Vol. 25 No. 6
November-December 2003

Atomic Weights of the Elements 2001 (IUPAC Technical Report)

R. D. Loss

Pure and Applied Chemistry
Vol. 75, No. 8, pp. 1107–1122 (2003)

The Commission on Atomic Weights and Isotopic Abundances wishes to emphasize the need for new precise calibrated isotope composition measurements in order to improve the atomic weights of a number of elements, which are still not known to a satisfactory level of accuracy. However, for many elements the limited accuracy of measurements is overshadowed by terrestrial variability, which is included in the tabulated uncertainty of the atomic weights.

The variation in the atomic weight of sulfur due to natural variation in its isotopic composition (shown here for the fraction of the 34S isotope). The natural variation in the atomic weight of S places a significant limit on the final uncertainty with which the atomic weight for this element may be stated. [fig reproduced from T.B. Coplen et al., Pure Appl. Chem. 74(10), 1987–2017, 2002]

The range of terrestrial variation observed in the atomic weights for most elements is generally small and does not affect most chemists in their day-to-day work. However, as improvements in instrumentation and analyst skill continue to produce more accurate and precise results, analysts may need to seriously consider atomic weight variations, and in some cases actually measure the atomic weights of specific elements in the material they are analyzing. Variations in the atomic weights of elements down to the microscale level also have major benefits, such as the ability to characterize materials not only on their chemical composition but also by their isotopic or atomic weight variability. This variation has been used successfully for many years in fields such as isotope geochemistry and nuclear astrophysics, and is now opening up whole new fields of study in medicine, forensics, and human nutrition. The Commission’s task of evaluating atomic weight data has thus expanded into reviewing isotopic abundance literature in increasingly diverse fields. The Commission is therefore seeking the assistance of all isotopic analysts in reporting isotopic data in a specific and comprehensive manner. This issue is being addressed in a current project [# 2001-019-2-200] on “Guidelines for mass spectrometric isotope ratio measurement.”

www.iupac.org/publications/pac/2003/7508/
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