Towards the compatibility of stable isotope data: review of related concepts, recent progress with reference materials and current challenges

ABSTRACT

Comparing and combining stable isotope datasets from different laboratories and different years is essential for many research areas, such as isotope hydrology, greenhouse gas observations, food studies, isotope forensics, palaeo-reconstructions, etc. Data compatibility (i.e. the ability to combine data) is related to the data quality. The prerequisite for data comparability is data normalization to a common stable isotope scale (often referred to as calibration) based on reliable reference materials (RMs) with accurately assigned values and uncertainties. Still, that does not guarantee the data compatibility (mutual agreement). Albeit metrological concepts related to data compatibility and measurement uncertainty have been developed and applied to analytical chemistry in general, these concepts have not yet been fully applied to stable isotope research. This can affect daily calibrations, analytical data and, therefore, data compatibility. In addition, IRMS users often prepare different laboratory standards themselves. Thereafter, users should then understand the contemporary concepts used for assigning RM value and uncertainty, as well as the limitations and potential problems associated with RMs. The history of RMs, preparation reports and also some problems in the past provide lessons to be learned. These include the δ¹³C drift of LSVEC (the second anchor on the δ¹³C scale before 2017), revisions to the value assignment principles, the introduction of replacements for LSVEC, related disputes and the potential underestimation of uncertainties for secondary RMs. The review describes metrological concepts related to isotopic scales, RMs and calibration hierarchies and data compatibility. The main RMs and their uncertainties are reviewed through the lens of metrology concepts. Additional focus is given to the VPDB scale for δ¹³C and issues of scale discontinuity, which can significantly reduce data compatibility in δ¹³C. The given examples of value and uncertainty assignment for RMs should be viewed as an example of value and uncertainty calculation in daily practice.

KEYWORDS:

• Stable isotopes
• data compatibility
• stable isotope scales
• reference materials
• use of metrology
• uncertainty

Acknowledgements

Discussions with former colleagues at the Institute for Reference Materials and Measurements (Belgium), the IAEA and during GAW-WMO meetings are greatly appreciated. Discussions with Drs M. Verkouterren (NIST, US), P. Tans (NOAA, later at University of Colorado, US) and D. Malinovskiy (LGC, UK) were particularly motivating. Comments by anonymous reviewers helped to improve the readability of the paper. The review paper was triggered by the invitation to give a keynote talk at GASIR-2023 and follow-up discussions.

Disclaimer

All RM values given in this paper are for informative purposes only. For official RM values and recommendations on the use of RMs, users should always refer to official certificates issued by the IAEA, NIST, USGS or other RM producers and ensure they possess valid/current certificates.

The review paper represents a scientific opinion and summary by the author that is supported by numerous publications. This opinion has been developed during his work at the Institute for Reference Materials and Measurements (formerly - Institute for Reference Materials and Measurements, now - JRC-Geel, EU, Belgium), and numerous discussions at the Global Atmospheric Watch (GAW-WMO) meetings for greenhouse gas measurement techniques, and during metrology meetings. During his employment at the IAEA, the author was responsible for developing new IAEA carbonate RMs for δ¹³C [25,26,28], organizing the IAEA technical meetings [23,27] and contributed to developing organic-matrix RMs [34].

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 The review is based on the talk presented at the GASIR Meeting 2023, Bayreuth, Germany, 27–29 September 2023.