Mid‐infrared polarization spectroscopy: A tool for in situ measurements of toxic gases in smoke‐laden environments

Infrared polarization spectroscopy (IRPS) was used to detect HCl in an 800 mm long tube furnace. Pieces of a polyvinyl chloride‐carpet were continuously fed into the furnace producing a heavy smoke, which is exemplified by the fact that the smoke completely obscured a red laser beam from a He‐Ne laser. This constitutes a very harsh environment from a diagnostic point of view due to the high smoke density and relatively long path through the furnace. Despite this it was still possible to measure HCl concentrations in the smoke down to a level of ∼50ppm using IRPS. The explanation for this success is twofold. First, the IRPS method is inherently almost noise free due to the use of crossed polarizers, creating a virtually zero background. Second, the laser beam attenuation due to non‐resonant absorption and scattering in the smoke, especially with soot particles, decreases with increasing laser wavelength. Therefore, this type of measurements would have been much more difficult to perform in the visible regime (with wavelengths ∼0.5µm) than in the infrared regime (with wavelengths ∼3µm).