Detection of HCl and HF by TTFMS and WMS

In this work we discuss on a compact spectrometer based on DFB diode lasers for detection of chloridric and fluoridric acids. HCl and HF concentrations are determined through optical absorption of the P(4) line (λ = 1.7 μm) and the R(3) line (λ = 1.3 μm), respectively. Both lines belong to first overtone vibrational bands and their linestrengths are 7.8 × 10−21 cm/molecule for HCl and 2.8 × 10−20 cm/molecule for HF. We chose these lines for their relative high intensities and because they are quite far from water vapour lines which represent the main interfering gas for trace-gases analysis. To detect these species we used two different high frequency modulation techniques: two-tone frequency modulation spectroscopy (f1 = 800 MHz and f2 = 804 MHz) was used for HCl while for HF we followed a simpler approach based on wavelength modulation spectroscopy (f = 600 kHz). We demonstrate that the two techniques provide comparable detection limit of about 80 ppbV at atmospheric pressure. Positive testing of our spectrometer makes it suitable for in situ measurements of exhaust gases coming from waste incinerators.