Infrared laser-spectroscopic analysis of 14NO and 15NO in human breath

We report on monitoring of nitric oxide (NO) traces in human breath via infrared cavity leak-out spectroscopy. Using a CO sideband laser near 5 μm wavelength and an optical cavity with two high-reflectivity mirrors (R=99.98%), the minimum detectable absorption is 2×10⁻¹⁰ cm⁻¹ Hz^1/2. This allows for spectroscopic analysis of rare NO isotopologues with unprecedented sensitivity. Application to simultaneous online detection of ¹⁴NO and ¹⁵NO in breath samples collected in the nasal cavity is described for the first time. We achieved a noise-equivalent detection limit of 7 parts per trillion for nasal ¹⁵NO (integration time: 70 s).