Project Details Developing a procedure for using intervals instead of fixed values for atomic weights

Project No.:
Start Date:
01 April 2012
End Date:
01 November 2018


This proposal meets a long range goal of the IUPAC for the advancement of research in the chemical sciences throughout the world by developing a procedure for the assignment of correct atomic weights using intervals instead of fixed values for atomic weights.

The introduction of intervals for elements of some standard atomic weights with a large variation in isotopic composition of natural materials has forced some users of the tables to change the way they use the data. This has inter alia been expressed as a concern by scientists from the gas mixing and analysis community, who need to know the molecular weight of gases for an uncertainty assessment of produced gas mixtures. This community is under pressure to justify its uncertainties because they directly underpin extremely high-value measurements of the energy content of fuel gases as well as the stability of long-term records of greenhouse gases in the atmosphere. The solution to the problem is to select the respective atomic weight from the known or assumed isotopic composition.

This would normally be done by looking at one of the diagrams in the 2002 Compilation of minimum and maximum isotope ratios of selected elements in naturally occurring terrestrial materials and reagents: U.S. Geological Survey Water-Resources Investigations Report 01-4222, or 98p., which has both delta values and atomic weight values for specific materials.

This procedure is not well established and needs to be developed. It is intended to do this by contacting key experts in the field and agree to standard approaches for extracting and using precise atomic weight information from the broader ranges of isotopic composition found in nature.


Most elements have more than one stable isotope. The mixture of isotopic compositions determines the atomic weight of a specific material. Since fractionation processes or radiogenic production can alter materials over time, the atomic weight often covers a wide range. This has been recognized by the CIAAW and a corresponding decision was made in 2009 to list lower and upper bounds of the atomic weights, instead of providing a mean atomic weight for applicable elements (Pure Appl. Chem., 2011, Vol. 83, No. 2, pp. 359-396; This is especially useful when no material with the average weight can be found in nature or when a particular reservoir has a narrow isotopic range with the mean differing from the total mean considerably.

Gases are a good example. O2 in air, for instance, has an 18O abundance, which is 2.39 % higher than sea water, leading to a correspondingly higher atomic weight of O2 in air. Likewise, precipitation, especially in the polar regions is considerably depleted in 18O, exceeding depletions in mole fraction of 5 % in some cases. Methane can vary considerably, depending on its source; it is, however, almost always substantially depleted in 13C and in 2H compared to the most carbon and hydrogen-bearing materials. When mixing gases gravimetrically, the final product should include a certificate with mole fractions and mass fractions of each constituent (and their uncertainty). Since it is often known that gases come from particular sources, the use of the full intervals indicated in the 2009 tables can represent an overestimate of the true uncertainty in the atomic weight. Alternatively, the atomic weight of each gas constituent can be provided. For establishing this relation firmly, one commonly must measure the isotopic composition because often it is not available. However, with a detailed assessment of the atomic weights of typical reservoirs and materials, the uncertainty in isotopic composition can be narrowed down, and provided as a mean value with range margins given for specified materials, and a traceability specification can be provided.


Two initial publications serving to educate the scientific public in these matters, have been completed:

– Brand W. A. (2012) Atomic weights: not so constant after all. Analytical and Bioanalytical Chemistry 405(9), 2755-2761. (March 2013)

– Brand W. A. and Coplen T. B. (2012) Stable isotope deltas: tiny, yet robust signatures in nature. Isotopes in Environmental and Health Studies 48(3), 393-409.

August 2013 update – The task group members have been working to solicit the wider opinion of the scientific community, in particular, the opinion of several ISO Technical Committees, and that of the JCGM WG-1 (BIPM). Position paper will be formulated after careful consideration of the presented advice and opinions.

A feature article titled “Atomic Weights: Not So Constant After All” has been published in Anal. Bioanal. Chem. 2013, 405, 2755-2761, DOI 10.1007/s00216-012-6608-0 (first online 18 Dec 2012)

July 2014 update – The project is being worked out in concerted efforts with project 2013-032-1-200, titled “Guidelines for the derivation of values and uncertainties from standard atomic weight intervals” (chaired by Adriaan van der Veen from the Dutch Metrology Institute). Together with BIPM and JCGM, work is well-underway to generate a manuscript by the end of 2014.

Page last updated: 7 Aug 2014