Acetylene-enhanced methane-QEPAS sensing

Abstract

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.