Selective BTEX detection using laser absorption spectroscopy in the CH bending mode region

BTEX (benzene, toluene, ethylbenzene, and xylene) are simple aromatics that play a major role in soot formation in combustion systems. They are widely used as aromatic representatives in fuel surrogates. Measuring BTEX in chemical reactors can give insight into the soot formation mechanisms and help mitigate its production. The similar chemical structures of BTEX make their selective detection highly challenging. These species give rise to broad and similar absorption features across most of the electromagnetic spectrum. In this work, we report the development of a selective BTEX laser-based sensor in the long-wave mid-infrared (LW-MIR) spectral region. The sensor probes the rarely accessed CH bending modes of BTEX molecules and takes advantage of the strong, spectrally isolated absorption features of aromatics in this spectral region. The developed sensor is based on a custom-designed difference-frequency generation (DFG) laser source. Wavelength selection, and the development and evaluation of the performance of the custom-designed sensor are discussed. The sensor was found to perform excellently over the mole fraction range of 0 - 1,000 ppm of each of the BTEX species. Estimated detection limits of our developed sensor are 60, 15, 40, and 50 ppm for benzene, toluene, ethylbenzene, and m-xylene, respectively.