Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) Detection of the ν₇ Band of Ethylene at Low Pressure with CO₂ Interference Analysis

Ethylene (C₂H₄) was detected using quartz-enhanced photoacoustic spectroscopy (QEPAS) at 10.5 µm with a continuous wave, distributed-feedback quantum cascade laser as the light source. The QEPAS sensor was operated at low pressures (≤200 torr) to eliminate the cross-talk spectral interference between C₂H₄ and CO₂, a major interfering species in practical applications. The sensor was calibrated to show a good linear response to C₂H₄ concentration and the Allan deviation analysis demonstrated a minimum detection limit of 8 ppb at an integration time of 90 s. Although no spectral overlap between C₂H₄ and CO₂ was confirmed at the pressure ≤200 torr by the direct absorption measurement using a 28-m multipass cell, we observed the apparent influence of the CO₂ addition to the C₂H₄/N₂ mixture on the photoacoustic signal of C₂H₄. An energy transfer model involving the vibration-vibration (VV) and vibration-translation (VT) transitions in the C₂H₄-CO₂-N₂ system was constructed to interpret the experimental data. Additionally, the vibrational relaxation times of C₂H₄ were obtained based on the QEPAS technique and the energy transfer model, which were in good agreement with the previous studies.