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

Mohn Joachim 1, Gutjahr Wilhelm 1, Toyoda Sakae 2, Harris Eliza 1, Ibraim Erkan 1, Geilmann Heike 3, Schleppi Patrick 4, Kuhn Thomas 5, Lehmann Moritz F. 5, Decock Charlotte 6, Werner Roland A. 7, Yoshida Naohiro 2,8, Brand Willi A. 3

1 Laboratory for Air Pollution & Environmental Technology, Swiss Federal Laboratories for Materials Science and Technology (EMPA), Überlandstr. 129, CH-8600 Dübendorf, Switzerland
2 Department of Chemical Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
3 Stable Isotope Laboratory, Max-Planck-Institute for Biogeochemistry (MPI-BGC), Hans-Knöll-Str. 10, DE-07745 Jena, Germany
4 Forest Soils and Biogeochemistry, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland
5 Biogeochemistry, University of Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland
6 Sustainable Agroecosystems, ETH Zürich, Tannenstrasse 1, CH-8092 Zürich, Switzerland
7 Institute of Agricultural Sciences, ETH Zürich, Universitätstrasse 2, CH-8092 Zürich, Switzerland
8 Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan

Rapid Commun. Mass Spectrom. 30 (2016): 2487-2496

DOI: 10.1002/rcm.7736


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 site-specific nitrogen isotopic composition of N2O was confirmed. However, the accuracy of this approach for calibration of δ15Nα and δ15Nβ 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 confines the anchoring of N2O site specific isotopic composition to the international isotope ratio scale AIR-N2. The authors suggest to establish a set of N2O isotope reference materials with appropriate site-specific isotopic composition, as community standards, to improve inter-laboratory compatibility.

Keywords: nitrous oxide, isotopic composition, site-preference, NH4NO3 thermal decomposition, isotope fractionation