Isotopically characterised N₂ O reference materials for use as community standards

Information on the isotopic composition of nitrous oxide (N₂ O) at natural abundance supports the identification of its source and sink processes. In recent years, a number of mass spectrometric and laser spectroscopic techniques have been developed and are increasingly used by the research community. Advances in this active research area, however, critically depend on the availability of suitable N₂ O isotope Reference Materials (RMs).

Within the project Metrology for Stable Isotope Reference Standards (SIRS), seven pure N₂ O isotope RMs have been developed and their ¹⁵ N/¹⁴ N, ¹⁸ O/¹⁶ O, ¹⁷ O/¹⁶ O ratios and ¹⁵ N site preference (SP) have been analysed by specialised laboratories against isotope reference materials. A particular focus was on the ¹⁵ N site-specific isotopic composition, as this measurand is both highly diagnostic for source appointment and challenging to analyse and link to existing scales.

The established N₂ O isotope RMs offer a wide spread in delta (δ) values: δ¹⁵ N: 0 to +104‰, δ¹⁸ O: +39 to +155‰, and δ¹⁵ N^SP : -4 to +20‰. Conversion and uncertainty propagation of δ¹⁵ N and δ¹⁸ O to the Air-N₂ and VSMOW scales, respectively, provides robust estimates for δ¹⁵ N(N₂ O) and δ¹⁸ O(N₂ O), with overall uncertainties of about 0.05‰ and 0.15‰, respectively. For δ¹⁵ N^SP , an offset of >1.5‰ compared with earlier calibration approaches was detected, which should be revisited in the future.

A set of seven N₂ O 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 δ¹⁷ O data for N₂ O RMs is expected to improve data quality/correction algorithms with respect to δ¹⁵ N^SP and δ¹⁵ N analysis by mass spectrometry. We anticipate that the N₂ O isotope RMs will enhance compatibility between laboratories and accelerate research progress in this emerging field.