|Isotope||Atomic mass (Da)||Isotopic abundance (amount fraction)|
|54Fe||53.939 609(3)||0.058 45(105)|
|56Fe||55.934 936(3)||0.917 54(106)|
|57Fe||56.935 393(3)||0.021 19(29)|
|58Fe||57.933 274(3)||0.002 82(12)|
In 1961, the Commission recommended Ar(Fe) = 55.847(3) based on the average value of two reported mass-spectrometric determinations. In 1993, the Commission changed the recommended value for the standard atomic weight to Ar(Fe) = 55.845(2) based on calibrated mass-spectrometric measurements carried out on a metallic iron sample of high purity.
Several studies have indicated natural isotope fractionation in iron-containing materials. The magnitude of the uncertainty assigned to the atomic-weight value was based mainly on the reported variations of Fe isotopic composition; however, subsequent studies now indicate somewhat different ranges. According to the compilation by the Commission, reported δ56Fe values range from −2.9 ‰ with Ar(Fe) = 55.8448 in human blood to +1.36 ‰ with Ar(Fe) = 55.8453 in part of a banded iron formation. Here δ56Fe refers to n(56Fe)/n(54Fe) relative to the reference material IRMM-014.
© IUPAC 2003