Reassessment of the NH4 NO3 thermal decomposition technique for calibration of the N2 O isotopic composition.

Abstract

In the last few years, the study of N2 O site-specific nitrogen isotope composition has been established as a powerful technique to disentangle N2 O 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). The ammonium nitrate (NH4 NO3 ) decomposition technique provides a strategy to scale the 15 N site-specific (SP≡δ15 Nα - δ15 Nβ ) and bulk (δ15 Nbulk =(δ15 Nα +δ15 Nβ )/2) isotopic composition of N2 O against the international standard for the 15 N/14 N isotope ratio (AIR-N2 ). Within the current project 15 N fractionation effects during thermal decomposition of NH4 NO3 on the N2 O site preference were studied using static and dynamic decomposition techniques. The validity of the NH4 NO3 decomposition technique to link NH4+ and NO3- moiety-specific δ15 N analysis by IRMS to the site-specific nitrogen isotopic composition of N2 O was confirmed. However, the accuracy of this approach for the calibration of δ15 Nα and δ15 Nβ values was found to be limited by non-quantitative NH4 NO3 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 N2 O molecule. The study reveals that the completeness and reproducibility of the NH4 NO3 decomposition reaction currently confine the anchoring of N2 O site-specific isotopic composition to the international isotope ratio scale AIR-N2 . The authors suggest establishing a set of N2 O isotope reference materials with appropriate site-specific isotopic composition, as community standards, to improve inter-laboratory compatibility. Copyright © 2016 John Wiley & Sons, Ltd.