Project Details Terminology and definition of quantities related to the isotope distribution in elements with more than two stable isotopes

Project No.:
Start Date:
01 July 2010
End Date:


To define terminology and to identify the most suitable definitions of quantities that characterize the isotope distribution in elements with more than two stable isotopes, including so-called mass-independent fractionation, non-mass dependent fractionation, isotope anomaly, 17O excess, etc.


Most atmospheric oxygen-bearing species show deviations in their triple oxygen isotope ratios from mass-dependent fractionation (MDF) relationships predicted by the theories of Urey, Bigeleisen and Mayer. Similar deviations have also been found in sulfur and other elements with more than two stables isotopes (e.g. Hg, Cd, Zn), often preserved in non-atmospheric reservoirs, including rocks, minerals, soils, ice and waters.
Despite the ubiquity of this type of isotope anomaly, there has never been an attempt to clearly define the terminology and physical quantities used to measure these anomalies and the processes that lead to their formation. Terms like mass-independent fractionation, non-mass dependent fractionation, isotope anomaly, isotope excess etc. have been used in the historic and recent literature, but are often not carefully distinguished. The realization that MDF comprises a range of possible relationships between the isotopes of one element led to further complications because it meant that apparent isotope anomalies could be created by a combination of different MDF processes.
At the moment, at least 4 different definitions to quantify isotope anomalies are being used. Furthermore, coefficients used in these definitions vary, which makes the comparison of data from different sources very difficult.
A consistent set of recommendations on how to express and quantify the isotope distribution in elements with more than two stable isotopes is highly warranted. From our experience as academic teachers, we are woefully aware how impenetrable the field is for young researchers at the moment because of the lack of consistency and the lack of understanding between different groups. This project seeks to alleviate this.


September 2010 – project announcement published in Chem. Int. Sep-Oct, p. 23

June 2011 – Presentation at 10th Informal Conference on Atmospheric and Molecular Science, Copenhagen, Denmark

July 2014 – The task group gave a presentation on the project at the European Geosciences Union (EGU) meeting in Vienna this year. Discussions of the draft definition continued at the 7th International Symposium on Isotopomers (ISI2014), Tokyo, Japan where a presentation of the project was also made.

July 2016 – Informal task group meeting at Goldschmidt Conference, Yokohama, Japan.  A second draft was reviewed by the task group members.

March 2018 – A further draft is being worked on and shall be submitted to Pure and Applied Chemistry this year.

Page last updated: 13 March 2018