N2O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison
Authors: Harris Stephen J. , Liisberg Jesper , Xia Longlong , Wei Jing , Zeyer Kerstin , Yu Longfei , Barthel Matti , Wolf Benjamin , Kelly Bryce F. J. , Cendón Dioni I. , Blunier Thomas , Six Johan , Mohn Joachim 
Published: May 2020

Abstract. For the past two decades, the measurement of nitrous oxide (N2O)
isotopocules – isotopically substituted molecules 14N15N16O, 15N14N16O and 14N14N18O of the main isotopic species 14N14N16O – has been a promising technique for
understanding N2O production and consumption pathways. The coupling of non-cryogenic and tuneable light sources with different detection schemes, such as direct absorption quantum cascade laser absorption spectroscopy (QCLAS), cavity ring-down spectroscopy (CRDS) and off-axis integrated cavity output spectroscopy (OA-ICOS), 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. Here, we compared the performance of N2O isotope laser spectrometers with the three most common detection schemes: OA-ICOS (N2OIA-30e-EP, ABB – Los Gatos Research Inc.), CRDS (G5131-i, Picarro Inc.) and QCLAS (dual
QCLAS and preconcentration, trace gas extractor (TREX)-mini QCLAS, Aerodyne Research Inc.). For
each instrument, the precision, drift and repeatability of N2O mole
fraction [N2O] and isotope data were tested. The analyzers were then
characterized for their dependence on [N2O], gas matrix composition
(O2, Ar) and spectral interferences caused by H2O, CO2, CH4 and CO to develop analyzer-specific correction functions.
Subsequently, a simulated two-end-member mixing experiment was used to
compare the accuracy and repeatability of corrected and calibrated isotope
measurements that could be acquired using the different laser spectrometers. Our results show that N2O isotope laser spectrometer performance is
governed by an interplay between instrumental precision, drift, matrix
effects and spectral interferences. To retrieve compatible and accurate
results, it is necessary to include appropriate reference materials
following the identical treatment (IT) principle during every measurement.
Remaining differences between sample and reference gas compositions have to
be corrected by applying analyzer-specific correction algorithms. These
matrix and trace gas correction equations vary considerably according to
N2O mole fraction, complicating the procedure further. Thus,
researchers should strive to minimize differences in composition between
sample and reference gases. In closing, we provide a calibration workflow to guide researchers in the operation of N2O isotope laser spectrometers in order to acquire accurate N2O isotope analyses. We anticipate that
this workflow will assist in applications where matrix and trace gas
compositions vary considerably (e.g., laboratory incubations, N2O
liberated from wastewater or groundwater), as well as extend to future
analyzer models and instruments focusing on isotopic species of other
molecules.