Abstract
We describe a technique of simultaneous detection of 14NO and 15NO 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 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 \(/\sqrt{\mathrm{Hz}}\) for 15NO and 62 ppb \(/\sqrt{\mathrm{Hz}}\) for 14NO 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 (δ15N) has been determined 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 chemical reduction with potassium iodine.
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