Comparison of QEPAS and BF-QEPAS approaches for methane and ethane sequential detection in the near-IR spectral range

This paper presents a comparison between quartz-enhanced photoacoustic spectroscopy (QEPAS) and beat-frequency QEPAS (BF-QEPAS) techniques for the sequential detection of methane (C1) and ethane (C2) in the near-infrared spectral range. Both approaches exploit a T-shaped quartz tuning fork (QTF)—coupled with acoustic resonator tubes—as sensitive element but differ fundamentally in the signal generation and acquisition methods. While conventional QEPAS-based approach requires periodic QTF characterization and longer acquisition time, BF-QEPAS enables simultaneous measurement of target gas concentration, QTF resonance frequency and quality factor through analysis of transient response signals. Experiments were performed using a laser diode emitting at a central wavelength of 1683.53 nm, targeting C1 and C2 absorption features. Our results demonstrate that the BF-QEPAS approach significantly reduces measurement time from minutes to few seconds and maintains comparable detection sensitivity, also for broadband absorbers such as ethane. For methane, minimum detection limits (MDLs) of 1.7 parts-per-million (ppm) and 5 ppm were achieved with QEPAS and BF-QEPAS techniques, respectively, while for ethane MDLs of 20 ppm and 62 ppm were obtained, respectively. The BF-QEPAS technique enables continuous, uninterrupted monitoring of both target gases in sequential detection mode, with the simultaneous validation of the measurement through the evaluation of the QTF resonance parameters.