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), δ¹⁵N^α, δ¹⁵N^ß (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.

 

[1] Toyoda, S. and N. Yoshida (1999). "Determination of nitrogen isotopomers of nitrous oxide on a modified isotope ratio mass spectrometer." Anal. Chem. 71(20): 4711-4718.

[2] Brewer, P. J. et al. (2019). "Advances in reference materials and measurement techniques for greenhouse gas atmospheric observations." Metrologia 56(3).

[3] Ostrom, N. E. et al. (2018). "Preliminary assessment of stable nitrogen and oxygen isotopic composition of USGS51 and USGS52 nitrous oxide reference gases and perspectives on calibration needs." Rapid Commun. Mass Spectrom. 32(15): 1207-1214.

[4] Harris, S. J., J. Liisberg et al. (2019). "N₂O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison." Atmos. Meas. Tech. Discuss. (in review).