2025/10/30 by Hao Yan, Fengtai Tong, Yongbo Peng +2 · 1 voice
Earth and Planetary Sciences · #Atmospheric chemistry and aerosols #Groundwater and Isotope Geochemistry #Atmospheric Ozone and Climate
paper · doi:10.1021/acs.analchem.5c02966
openalex publication_date 2025/10/30 · openalex created_date 2025/11/01 · openalex updated_date 2026/06/14
Because atmospheric N 2 is highly enriched in 15 N 15 N compared to stochastic expectations, its clumped-isotope signature (Δ 30 ) can serve as a tracer for nitrogen cycling. Despite the environmental and geological significance of this proxy, accurate and precise Δ 30 measurement remains challenging due to the low abundance of 15 N 15 N in N 2 and the limitations of conventional isotope ratio mass spectrometry (IRMS), which cannot resolve typical interferences at mass 30. In this study, we employed a state-of-the-art high-resolution IRMS (Thermo Fisher Scientific Ultra HR-IRMS) to measure the clumped isotope composition of N 2 and to assess isobaric interference effects. We found that, when the intensity ratios of 12 C 16 O + / 14 N 14 N + and 14 N 16 O + / 15 N 15 N + exceed 6.4 × 10 –4 and 8, respectively, the measured Δ 30 exhibits a clear positive shift at a mass resolving power (MRP) of ∼46,000, underscoring the necessity of a high-purity N 2 analyte for reliable measurements. To achieve this, we developed a purification system combining cold traps and gas chromatography (GC) to isolate N 2 from various sample matrixes. Using this system and Ultra HR-IRMS, we analyzed Δ 30 values of N 2 produced via three methods: (i) Sr 3 N 2 -catalyzed reordering, (ii) thermal decomposition of Sr 3 N 2, and (iii) thermal decomposition of KN 3 . While all three methods can generate N 2 with isotopically stochastic distributions, the decomposition of KN 3 yielded the most consistent Δ 30 values. By measuring against isotopically stochastic N 2, we determined the Δ 30 value of atmospheric N 2 to be 19.4 ± 0.3‰ (1SD), consistent with previous measurements using the Nu Instruments Panorama mass spectrometer. Finally, we reported the Δ 30 values of dissolved N 2 from lakes, ranging from 12.9 ± 0.5‰ to 20.5 ± 0.5‰. The lower Δ 30 values in dissolved N 2 relative to atmospheric N 2 likely result from mixing with biogenic N 2 produced by bacterial denitrification and/or anammox. Using a simple mixing model, we estimated the fraction and nitrogen isotope composition (δ 15 N) of biogenic N 2, providing insights into its formation mechanisms in aquatic systems. Our findings suggest that the clumped isotope composition of dissolved N 2 is a promising proxy for tracing nitrogen cycling in the hydrosphere.