Analytical Sciences

High-resolution Spectra of Bromo Methane and Methyl Bromide Obtained by a Continuously Tunable 10-bar CO₂ Laser Based Photoacoustic Spectrometer

Albert Romann and Markus W. Sigrist

Swiss Federal Institute of Technology (ETH), Institute of Quantum Electronics, Laboratory for Laser Spectroscopy and Environmental Sensing, CH-8093 Zürich, Switzerland

High resolution spectral data of 100 ppmV (10^-6 per volume) concentrations of the trace gases bromo methane (BM, CH₃Br), and methyl bromide (DME, (CH₃)₂O), buffered in synthetic air (80% N₂, 20% O₂) at atmospheric pressure and room temperature are reported. The spectra are recorded with a continuously tunable 10-bar CO₂ laser based photoacoustic (PA) spectrometer. The tuning range covers 76 cm^-1 between 9.2 um (1087 cm^-1) and 10.7 um (935 cm^-1) at a constand narrow line-width of 0.018 cm^-1 (540 MHz). The non-resonant PA measuring cell employs an in-line 10-microphone array. The estimated detection limits for BM and DME are approximately 2 ppmV for a signal-to-noise ratio (SNR) of 3. This corresponds to a calculated detection limit of  ~76 ppbV for ethylene.

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