A High-Precision Analytical Technique for Dissolved N2 Isotopes in Aquatic Systems: Biogeochemical Applications and Determination of Solubility Equilibrium Isotope Effects

IF 1.7 3区化学 Q4 BIOCHEMICAL RESEARCH METHODS

Rapid Communications in Mass Spectrometry Pub Date : 2025-06-17 DOI:10.1002/rcm.10094

Katelyn McPaul, Scott D. Wankel, Alan M. Seltzer

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引用次数: 0

Abstract

Rationale

The 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 N₂ in these systems, high-precision (on the order of 0.001‰) measurements of N₂ isotopes paired with inert gas measurements are required to disentangle atmospheric and biogeochemical signals. Additionally, the solubility equilibrium isotope fractionation of N₂ and its temperature and salinity dependence are underconstrained at this level of precision.

Methods

We introduce a new technique for sample collection, processing, and dynamic dual-inlet mass spectrometry allowing for high-precision measurement of δ¹⁵N-N₂ and 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 N₂ across a range of temperatures and salinities.

Results

Our 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 N₂ solubility equilibrium isotope effect (ε = α/1000 − 1, where α = (²⁹N₂/²⁸N₂)_dissolved/(²⁹N₂/²⁸N₂)_gas) in water of ε(‰) = 0.753 − 0.004•T where T is 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.

Conclusions

Our new method allows for high-precision measurements of the isotopic composition of dissolved N₂ and Ar, and dissolved N₂/Ar and Kr/N₂ ratios, within the same sample. Pairing measurements of N₂ with inert gases facilitates the quantification of excess N₂ from 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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水生系统中溶解态氮同位素的高精度分析技术：生物地球化学应用和溶解度平衡同位素效应的测定

水中溶解二氮（δ15N-N2）的同位素组成可以为水生系统氮循环的来源和途径提供强有力的约束。然而，由于这些系统中存在大量来自大气的溶解N2，因此需要高精度（约0.001‰）的N2同位素测量与惰性气体测量相结合，以解开大气和生物地球化学信号。此外，在这个精度水平上，N2的溶解度平衡同位素分馏及其对温度和盐度的依赖是不受限制的。方法介绍了一种新的样品采集、处理和动态双入口质谱技术，可以高精度测量水中δ(40Ar/36Ar)和δ(Kr/N2)的δ 15n -N2和δ(N2/Ar)。我们评估了该技术的可重复性，并利用它来重新确定溶解N2在一系列温度和盐度下的溶解度平衡同位素效应。结果该方法的δ 15n -N2（0.006‰）和δ(N2/Ar)（0.41‰）测量重现性为1σ，适用于水体生物地球化学氮循环的示踪。通过一系列的空气-水平衡实验，我们发现在ε（‰）= 0.753 ~ 0.004•T （T为温度）的水中，N2溶解度平衡同位素效应(ε = α/1000−1，其中α = （29N2/28N2)溶解/(29N2/28N2)气体），在~2℃~ 23℃和~0 ~ 30 psu的温度范围内，不确定性约为0.001‰。我们发现ε对盐度没有明显的依赖性。结论该方法可以高精度测量同一样品中溶解态N2和Ar的同位素组成，以及溶解态N2/Ar和溶解态Kr/N2的比值。氮气与惰性气体的配对测量有助于量化来自生物地球化学来源的过量氮气及其同位素组成。该方法可以在海洋、沿海和淡水环境中广泛应用，以表征和定量限制潜在的氮循环源和途径，并区分这些系统中的物理和生物同位素信号。

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来源期刊

Rapid Communications in Mass Spectrometry 化学-分析化学

CiteScore

4.10

自引率

5.00%

发文量

219

审稿时长

2.6 months

期刊介绍： Rapid Communications in Mass Spectrometry is a journal whose aim is the rapid publication of original research results and ideas on all aspects of the science of gas-phase ions; it covers all the associated scientific disciplines. There is no formal limit on paper length ("rapid" is not synonymous with "brief"), but papers should be of a length that is commensurate with the importance and complexity of the results being reported. Contributions may be theoretical or practical in nature; they may deal with methods, techniques and applications, or with the interpretation of results; they may cover any area in science that depends directly on measurements made upon gaseous ions or that is associated with such measurements.