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N2 Clumped Isotope Measurements with Thermo Fisher Scientific Ultra High-Resolution IRMS and Applications to the
Hao Yan1,2, Fengtai Tong1,2, Yongbo Peng1,2
1International Center for Isotope Effects Research, State Key Laboratory of Critical Earth Material Cycling and Mineral Deposits, Nanjing University, Nanjing 210023, China.
Analytical Chemistry
|October 30, 2025
Summary
The clumped-isotope signature (Δ30) of nitrogen gas (N2) can track nitrogen cycling. This study uses high-resolution mass spectrometry and a new purification system to accurately measure Δ30 in atmospheric and aquatic N2 samples.
Area of Science:
- Geochemistry and Environmental Science
- Isotope Geochemistry
- Biogeochemical Cycles
Background:
- Atmospheric N2 is enriched in 15N15N, making its clumped-isotope signature (Δ30) a valuable tracer for nitrogen cycling.
- Accurate Δ30 measurement is challenging due to low 15N15N abundance and limitations of conventional isotope ratio mass spectrometry (IRMS) in resolving interferences.
- High-resolution IRMS is necessary for precise Δ30 measurements, but requires highly pure N2 samples.
Purpose of the Study:
- To employ a state-of-the-art high-resolution IRMS to measure the clumped isotope composition of N2.
- To assess isobaric interference effects on Δ30 measurements and develop a purification system for high-purity N2.
- To determine the Δ30 value of atmospheric N2 and report Δ30 values for dissolved N2 from lakes.
Main Methods:
- Utilized a Thermo Fisher Scientific Ultra HR-IRMS for high-resolution analysis of N2 clumped isotopes (Δ30).
- Developed a purification system combining cold traps and gas chromatography (GC) to isolate N2 from sample matrices.
- Analyzed N2 produced via Sr3N2-catalyzed reordering, thermal decomposition of Sr3N2, and thermal decomposition of KN3.
Main Results:
- Identified critical intensity ratios for isobaric interferences (12C16O+/14N14N+ and 14N16O+/15N15N+) that cause positive Δ30 shifts at ~46,000 MRP.
- Determined the Δ30 value of atmospheric N2 to be 19.4 ± 0.3‰, consistent with previous studies.
- Reported Δ30 values for dissolved N2 in lakes ranging from 12.9 ± 0.5‰ to 20.5 ± 0.5‰, indicating mixing with biogenic N2.
Conclusions:
- High-purity N2 is essential for reliable Δ30 measurements using high-resolution IRMS.
- The decomposition of KN3 provides consistent Δ30 values, suitable for calibrating measurements.
- Clumped isotope composition of dissolved N2 is a promising proxy for tracing nitrogen cycling and estimating biogenic N2 contributions in aquatic systems.
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