Correction and Calibration Protocol for Isotope Data via CRDS: A Study Case for N₂O and Other Isotope Systems

Abstract. Advances in laser spectroscopy have significantly simplified the measurement of N₂O isotopologues (¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁴N¹⁸O), but the raw data require extensive post-processing. This problem arises from the complexity of spectral fitting, which is controlled by an intricate interplay between the physics of vibrational spectroscopy, gas composition, fitting algorithm, and instrumental parameters. Following the general principles of identical treatment, the highest precision and accuracy is achieved when reference gases mimic the sample composition, which underpins our correction and calibration protocol.

This study presents a comprehensive and detailed correction and calibration protocol to post-process N₂O isotopic data, exemplified by data obtained from three commercial cavity ring-down spectroscopy (CRDS) analysers (G5131-i, Picarro Inc.). Experimental correction functions for delta values on changes in N₂O, CH₄, CO₂ and O₂ concentrations were determined for individual analysers to derive a mathematical framework, which was verified with spectral simulations. We confirm that the apparent δ-values scale inversely with the N₂O concentration, with the slope being analyser-specific and highly variable over short time intervals. Consequently, any instrument must be routinely characterised to maintain high-quality data. Furthermore, when CH₄ and CO₂ concentrations vary simultaneously, their combined spectral interference displays a non-additive interaction. We strongly advise removing CO₂ from the sample gas before analysis to ensure optimal data quality unless CH₄ / CO₂ variations are very small such as for N₂O emissions from upland soils).

We provide an end-to-end, stand-alone MATLAB application with a user-friendly interface for standardised data reduction, which was validated by analysis of several known target gases but with different gas compositions. This protocol/MATLAB application aims to support researchers in efficiently obtaining high-quality and reliable N₂O isotope data from the tested CRDS analyser model, while also providing a study case for data correction for other analyser models and detection schemes. Therefore, the code can be readily adapted to any isotope system for routine application.