Time: Monday, 16/June/2025: 2:20pm - 3:10pm Session Chair: Harro A.J. Meijer Session Chair: Federica Camin |
Location: 5161.0151 Bernoulliborg, Nijenborgh 9, 9747 AG Groningen |
Keynote: Recent developments concerning VPDB, SI-traceability and measurements of carbon isotope delta
National Measurement Laboratory, LGC Limited, Teddington, UK
Isotope delta (d) is a quantity that is not traceable to the SI because specific artefacts are used to define isotope delta scales. There are currently two carbon isotope delta scales that both have Vienna Peedee belemnite (VPDB) as the reference. The VPDB scale is defined by the exact carbon isotope delta value assigned to the NBS 19, while the VPDB-LSVEC scale is defined by both NBS 19 and LSVEC. This is not an ideal situation, but highlights the shortcomings of artefact-based measurement scales that evolve over time.
The carbon isotope delta scales could be made SI-traceable through determination “absolute” isotope ratio of VPDB. One might consider RVPDB(13C/12C) as the fundamental constant underpinning scales. To achieve this it is necessary to determine the SI-traceable isotope ratio and the VPDB-traceable carbon isotope delta of one or more specific materials.
The use of gravimetric mixtures of isotopes/isotopologues to correct for instrumental mass fractionation is a primary method for SI-traceable isotope ratios. It has been implemented several times using a variety of 12C- and 13C-enriched isotopologues with both IRMS and MC-ICP-MS instrumentation. SI-traceable carbon isotope ratios can also be determined by ab initio optical spectroscopy or using 13C-satellites in 1H-NMR.
The use of gravimetric mixtures has recently been improved through selection of 12C- and 13C-enriched glucoses as the starting compounds; careful planning of weighing steps; significant improvements in the determination of the presence of co-enrichment of oxygen and hydrogen; and external validation. This has afforded a reduction in uncertainty for RVPDB(13C/12C) by an order-of-magnitude.
This work has been recognized by the IUPAC CIAAW as the “best measurement” of the isotopic composition of carbon and prompted revision of their recommendation for the isotopic composition of VPDB for the first time since 1990 for carbon and 2010 for oxygen.
Calcium carbonate and water pyrolysis measurements suggest minor adjustment to the VPDB and VSMOW-SLAP δ18O scale relation.
University Groningen, Netherlands, The
Measurements of the isotopic oxygen composition of materials are widely used in many fields. These 18O compositions are expressed using different scales. These scales are only linked indirectly, because water and calcium carbonate reference materials, used to establish these 18O scales, have to be converted to CO2 first, and the isotopic fractionation of these conversions has only been measured a few times in the past. The anchoring of the two 18O scales is therefore currently sub-optimal.
Primary reference materials, both waters and calcium carbonates, were pyrolyzed within a single measurement sequence in a high-temperature elemental analyser–pyrolysis system connected to an isotope ratio mass spectrometer (IRMS) in continuous-flow mode. As calcium carbonate is difficult to pyrolyze completely, additives were added to reach a 100% yield. The d18O of the calcium carbonates were normalised on the VSMOW-SLAP scale using VSMOW2 and in-house water references.
The average results of 6 international calcites references measured in five independent sequences of pyrolysis measurements showed a difference of 0.07 to 0.09‰ with the presently described relationship in literature between the d18O VPDB and d18O VSMOW-SLAP scale.
The outcome of the study made the direct comparison of the two different 18O scales possible. Our results demonstrate a small discrepancy in the presently recommended relation between the two 18O scales.