Detection of shock-heated hydrogen peroxide (H₂O₂) by off-axis cavity-enhanced absorption spectroscopy (OA-CEAS)

Cavity-enhanced absorption spectroscopy (CEAS) is a promising technique for studying chemical reactions due to its desirable characteristics of high sensitivity and fast time-response by virtue of the increased path length and relatively short photon residence time inside the cavity. Off-axis CEAS (OA-CEAS) is particularly suited for the shock tube applications as it is insensitive to slight misalignments, and cavity noise is suppressed due to non-overlapping multiple reflections of the probe beam inside the cavity. Here, OA-CEAS is demonstrated in the mid-IR region at 1310.068 cm⁻¹ to monitor trace concentrations of hydrogen peroxide (H₂O₂). This particular probe frequency was chosen to minimize interference from other species prevalent in combustion systems and in the atmosphere. The noise-equivalent detection limit is found to be 3.25 × 10⁻⁵ cm⁻¹, and the gain factor of the cavity is 131. This corresponds to a detection limit of 74 ppm of H₂O₂ at typical high-temperature combustion conditions (1200 K and 1 atm) and 12 ppm of H₂O₂ at ambient conditions (296 K and 1 atm). To our knowledge, this is the first successful application of the OA-CEAS technique to detect H₂O₂ which is vital species in combustion and atmospheric science.