Reassessment of the NH4NO3 thermal decomposition technique for calibration of the N2O isotopic composition

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 ≡ δ15Nα – δ15Nβ) and bulk (δ15Nbulk = (δ15Nα + δ15Nβ)/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 δ15Nα and δ15Nβ 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 α or β 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. Copyright © 2016 John Wiley & Sons, Ltd.