Multi-pass absorption spectroscopy for H₂O₂ detection using a CW DFB-QCL

Hydrogen peroxide (H₂O₂) detection was demonstrated with multi-pass absorption spectroscopy using a commercial 76-m astigmatic multi-pass absorption cell. An ∼7.73-μm continuous wave, distributed feedback quantum cascade laser (CW DFB-QCL) was employed for targeting a strong H₂O₂ line (1296.2 cm^-1) in the fundamental absorption band. Wavelength modulation spectroscopy combined with a second harmonic detection technique was utilized to increase the signal-to-noise ratio. By optimizing the pressure inside the multi-pass cell and the wavelength modulation depth, a minimum detection limit (1σ) of 13.4 ppbv was achieved for H₂O₂ with a 2-s sampling time. From an Allan-Werle deviation plot, the detection limit could be improved to 1.5 ppbv with an averaging time of 200 s. Interference effects of atmospheric air components are also discussed.