|Isotope||Atomic mass (Da)||Isotopic abundance (amount fraction)|
In its 1961 report, the Commission recommended Ar(Sr) = 87.62 based on the mass-spectrometric determination of Nier. This value was revised to Ar(Sr) = 87.62(1) in 1969 and it remains unchanged since.
Known natural variations in the abundance of 87Sr, the product isotope of radioactive 87Rb decay, prevent the recommendation of a more precise standard atomic-weight value. The n(87Sr)/n(86Sr) ratio is a convenient measure of that variability. Anomalous traces of almost pure 87Sr have been reported from rubidium ores, for which the atomic weight will not be in the tabulated standard atomic weight range.
The appreciable variability in n(87Sr)/n(86Sr) is the basis for Rb-Sr geochronology, and for analysis of source components in mixtures of water and geologic materials.
© IUPAC 2003