Precise isotope abundance ratio measurement of nitrous oxide using diode lasers

High-precision measurement of the isotope abundance ratio 14N15N16O/15N14N16O/14N14N18O/14N14N16O (∼0.37/0.37/0.2/100) in nitrous oxide by a laser-spectroscopic method is demonstrated. The abundance ratio is determined from the absorbance ratio of selected absorption lines near 2 μm of the respective isotopic molecular species. Four wavelength-modulated diode lasers whose center wavelengths are stabilized on the selected lines are made to travel different distances in a multipass cell to compensate the large abundance differences. In the test experiment using pure nitrous oxide samples the abundance ratio can be measured with relative uncertainties of ±(2–6)×10−4 (±0.2–0.6‰). The laser-spectroscopic method is expected to complement mass spectrometry in isotope analyses to trace the geochemical cycles of environmental substances.