Naturally occurring iron has four stable isotopes, Fe, Fe, Fe, and Fe.
Stable iron isotopes are described as the relative abundance of each of the stable isotopes with respect to Fe. The standard for iron is elemental iron, IRMM-014, and it is distributed by the Institute for Reference Materials and Measurement. The delta value is compared to this standard, and is defined as:
Delta values are often reported as per mil values (‰), or part-per-thousand differences from the standard. Iron isotopic fractionation is also commonly described in units of per mil per atomic mass unit.
In many cases, the δFe value can be related to the δFe and δFe values through mass-dependent fractionation: