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

Abstract We describe a technique of simultaneous detec-tion of 14NO and 15NO by means of Faraday Modulation Spectroscopy (FAMOS) based on a cw distributed feed-back quantum cascade laser (QCL) operating near 5.4 µm. FAMOS is a spectroscopic method for selective, sensi-tive, and time-resolved detection of free radical molecules such as NO, in the mid-infrared spectral region. The se-lected spectral lines are the Q (1.5) for 15NO located at 1842.76 cm−1 and the P (9.5) for 14NO located at 1842.93 cm−1. The detection limit (1σ) of 6 ppb/√Hz for 15NO and 62 ppb/ Hz for 14NO has been achieved. The simultaneous detection was performed using a fast laser fre-quency switching between the two isotopologues with a time resolution of 2 s. The isotope ratio (δ15N) has been deter-mined with a precision (1σ) of 0.52 ‰ at 800-s averaging time for 100 ppm NO-gas with a time resolution of 2 s. δ15N is determined after NO release from nitrite by chemi-cal reduction with potassium iodine