Antonio Possolo, Adriaan M. H. van der Veen, Juris Meija and D. Brynn Hibbert
Pure and Applied Chemistry, 2018Volume 90, Issue 2, pp. 395-424
In 2009, the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW) introduced the interval notation to express the standard atomic weights of elements whose isotopic composition varies significantly in nature. However, it has become apparent that additional guidance on how representative values should be derived from these intervals would be helpful, as well as on how the associated uncertainty should be characterized and propagated to cognate quantities, such as relative molecular masses. The assignment of suitable probability distributions to the atomic weight intervals is consistent with the CIAAW’s goal of emphasizing the variability of the atomic weight values in nature. These distributions, however, are not intended to reflect the natural variability of the abundances of the different isotopes in the earth’s crust or in any other environment. Rather, they convey states of knowledge about the elemental composition of “normal” materials generally, or about specific classes of such materials. In the absence of detailed knowledge about the isotopic composition of a material, or when such details may safely be ignored, the probability distribution assigned to the standard atomic weight intervals may be taken as rectangular (or, uniform). This modeling choice is a reasonable and convenient default choice when a representative value of the atomic weight and associated uncertainty are needed in calculations involving atomic and relative molecular masses. When information about the provenance of the material or other information about the isotopic composition needs to be taken into account, then this distribution may be non-uniform. In this report, several examples are presented of how the probability distribution of an atomic weight or relative molecular mass may be characterized, and also how it may be used to evaluate the associated uncertainty.
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