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RationaleNitrogen isotopic compositions (δ15N) of source and trophic amino acids (AAs) are crucial tracers of N sources and trophic enrichments in diverse fields, including archeology, astrobiochemistry, ecology, oceanography, and paleo‐sciences. The current analytical technique using gas chromatography‐combustion‐isotope ratio mass spectrometry (GC/C/IRMS) requires derivatization, which is not compatible with some key AAs. Another approach using high‐performance liquid chromatography‐elemental analyzer‐IRMS (HPLC/EA/IRMS) may experience coelution issues with other compounds in certain types of samples, and the highly sensitive nano‐EA/IRMS instrumentations are not widely available. MethodsWe present a method for high‐precision δ15N measurements of AAs (δ15N‐AA) optimized for canonical source AA‐phenylalanine (Phe) and trophic AA‐glutamic acid (Glu). This offline approach entails purification and separation via high‐pressure ion‐exchange chromatography (IC) with automated fraction collection, the sequential chemical conversion of AA to nitrite and then to nitrous oxide (N2O), and the final determination of δ15N of the produced N2O via purge‐and‐trap continuous‐flow isotope ratio mass spectrometry (PT/CF/IRMS). ResultsThe cross‐plots of δ15N of Glu and Phe standards (four different natural‐abundance levels) generated by this method and their accepted values have a linear regression slope of 1 and small intercepts demonstrating high accuracy. The precisions were 0.36‰–0.67‰ for Phe standards and 0.27‰–0.35‰ for Glu standards. Our method and the GC/C/IRMS approach produced equivalent δ15N values for two lab standards (McCarthy Lab AA mixture and cyanobacteria) within error. We further tested our method on a wide range of natural sample matrices and obtained reasonable results. ConclusionsOur method provides a reliable alternative to the current methods for δ15N‐AA measurement as IC or HPLC‐based techniques that can collect underivatized AAs are widely available. Our chemical approach that converts AA to N2O can be easily implemented in laboratories currently analyzing δ15N of N2O using PT/CF/IRMS. This method will help promote the use of δ15N‐AA in important studies of N cycling and trophic ecology in a wide range of research areas.
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A High‐Precision Analytical Technique for Dissolved N 2 Isotopes in Aquatic Systems: Biogeochemical Applications and Determination of Solubility Equilibrium Isotope Effects
ABSTRACT RationaleThe isotopic composition of dissolved dinitrogen gas (δ15N‐N2) in water can offer a powerful constraint on the sources and pathways of nitrogen cycling in aquatic systems. However, because of the large presence of atmosphere‐derived dissolved N2in these systems, high‐precision (on the order of 0.001‰) measurements of N2isotopes paired with inert gas measurements are required to disentangle atmospheric and biogeochemical signals. Additionally, the solubility equilibrium isotope fractionation of N2and its temperature and salinity dependence are underconstrained at this level of precision. MethodsWe introduce a new technique for sample collection, processing, and dynamic dual‐inlet mass spectrometry allowing for high‐precision measurement of δ15N‐N2and δ(N2/Ar) with simultaneous measurement of δ(40Ar/36Ar) and δ(Kr/N2) in water. We evaluate the reproducibility of this technique and employ it to redetermine the solubility equilibrium isotope effects for dissolved N2across a range of temperatures and salinities. ResultsOur technique achieves measurement reproducibility (1σ) for δ15N‐N2(0.006‰) and δ(N2/Ar) (0.41‰) suitable for tracing biogeochemical nitrogen cycling in aquatic environments. Through a series of air–water equilibration experiments, we find a N2solubility equilibrium isotope effect (ε = α/1000 − 1, where α = (29N2/28N2)dissolved/(29N2/28N2)gas) in water of ε(‰) = 0.753 − 0.004•TwhereTis the temperature (°C), with uncertainties on the order of 0.001‰ over the temperature range of ~2°C–23°C and salinity range of ~0–30 psu. We find no apparent dependence of ε on salinity. ConclusionsOur new method allows for high‐precision measurements of the isotopic composition of dissolved N2and Ar, and dissolved N2/Ar and Kr/N2ratios, within the same sample. Pairing measurements of N2with inert gases facilitates the quantification of excess N2from biogeochemical sources and its isotopic composition. This method allows for a wide range of applications in marine, coastal, and freshwater environments to characterize and quantitatively constrain potential nitrogen‐cycling sources and pathways and to differentiate between physical and biological isotope signals in these systems.
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- Award ID(s):
- 2122427
- PAR ID:
- 10610817
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Rapid Communications in Mass Spectrometry
- Volume:
- 39
- Issue:
- 19
- ISSN:
- 0951-4198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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