Isotopically characterised N2O reference materials for use as community standards

Rationale Information on the isotopic composition of nitrous oxide (N2O) 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 N2O isotope Reference Materials (RMs). Methods Within the project Metrology for Stable Isotope Reference Standards (SIRS), seven pure N2O isotope RMs have been developed and their N-15/N-14, O-18/O-16, O-17/O-16 ratios and N-15 site preference (SP) have been analysed by specialised laboratories against isotope reference materials. A particular focus was on the N-15 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 (delta) values: delta N-15: 0 to +104 parts per thousand, delta O-18: +39 to +155 parts per thousand, and delta N-15(SP): -4 to +20 parts per thousand. Conversion and uncertainty propagation of delta N-15 and delta O-18 to the Air-N-2 and VSMOW scales, respectively, provides robust estimates for delta N-15(N2O) and delta O-18(N2O), with overall uncertainties of about 0.05 parts per thousand and 0.15 parts per thousand, respectively. For delta N-15(SP), an offset of >1.5 parts per thousand compared with earlier calibration approaches was detected, which should be revisited in the 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 delta O-17 data for N2O RMs is expected to improve data quality/correction algorithms with respect to delta N-15(SP) and delta N-15 analysis by mass spectrometry. We anticipate that the N2O isotope RMs will enhance compatibility between laboratories and accelerate research progress in this emerging field.