Simultaneous detection of 14NO and 15NO using Faraday modulation spectroscopy

We describe a technique of simultaneous detection of 14 NO and 15 NO by means of Faraday Modulation Spectroscopy (FAMOS) based on a cw distributed feedback quantum cascade laser (QCL) operating near 5.4 μm. FAMOS is a spectroscopic method for selective, sensitive, and time-resolved detection of free radical molecules such as NO, in the mid-infrared spectral region. The selected spectral lines are the Q (1.5) for 15 NO located at 1842.76 cm −1 and the P (9.5) for 14 NO located at 1842.93 cm −1 . The detection limit (1 σ ) of 6 ppb for 15 NO and 62 ppb for 14 NO has been achieved. The simultaneous detection was performed using a fast laser frequency switching between the two isotopologues with a time resolution of 2 s. The isotope ratio ( δ 15 N) has been determined with a precision (1 σ ) of 0.52‰ at 800-s averaging time for 100 ppm NO-gas with a time resolution of 2 s. δ 15 N is determined after NO release from nitrite by chemical reduction with potassium iodine.