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  • Title: Reassessment of the NH4 NO3 thermal decomposition technique for calibration of the N2 O isotopic composition.
    Author: Mohn J, Gutjahr W, Toyoda S, Harris E, Ibraim E, Geilmann H, Schleppi P, Kuhn T, Lehmann MF, Decock C, Werner RA, Yoshida N, Brand WA.
    Journal: Rapid Commun Mass Spectrom; 2016 Dec 15; 30(23):2487-2496. PubMed ID: 27605461.
    Abstract:
    RATIONALE: 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). METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.
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