Sensitive measurements of trace gas of formaldehyde using a mid-infrared laser spectrometer with a compact multi-pass cell

A compact multi-pass cell with a pair of cylindrical mirrors for sensitive detection of trace gases in emission from combustion was constructed. The cell path-length was 9.8 m and its volume was 0.13 L. Each mirror shape was a square with a side length of 25.4 mm and the mirrors were placed 100 mm apart. The cell was applied to detection of formaldehyde (HCHO), which is formed during fuel combustion and is harmful to the environment. The direct absorption spectrum in the range 2979.06-2981.2 cm−1 was recorded with a mid-infrared distributed feedback (DFB) interband cascade laser. The recorded spectrum of HCHO was in good agreement with a spectrum simulated using the HITRAN 2012 database. An absorption line at 2979.663 cm−1 (4.26 × 10−21 cm2 molecule−1 cm−1, ν5, 1184-1073), which showed the strongest absorption in the emission frequency range of the DFB interband cascade laser, was selected for HCHO detection. We also confirmed that there were no interferences of absorption peaks of major combustion products in the selected HCHO absorption peaks. At a signal-to-noise ratio of two and 3 kPa using 2f wavelength modulation spectroscopy at less than 1 MHz bandwidth, the limit of detection for HCHO was 73 ppb by volume.