A user’s guide for accurate N2O isotopocule measurements using laserspectroscopy

Abstract

For the past two decades, N2O isotopocules – isotopically substituted molecules 14N15N16O, 15N14N16O and 14N14N18O of the main isotopic species 14N14N16O – have been identified as promising tools for understanding N2O production and consumption pathways. The application of midinfrared laser spectroscopy to the study of N2O isotopocules continues to grow due to recent progress in analyzer development. The coupling of non‐cryogenic and tuneable light sources with differentdetection schemes, such as direct absorption quantum cascade laser spectroscopy (QCLAS), cavity ring‐down spectroscopy (CRDS) and off‐axis integrated cavity output spectroscopy (OAICOS), has enabled the production of commercially‐available and field‐deployable N2O isotopic analyzers.In contrast to traditional isotope‐ratio mass‐spectrometry (IRMS), these instruments are inherently selective for position‐specific 15N substitution and provide real‐time data, with minimal or no sample pretreatment, which is highly attractive for process studies.In this workshop, we will present the results of an intercomparison study that we conducted on various N2O isotope laser spectrometers that use the three most common detection schemes: OAICOS (N2OIA‐30e‐EP, ABB‐Los Gatos Research Inc.), CRDS (G5131‐i, Picarro Inc.) and QCLAS (dual QCLAS, preconcentration – mini QCLAS, Aerodyne Research Inc.). Analyzers were tested for their concentration dependence, and gas matrix (N2, O2, Ar) and trace gas (H2O, CO2, CH4, CO) interference effects to compare the magnitude of these effects across instruments, and to develop analyzer‐specific correction functions. The instruments were also characterized for precision, repeatability and instrumental, and the accuracy of corrected results standardized to international scales were compared among laser spectrometers and to IRMS in a simulated two end‐member mixingexperiment.We will show that N2O isotope laser spectrometer performance is governed by a complex interplay between instrumental precision, drift, matrix effects and spectral interferences – and that these ultimately vary as a function of N2O mole fraction. To retrieve compatible and accurate results, appropriate reference materials following the identical treatment (IT) principle are required. Remaining differences in gas composition between sample and reference gas have to be corrected by applying analyzer‐specific correction algorithms. However, these matrix and trace gas correction equations vary considerably according to the co‐measured N2O mole fraction, complicatingthis procedure further. We therefore recommend that researchers strive to implement measurement setups that require as few corrections as possible