Preliminary assessment of stable nitrogen and oxygen isotopic composition of USGS51 and USGS52 nitrous oxide reference gases and perspectives on calibration needs

Rationale

Despite a long history and growing interest in isotopic analyses of N₂O, there is a lack of isotopically characterized N₂O isotopic reference materials (standards) to enable normalization and reporting of isotope‐delta values. Here we report the isotopic characterization of two pure N₂O gas reference materials, USGS51 and USGS52, which are now available for laboratory calibration (https://isotopes.usgs.gov/lab/referencematerials.html).

Methods

A total of 400 sealed borosilicate glass tubes of each N₂O reference gas were prepared from a single gas filling of a high vacuum line. We demonstrated isotopic homogeneity via dual‐inlet isotope‐ratio mass spectrometry. Isotopic analyses of these reference materials were obtained from eight laboratories to evaluate interlaboratory variation and provide preliminary isotopic characterization of their δ¹⁵N, δ¹⁸O, δ¹⁵N^α, δ¹⁵N^β and site preference (S_P) values.

Results

The isotopic homogeneity of both USGS51 and USGS52 was demonstrated by one‐sigma standard deviations associated with the determinations of their δ¹⁵N, δ¹⁸O, δ¹⁵N^α, δ¹⁵N^βand S_P values of 0.12 mUr or better. The one‐sigma standard deviations of S_Pmeasurements of USGS51 and USGS52 reported by eight laboratories participating in the interlaboratory comparison were 1.27 and 1.78 mUr, respectively.

Conclusions

The agreement of isotope‐delta values obtained in the interlaboratory comparison was not sufficient to provide reliable accurate isotope measurement values for USGS51 and USGS52. We propose that provisional values for the isotopic composition of USGS51 and USGS52 determined at the Tokyo Institute of Technology can be adopted for normalizing and reporting sample data until further refinements are achieved through additional calibration efforts.