Stable isotopes and the international system of units

It is now over 60 years since Nier built the first isotope ratio mass spectrometer. The introduction of continuous-flow techniques heralded a huge expansion in the use of stable isotopes in biomedical and environmental sciences, yet there is no consensus on the appropriate units, especially in the biomedical field. Most isotope ratio mass spectrometry (IRMS) instruments calculate isotopic abundance in terms of delta notation (delta, per thousand, per mil), which is a convention determined by geochemistry, because most of the original IRMS instruments were developed in isotope geochemistry laboratories to measure natural abundance variations. Delta units are not SI units. This paper considers the appropriate units for studies using stable isotopes based on the International System of Units (SI). The SI base unit for concentration is the mol, from which atom fraction and mol fraction are derived. The units of stable isotope abundance, atom % and mol %, are the atom and mol fractions expressed as percentages. Atom % excess and mol % excess are the SI units of enrichment and are to be recommended for use in tracer studies.