Towards a calibration-free analysis of 15N site preference in N2O reference materials using matrix-isolation infrared spectroscopy

In the global nitrous oxide (N₂O) budget, various processes can influence the natural isotope abundances, often enriched with ¹⁵N with a site-specific preference δ¹⁵N^SP that serves as a unique natural isotope tracer. Unlike δ¹⁸O and δ¹⁵N^bulk, δ¹⁵N^SP is independent of the substrate’s isotopic signature and remains unchanged during N₂O diffusion. However, while δ ¹⁵N^SP can reveal mechanisms of N₂O formation and reduction [1], distinguishing between production and consumption processes remains challenging due to overlapping isotopic signatures and variable fractionation factors. Current approaches, such as dual isotope plots (e.g., δ¹⁵N^SP/δ¹⁵N^bulk), help constrain dominant pathways but rely on experimental fractionation data. Which can be difficult considering that for the determination of ¹⁵N^SP values with isotope ratio mass spectrometry (IRMS) methods it was shown that they are highly reliant on the choice of calibration with differences of up to 30 ‰ [2]. At the same time, laser absorption spectroscopy (LAS) of rotational-vibrational transition is prone to interferences by other trace gases, requires rigorous calibration and needs preconcentration units [3-4]. We propose using matrix-isolation Fourier-transform infrared (MI-FTIR) spectroscopy, which provides a calibration-free measurement of site-specific N₂O isotopic composition by determining the absorption cross-section of the pure vibrational features of the respective isotopocules

 

[1] Toyoda, S., Yoshida, N. and Koba, K. (2017), Isotopocule analysis of biologically produced nitrous oxide in various environments. Mass. Spec. Rev., 36: 135-160

[2] Westley, M.B., Popp, B.N. and Rust, T.M. (2007), The calibration of the intramolecular nitrogen isotope distribution in nitrous oxide measured by isotope ratio mass spectrometry†. Rapid Commun. Mass Spectrom., 21: 391-405.

[3] Harris, E., Zeyer K., Kegel R., et al. (2015), Nitrous oxide and methane emissions and nitrous oxide isotopic composition from waste incineration in Switzerland. Waste Management, 35: 135-140

 

[4] Ostrom, N.E., Ostrom, P.H. (2017), Mining the isotopic complexity of nitrous oxide: a review of challenges and opportunities. Biogeochemistry, 132: 359–372.