N${}_2$O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison

For the past two decades, the measurement of nitrous oxide (N2O) isotopocules – isotopically substituted molecules ${}^{14}$N${}^{15}$N${}^{16}$O, ${}^{15}$N${}^{14}$N${}^{16}$O and ${}^{14}$N${}^{14}$N${}^{18}$O of the main isotopic species ${}^{14}$N${}^{14}$N${}^{16}$O – 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 N${}_2$O isotopic analyzers. In contrast to traditional isotope-ratio mass spectrometry (IRMS), these instruments are inherently selective for position-specific ${}^{15}$N 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 N${}_2$O isotope laser spectrometers with the three most common detection schemes: OA-ICOS (N${}_2$OIA-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.). ... mehrFor each instrument, the precision, drift and repeatability of N${}_2$O mole fraction [N${}_2$O] and isotope data were tested. The analyzers were then characterized for their dependence on [N${}_2$O], gas matrix composition (O${}_2$, Ar) and spectral interferences caused by H${}_2$O, CO${}_2$, CH${}_4$ 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 N${}_2$O 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 N${}_2$O 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 N${}_2$O isotope laser spectrometers in order to acquire accurate N${}_2$O isotope analyses. We anticipate that this workflow will assist in applications where matrix and trace gas compositions vary considerably (e.g., laboratory incubations, N${}_2$O liberated from wastewater or groundwater), as well as extend to future analyzer models and instruments focusing on isotopic species of other molecules.