Characterisation of new reference materials IAEA‐610, IAEA‐611 and IAEA‐612 aimed at the VPDB δ<sup>13</sup>C scale realisation with small uncertainty

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Rationale In recent years, the primary reference material (RM) for the VPDB scale, NBS19, has become unavailable, and the RM used for low‐end scale‐anchoring, LSVEC, was found unsuitable due a drift in the δ13C value. Given these problems, new RMs aimed at realising the VPDB δ13C scale with low uncertainty were produced. Establishing the consistency of the new RMs with the “old” RMs prompted our revision of the underlying principles of RM value assignments, and the VPDB δ13C scale realisation and its long‐term sustainability. Methods Analysis of major developments of the VPDB scale, a review of the contemporary requirements for RMs, and comparison with well‐established measurement scales have been performed, with the aim of revising the VPDB δ13C scale, principles of RM value assignments, and calibrator hierarchy. Requirements for scale‐anchoring RMs with low uncertainty and measures to establish the scale sustainability have been formulated. Results The revised scale realisation is based on multiple reference points, well‐defined calibration hierarchy and the use of well‐understood methods for value assignment. The realisation scheme includes the new primary RM IAEA‐603 and scale‐anchoring RMs IAEA‐610, IAEA‐611 and IAEA‐612, covering δ13C from +2.46 to −36.7 ‰ VPDB, with uncertainties, including inhomogeneity and stability assessment, of less than 0.015 ‰. The values of these four RMs were assigned in a mutually consistent way; agreement between measurements made using this realisation with those made using the VPDB scale of 2006 has been demonstrated on NIST CO2 RMs 8562–8564. Conclusions Multipoint‐anchoring of the VPDB δ13C scale provides several distinct “points” on the scale as means for cross‐measurements to check the stability and viability of RMs and detect drift of values, if any. This ensures that the δ13C scale is suitable for the most demanding applications, and provides options for developing further RMs with high accuracy inside a robust scale realisation scheme.

mentioning

confidence: 74%

“…Details of the reference method used for RMs IAEA-603 and IAEA-610/612 value assignment by DI-IRMS can be found in section S4, supporting information, and in the literature 25,26 ).…”

Section: Methods For Rm Value Assignmentmentioning

confidence: 99%

Section: Methods For Rm Value Assignmentmentioning

confidence: 99%

Section: Considerations For Re-establishing the Vpdb Scale Realisation Schemementioning

confidence: 99%

Section: Considerations For Re-establishing the Vpdb Scale Realisation Schemementioning

confidence: 99%

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On the metrological traceability and hierarchy of stable isotope reference materials aimed at realisation of the VPDB scale: Revision of the VPDBδ¹³C scale based on multipoint scale‐anchoring RMs

Assonov

Fajgelj

Allison

et al. 2021

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Assessment of the accuracy of the ¹⁷O correction algorithm used in δ¹³C determinations by CO₂ mass spectrometry

Assonov¹

2023

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High-accuracy δ 13 C values are required for observations of greenhouse gases CO 2 and methane, and, consequently, for international reference materials (RMs). Recently, another application, clumped isotope geothermometry of natural carbonates, has demonstrated the requirement for high-accuracy δ 13 C values. δ 13 C determinations by mass spectrometry use an 17 O isobaric correction on m/z 45, where 17 O abundance is calculated from the measured 18 O with the 17 O-18 O relationship assumed with λ = 0.528. This relationship is the key assumption of the algorithm proposed in 2003 and accepted by IUPAC in 2010. However, to date, this relationship and potential δ 13 C biases have not been verified using 17 O measurements. Methods: To verify the 17 O correction and to estimate potential δ 13 C biases, we compile measured 17 O data for carbonate RMs, for a range of natural carbonates that are typically analyzed in clumped isotope geothermometry and for CO 2 in isotope equilibrium with natural waters including plants and biota. δ 13 C biases are calculated based on 17 O deviation from the 17 O-18 O relationship assumed in the 17 O correction.Results: To estimate δ 13 C biases accurately, the VPDB-CO 2 framework for expressing 17 O excess is defined and linked to the δ 13 C scale definition. δ 13 C biases estimated for carbonate RMs are found within ±0.004‰; the biases estimated for natural carbonates and CO 2 in equilibrium with natural waters are mostly within ±0.010‰ (bidirectional distribution around zero). In all cases, the estimated biases are found within the best instrumental uncertainty of modern isotope ratio mass spectrometry (IRMS) (around ±0.014‰, k = 2). Conclusions:For the first time, high accuracy of δ 13 C data obtained by CO 2 mass spectrometry using the 17 O correction with fixed λ = 0.528 has been demonstrated using measured 17 O data. δ 13 C biases estimated are within the best IRMS precision (±0.014‰, k = 2) and can be neglected in most practical applications. To obtain high-quality δ 13 C data, it is strictly necessary that all data are treated on the VPDB-CO 2 scale. | INTRODUCTIONIsotopic compositions, such as δ 13 C and δ 18 O, are reported as relative deviations and are only meaningful when placed within a wellestablished and robust reference frame(s). In practice, a set of reference materials (RMs), such as the primary RM, scale-anchoring RMs and potentially other RMs, 1 defines the reference frame (or the "scale") in use and sample data are placed within this reference frame.Applications that aim for very high precision and accuracy, such as δ 13 C measurements of atmospheric CO 2 and CH 4 (major

Perspective: Hidden biases in isotope delta results and the need for comprehensive reporting

Dunn,

Skrzypek

2023

Measurements of stable‐isotope composition on an isotope‐delta scale can be subject to bias between laboratories or over time within a single laboratory. This bias can arise not just from differences in method protocol but also from changes in reporting guidelines, or even to the isotope‐delta scales themselves. Without a clear description of method protocols, including all sample preparation steps, instrumental parameters and settings, data processing including calibration of results and estimation of measurement uncertainty, the traceability and comparability of isotope‐delta values cannot be assured as bias(es) may remain hidden. To address this need, there are now clear guidelines published by IUPAC for reporting isotope‐delta values for the “light” elements hydrogen, carbon, nitrogen, oxygen and sulfur.1 We recommend that authors and reviewers adhere to those guidelines when preparing and reviewing future publications.