Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) Detection of the ν 7 Band of Ethylene at Low Pressure with CO 2 Interference Analysis.

Ethylene (C2 H4 ) 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 C2 H4 and CO2 , a major interfering species in practical applications. The sensor was calibrated to show a good linear response to C2 H4 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 C2 H4 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 C2 H4 /N2 mixture on the photoacoustic signal of C2 H4 . An energy transfer model involving the vibration-vibration (VV) and vibration-translation (VT) transitions in the C2 H4 -CO2 -N2 system was constructed to interpret the experimental data. Additionally, the vibrational relaxation times of C2 H4 were obtained based on the QEPAS technique and the energy transfer model, which were in good agreement with the previous studies.