Rationale: In the last few years, the study of N₂O site-specific nitrogen isotope composition has been established as a powerful technique to disentangle N₂O emission pathways. This trend has been accelerated by significant analytical progress in the field of isotope ratio mass spectrometry (IRMS) and more recently quantum cascade laser absorption spectroscopy (QCLAS).
Methods: The ammonium nitrate (NH₄NO₃) decomposition technique provides a strategy to scale the ¹⁵N site-specific (SP ≡ δ¹⁵N^α – δ¹⁵N^β) and bulk (δ¹⁵N^bulk = (δ¹⁵N^α + δ¹⁵N^β)/2) isotopic composition of N₂O against the international standard for the ¹⁵N/¹⁴N isotope ratio (AIR-N₂). Within the current project ¹⁵N fractionation effects during thermal decomposition of NH₄NO₃ on the N₂O site preference were studied using static and dynamic decomposition techniques.
Results: The validity of the NH₄NO₃ decomposition technique to link NH₄⁺ and NO₃^– moiety-specific δ¹⁵N analysis by IRMS to the site-specific nitrogen isotopic composition of N₂O was confirmed. However, the accuracy of this approach for the calibration of δ¹⁵N^α and δ¹⁵N^β values was found to be limited by non-quantitative NH₄NO₃ decomposition in combination with substantially different isotope enrichment factors for the conversion of the NO₃^– or NH₄⁺ nitrogen atom into the or position of the N₂O molecule.
Conclusions: The study reveals that the completeness and reproducibility of the NH₄NO₃ decomposition reaction currently confine the anchoring of N₂O site-specific isotopic composition to the international isotope ratio scale AIR-N₂. The authors suggest establishing a set of N₂O isotope reference materials with appropriate site-specific isotopic composition, as community standards, to improve inter-laboratory compatibility.