Clumped isotope analysis in nitrous oxide by mid-IR laser spectroscopy: analytical developments and validation

Nitrous oxide (N₂O) has been for long a major focus of all greenhouse gas accounting agreements. Understanding the mechanisms of its formation and clarifying its sources and sinks are highly important for mitigating N₂O emissions. In this context, measuring the doubly substituted isotopocules of N₂O can add new and unique opportunities to fingerprint and constrain the biogeochemical N₂O cycle, similar to other atmospheric species such as CO₂, CH₄, and O₂.

We address this challenging research field by developing and validating a laser spectroscopic technique for selective analysis of the eight most abundant N₂O isotopic species including the doubly substituted isotopocules ¹⁴N¹⁵N¹⁸O, ¹⁵N¹⁴N¹⁸O, and ¹⁵N¹⁵N¹⁶O. This method is based on quantum cascade laser absorption spectroscopy (QCLAS) and reaches a precision of 0.01 – 0.20 ‰ with 1 – 2 min spectral averaging on samples of 4 μmol of N₂O in N₂ at 4 hPa.

Furthermore, we have established a new reference frame combining two independent approaches: (1) clumped N₂O isotopocule abundances were linked to stochastic distribution by equilibrating the N–O bond in the N₂O molecule over activated Al₂O₃ at 100 and 200 °C, and (2) individual isotopocule concentrations were calibrated using a set of high-accuracy gravimetric N₂O-in-N₂ gas mixtures. The latter approach, applied for the first time to clumped isotope measurements, has a particular potential in realizing regular multi-point calibration for species like ¹⁵N¹⁵N¹⁶O, because no procedure for equilibration of the N–N bond has been successful yet.

Results of the validation measurements, using the QCLAS method and calibration approach, are presented for a large range of δ values (approx. 100 ‰ for d¹⁵N and d¹⁸O). Inter-comparison measurements with high-resolution mass spectrometry show compatible results for bulk isotopic composition (d¹⁵N, d(458+548)), but superior performance of QCLAS for determining site-selectivity (SP, SP¹⁸). In summary, this work provides new methodological basis for the measurements of clumped N₂O isotopes and has a high potential to stimulate future research in the N₂O community by establishing a new class of reservoir-insensitive tracers and molecular-scale insights.