Construction and Optimization of an Apparatus for Detection of Nitric Oxide through Faraday Modulation Spectroscopy

Sammanfattning: Faraday modulation spectroscopy (FAMOS) is a technique for detection of paramagnetic molecules. By applying a magnetic field over a gaseous sample, the presence of paramagnetic species will rotate the polarization plane of light, addressing a transition in such a species. By placing a gas containing paramagnetic molecules between almost crossed polarizers and modulating the magnetic field, the intensity of the transmitted light will consequently be modulated. Since the rotation of the polarization plane of light is proportional to the concentration of species, this technique can be used for quantitative analysis of paramagnetic molecules. Since FAMOS is solely sensitive to paramagnetic molecules it is superior to many other types of laser-based detection techniques, drastically decreasing various types of noise, background signals, as well as signals from other molecules; e.g. flicker noise, etalon effects and signals from water and CO2 molecules. An experimental setup for detection of nitric oxide (NO) by FAMOS has been developed and optimized. This system is based on a quantum cascade laser emitting light at 5.331 μm, addressing the—for FAMOS—most sensitive transition in NO, Q3/2(3/2). Optimized parameters include a pressure of 60 mbar, a magnetic field of 190 G and a polarizer uncrossing angle of 0.75°. In its present configuration, this system has demonstrated a detection of NO down to 200 ppb for a detection rate of 10 measurements per second. It is very possible that the limit of detection is even lower than this number since this lowest acquirable concentration is limited by the specifications of the gas mixer. A standard deviation between subsequent measurements, of 15 s time separation, is calculated to 30 ppb. However, this is far from the expected ultimate detection limit of this system and this technique in general. One process that causes a weakening of the signal is outgassing. When measuring on an emptied system this phenomenon is greatly reduced and a standard deviation of measurements is then measured and calculated to 7.6 ppb. The detection limit is presumed to be in the very low ppb, or sub-ppb, regime and this limit should be obtainable by further optimization of the system.