Project Details Assessment of absolute isotope ratios for the international isotope delta measurement standards

Project No.:
Start Date:
22 May 2020
End Date:


This project seeks to assess the published absolute isotope ratios or isotopic abundances for the international measurement standards for isotope delta scales (i.e. the zero-points) in order to update the recommended values for these fundamental quantities and facilitate linking the isotope delta scales to the international system of units (SI). The assessment will consider the type of measurement (e.g. fully calibrated vs partially calibrated) and measurement uncertainty. Where more than one value has been reported for a particular absolute isotope ratio, these will be combined into a best estimate using appropriate statistical methods.


Currently, isotope delta measurements are reported relative to an international measurement standard that forms the zero-point of the scale and the base of the traceability chain.

The absolute isotope ratios of these measurement standards are the fundamental values which allow conversion of isotope delta values to other expressions of isotopic composition such as isotopic abundances, absolute isotope ratios or atomic weights that are often used in studies involving stable isotopes as tracers. At least some of the discrepancies between laboratories in these measurements is due to application of different values for the absolute isotope ratio of the zero-point materials. These isotope ratios are also the quantities which link the isotope delta scale to the international system of units.

The currently-recommended values of these absolute isotope ratios are derived from publications reporting measurement results, however in some instances there are multiple publications each reporting different values. Absolute values underpinning carbon or silicon isotope delta scales, for example, are not consistent and uniform recommendations are needed. Collating these measurement results and assessing them will allow new best-estimates of these absolute isotope ratios to be determined that reflect all previous measurements, rather than selecting a single publication as the source of the recommended value.

Considerations during such assessment include the nature of the calibration employed during measurement as well as the uncertainty budget.

The new best estimates will be distributed in the form of an IUPAC Technical Report and will also be available on the CIAAW website in machine-readable form.


Project announcement published in Chemistry International, vol. 43, no. 1, 2021, pp. 34-35.

Page last updated 22 Jan 2021