Time resolved simultaneous detection of 14NO and 15NO via mid-infrared cavity leak-out spectroscopy

Abstract

We present a ring-down absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near λ = 5 μm. Using a linear ring-down cavity (length: 0.5 m) with high reflective mirrors (R = 99.988 %), we observed a noise-equivalent absorption coefficient of 3 × 10−10 cm−1Hz−1/2. This corresponds to a noise-equivalent concentration of 800 parts per trillion (ppt) for 14NO and 40 ppt for 15NO in 1 s averaging time. We achieve a time resolution of 1 s which allows time resolved simultaneous detection of the two N isotopes. The δ15N value was obtained with a precision of ±1.2‰ in a sample with a NO fraction of 11 ppm. The simultaneous detection enables the use of 15NO as a tracer molecule for endogenous biomedical processes.