently changes and causes fluctuations in both the QEPAS signal level and the characteristics of the quartz tuning fork (QTF), C₂H₂ accelerates CH₄ relaxation without inducing significant shifts in the QTF frequency. Firstly, the catalytic effect of C₂H₂ molecules on the CH₄ relaxation process was analyzed. Systematic investigations revealed that increasing C₂H₂ concentration enhances CH₄-QEPAS signal intensity in three distinct phases: rapid growth, gradual saturation, and eventual stabilization. At 6000 ppm C₂H₂, the CH₄-QEPAS signal amplitude increased by 2.53-fold compared to the situation without C₂H₂. The C₂H₂-enhanced CH₄-QEPAS system maintained excellent linearity (R² = 0.9999) across 1000-12,000 ppm CH₄. Allan deviation analysis confirmed a minimum detection limit (MDL) of 540 ppb at 1000 s average time, demonstrating excellent long-term stability. This work not only provides a robust strategy for CH₄ detection but also expands the application of QEPAS in gas relaxation dynamics modulation, highlighting C₂H₂ as a superior relaxation promoter for other molecules with a slow relaxation rate.