For each element, one stable isotope form is much more rare than the other. The stable isotopic composition of a material is expressed as a ratio (R) between these two forms. For example, hydrogen stable isotopic composition would be calculated as:
R = 2H/1H
The absolute proportion of rare to common isotopes in a sample can be quite small and difficult to measure. It is easier to compare the difference between the stable isotope composition of a material of interest and the composition of an international standard. Using the standard for reference, stable isotope ratios are expressed as:
δ = (Rsample/Rstandard – 1) * 1000
Stable isotope ratios are expressed in ‰ (read “per mil”) or parts per thousand. Ratios are often referred to as delta (δ) values.
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