Challenges in measuring nitrogen isotope signatures in inorganic nitrogen forms: An interlaboratory comparison of three common measurement approaches

A - Papers appearing in refereed journals

Biasi, C., Jokinen, S., Prommer, J., Ambus, P., Dorsch, P., Yu, L., Granger, S. J., Boeckx, P., Nieuland, K. V., Bruggemann, N. ., Wissel, H., Voropaev, A., Zilberman, T., Jantti, H., Trubnikova, T., Welti, N., Voigt, C., Gebus-Czupyt, B., Czupyt, Z. and Wanek, W. 2022. Challenges in measuring nitrogen isotope signatures in inorganic nitrogen forms: An interlaboratory comparison of three common measurement approaches. Rapid Communications in Mass Spectrometry. 36 (22), p. e9370. https://doi.org/10.1002/rcm.9370

AuthorsBiasi, C., Jokinen, S., Prommer, J., Ambus, P., Dorsch, P., Yu, L., Granger, S. J., Boeckx, P., Nieuland, K. V., Bruggemann, N. ., Wissel, H., Voropaev, A., Zilberman, T., Jantti, H., Trubnikova, T., Welti, N., Voigt, C., Gebus-Czupyt, B., Czupyt, Z. and Wanek, W.
Abstract

Rationale
Stable isotope approaches are increasingly applied to better understand the cycling of inorganic nitrogen (Ni) forms, key limiting nutrients in terrestrial and aquatic ecosystems. A systematic comparison of the accuracy and precision of the most commonly used methods to analyze δ15N in NO3− and NH4+ and interlaboratory comparison tests to evaluate the comparability of isotope results between laboratories are, however, still lacking.

Methods
Here, we conducted an interlaboratory comparison involving 10 European laboratories to compare different methods and laboratory performance to measure δ15N in NO3− and NH4+. The approaches tested were (a) microdiffusion (MD), (b) chemical conversion (CM), which transforms Ni to either N2O (CM-N2O) or N2 (CM-N2), and (c) the denitrifier (DN) methods.

Results
The study showed that standards in their single forms were reasonably replicated by the different methods and laboratories, with laboratories applying CM-N2O performing superior for both NO3− and NH4+, followed by DN. Laboratories using MD significantly underestimated the “true” values due to incomplete recovery and also those using CM-N2 showed issues with isotope fractionation. Most methods and laboratories underestimated the at%15N of Ni of labeled standards in their single forms, but relative errors were within maximal 6% deviation from the real value and therefore acceptable. The results showed further that MD is strongly biased by nonspecificity. The results of the environmental samples were generally highly variable, with standard deviations (SD) of up to ± 8.4‰ for NO3− and ± 32.9‰ for NH4+; SDs within laboratories were found to be considerably lower (on average 3.1‰). The variability could not be connected to any single factor but next to errors due to blank contamination, isotope normalization, and fractionation, and also matrix effects and analytical errors have to be considered.

Year of Publication2022
JournalRapid Communications in Mass Spectrometry
Journal citation36 (22), p. e9370
Digital Object Identifier (DOI)https://doi.org/10.1002/rcm.9370
Open accessPublished as ‘gold’ (paid) open access
FunderBiotechnology and Biological Sciences Research Council
Publisher's version
Output statusPublished
Publication dates
Online29 Jul 2022
Publication process dates
Accepted28 Jul 2022
PublisherWiley
ISSN0951-4198

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