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 (14 N15 N16O, 15 N14 N16O, 14 N14 N18O), 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 G5131-i cavity ring-down spectroscopy (CRDS) analysers manufactured by Picarro Inc., USA. 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 G5131-i 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 case study for data correction for other analyser models and detection schemes. Therefore, the code can be readily adapted to any isotope system for routine application.