Detection of Formaldehyde in Flames Using UV and MIR Absorption Spectroscopy

Abstract

Absorption spectroscopy in the ultraviolet (UV) and mid-infrared (MIR) spectral region has been used in a comparative study for the detection of formaldehyde in laminar low pressure flames of dimethyl ether (DME) and methane. Both spectral regions were tested to explore respective advantages and limitations, especially for the detection of stable molecules in flames. In the UV, cavity ring-down spectroscopy (CRDS), a highly sensitive multi-pass absorption technique, has been used for the detection of formaldehyde in the A − X4₁⁰ hot band at 370 nm. For measurements in the mid-infrared, the fundamental CO-stretch vibration ν₂ of formaldehyde at 5.9 μm was used. MIR laser radiation was generated with a pulsed quantum cascade laser (QCL), a novel laser type which offers high output energies and excellent beam quality in the MIR. It was shown that MIR-QCL absorption spectroscopy and UV-CRDS have a comparable sensitivity, even though only single-pass absorption was used in the MIR. Additionally, MIR-QCL is clearly superior in terms of the simplicity of the experimental setup. This shows the large potential of QCL based laser absorption spectroscopy for the sensitive detection of trace species in the MIR.