Terminology and Definition of Quantities Related to the Isotope Distribution in Elements with More than Two Stable Isotopes

Terminology and Definition of Quantities Related to the Isotope Distribution in Elements with More than Two Stable Isotopes

This new project, coordinated by the Inorganic Chemistry Division, will seek 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, nonmass dependent fractionation, isotope anomaly, 17O excess, and other terms.

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 nonatmospheric 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, nonmass dependent fractionation, isotope anomaly, and isotope excess, 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 four 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 situation.

For more information contact the task group chair Jan Kaiser <j.kaiser@uea.ac.uk>.

www.iupac.org/web/ins/2009-046-2-200

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