Methods: Within the project Metrology for Stable Isotope Reference Standards (SIRS), seven pure N2O isotope RMs have been developed and their 15N/14N, 18O/16O, 17O/16O ratios and 15N site preference (SP) have been analysed by specialised laboratories against isotope reference materials. A particular focus was on the 15N site-specific isotopic composition, as this measurand is both highly diagnostic for source appointment and challenging to analyse and link to existing scales.
Results: The established N2O isotope RMs offer a wide spread in delta (δ) values: δ15N: 0 to +104 ‰, δ18O: +39 to +155 ‰, and δ15NSP: −4 to +20 ‰. Conversion and uncertainty propagation of δ15N and δ18O to the Air-N2 and VSMOW scales, respectively, provides robust estimates for δ15N(N2O) and δ18O(N2O), with overall uncertainties of about 0.05 ‰ and 0.15 ‰, respectively. For δ15NSP, an offset of > 1.5 ‰ compared to earlier calibration approaches was detected, which should be revisited in future.
Conclusions: A set of seven N2O isotope RMs anchored to the international isotope-ratio scales was developed that will promote the implementation of the recommended two-point calibration approach. Particularly, the availability of δ17O data for N2O RMs is expected to improve data quality/correction algorithms with respect to δ15NSP and δ15N analysis by mass spectrometry. We anticipate that the N2O isotope RMs will enhance compatibility between laboratories and accelerate research progress in this emerging field.