Comparison of N2O isotope spectrometers for high-precision measurements in ambient air and incubation experiments

Over the last two decades, research involving N2O site specific isotopic analysis has been stimulated by continuing analytical progress in isotope-ratio mass-spectrometry (IRMS) and more recently mid-infrared laser spectroscopy. This development has been triggered by the invention and availability of quantum cascade lasers (QCL), which offer high optical power in continuous wave operation at room temperature. QCL light sources have been combined with different detection schemes such as direct absorption (QCLAS), cavity ring down (CRDS) and off-axis integrated cavity output (OA-ICOS) to realize compact, field-deployable analyzers.The availability of temporal resolved N2O isotopic information in real-time will deepen our process-level understanding of the nitrogen cycle. It will also open up entirely new research areas that will attract an increasing number of application-oriented scientists. Provided that the novel laser spectrometers produce compatible and thus accurate results (i.e. traceable to the international isotope ratio scales, AIR-N2 for 15N/14N and VSMOW for 18O/16O), the implementation of these instruments will lead to a further dissemination of N2O isotopic research.We will present results of an inter-comparison study on the three most common commercial N2O isotope analyzers, including Aerodyne Research (dual QCLAS, with/without TREX), Picarro (G5131-i) and Los Gatos Research (Model 914-0027). Most importantly, gas matrix effects were investigated by determining the dependence of N2O isotope deltas on the analysis in an “ambient” N2/O2/Ar/CO2/CH4/CO versus a simplified N2/O2/Ar or N2/O2 matrix. In addition, spectral interferences of enhanced trace gas concentrations (CO2, CH4, CO, H2O) were characterized and strategies for removal tested. Short-term and long-term repeatability, drift and dependence of isotope deltas on N2O concentrations were also quantified and compared among instruments. Based on these results a calibration strategy was established and the accuracy of individual analyzers assessed combining the studied uncertainty contributions.Our study will guide the selection of instruments for specific applications (e.g. ambient air versus incubation studies), and foster the development of N2O isotope reference gases optimized for laser spectrometers currently ongoing within the EMPIR project “Metrology for Stable Isotope Reference Standards (SIRS)”

A comparison of commercially-available quantum cascade laser spectrometers to measure N2O isotopocules (δ15Nα, δ15Nβ & δ18O) at ambient concentrations

Over the last two decades, research involving N2O isotopic analysis has been stimulated by continuing analytical progress in isotope-ratio mass-spectrometry (IRMS) and more recently by the development of mid-infrared laser spectroscopy. The advancement of laser spectroscopic techniques was enabled by the invention and availability of quantum cascade lasers (QCL), which have been coupled with different detection schemes such as direct absorption quantum cascade laser absorption (QCLAS), cavity ring down (CRDS) and off-axis integrated cavity output spectroscopy (OA-ICOS) to realize compact field-deployable analyzers. To improve the accuracy and reliability of these instrumental techniques, analyzer-specific calibration procedures and correction algorithms are required to account for matrix effects, spectral interferences and drift during measurements. We performed an inter-comparison study of three commercially-available N2O isotope laser spectrometers (δ15Nα, δ15Nβ & δ18O); Aerodyne Research (dual QCLAS, with/without pre-concentration, Picarro (G5131-i) and Los Gatos Research (Model 914-0027). Gas matrix effects were investigated by determining the dependence of N2O isotope deltas on the analysis in an “ambient” N2/O2/Ar/CO2/CH4/CO versus a simplified N2/O2/Ar or N2/O2 matrix. In addition, spectral interferences of enhanced trace gas concentrations (CO2, CH4, CO, H2O) were characterized and strategies for removal tested. Short-term and long-term repeatability, drift and dependence of isotope deltas on N2O concentrations were also quantified and compared among instruments. Using these results, we evaluated the accuracy of individual analyzers and developed calibration strategies tailored to each machine. Our study will guide the selection of instruments for specific applications (e.g. ambient air versus chamber measurements), and foster the development of N2O isotope reference gases optimized for laser spectrometers currently ongoing within the EMPIR project “Metrology for Stable Isotope Reference Standards (SIRS)”

N2O isotope laser spectrometers: towards reproducible and accurate measurements

The latest commercial laser spectrometers have the potential to revolutionise nitrous oxide (N2O) isotope analysis: they provide continuous and rapid analysis, measure at ambient concentrations without pre-concentration, and are field-deployable. However, to do so, they must be able to produce trustworthy data.Here, we present results from a series of experiments performed on N2O isotope laser spectrometers with different detection schemes: off-axis integrated cavity output spectroscopy (OA-ICOS, ABB-Los Gatos Research Inc.), cavity ring-down spectroscopy (CRDS, Picarro Inc.) and direct absorption quantum cascade laser absorption spectroscopy (QCLAS, Aerodyne Research Inc.). For each instrument, the precision, drift and repeatability of N2O mole fraction [N2O] and isotope data were tested. The analysers 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 analyser-specific correction functions. Based on these results, we developed a calibration workflow which allows for reproducible and accurate data to be generated using these instruments. We assessed the robustness of this workflow by measuring a series of gas mixtures of known N2O isotopic composition, but with varying N2O and other trace gas concentrations, to compare the accuracy and repeatability of calibrated measurements obtained by the different analysers. We show that, provided the compositional differences between reference gases and samples gases can be accounted for (either by implementing chemical traps or interference correction functions), reproducible and accurate data can be obtained.Our results also clearly highlight that N2O isotope laser spectrometers are not “plug and play” devices, and that N2O isotope analysis using laser spectroscopy is not straight-forward. Instead, researchers wishing to use these instruments need to carefully consider the desired application, precision and accuracy, and develop appropriate calibration strategies to achieve these outcomes

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

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

N2O isotope research: development of reference materials and metrological characterization of OIRS analyzers within the SIRS project

Measurements of the four most abundant stable isotopocules of N2O (14N14N16O, 15N14N16O, 14N15N16O, and 14N14N18O) can provide a valuable constraint on source attribution of atmospheric N2O. N2O isotopocules at natural abundance levels can be analyzed by isotope-ratio mass-spectrometry (IRMS) [1] and more recently optical isotope ratio spectroscopy (OIRS) [2]. OIRS instruments can analyze the N2O isotopic composition in gaseous mixtures in a continuous-flow mode, providing real-time data with minimal or no sample pretreatment, which is highly attractive to better resolve the temporal complexity of N2O production and consumption processes. Most importantly, OIRS laser spectroscopy is selective for position-specific 15N substitution due to the existence of characteristic rotational-vibrational spectra.By allowing both in-situ application and measurements in high temporal resolution, laser spectroscopy has established a new quality of data for research on N2O in particular and N cycling in general. However, applications remain challenging and are still scarce as a metrological characterization of OIRS analyzers, reporting factors limiting their performance is still missing. In addition, only since recently two pure N2O isotopocule reference materials have been made available through the United States Geological Survey (USGS), which however, only offer a small range of δ15N and δ18O values (< 1 ‰) and are therefore not suited for a two-point calibration approach [3].This presentation will highlight the recent progress achieved within the framework of the EMPIR project “Metrology for Stable Isotope Reference Standards (SIRS)”, namely:(1) The development of pure and diluted N2O reference materials (RMs), covering the range of isotope values required by the scientific community. These gaseous standards are available as pure N2O or N2O diluted in whole air. N2O RMs were analyzed by an international group of laboratories for δ15N, δ18O (MPI-BGC, Tokyo Institute of Technology, UEA), δ15Nα, δ15Nß (Empa, Tokyo Institute of Technology) and δ17O (UEA) traceable to the existing isotope ratio scales.(2) The metrological characterization of the three most common commercial N2O isotope OIRS analyzers (with/without precon QCLAS, OA-ICOS and CRDS) for gas matrix effects, spectral interferences of enhanced trace gas concentrations (CO2, CH4, CO, H2O), short-term and long-term repeatability, drift and dependence of isotope deltas on N2O concentrations [4].In summary, the authors suggest to include appropriate RMs following the identical treatment (IT) principle during every OIRS measurement to retrieve compatible and accurate results. Remaining differences between sample and reference gas composition have to be corrected, by applying analyzer-specific correction algorithms