Stable isotope delta For stable isotopes, isotope ratios are typically reported using the delta (?) notation where ? represents the ratio of heavy isotope to light isotope in the sample over the same ratio of a standard reference material, reported using units of in "per mil" (‰, parts per thousand) and reported relative to the specific standard reference material. Chemistry Wikipedia https://en.wikipedia.org/wiki/Isotope_geochemistry#Stable_isotope_geochemistry ODM2 Units Type Controlled Vocabulary ODM2 Working Group A vocabulary for describing the type of the Unit or the more general quantity that the Unit represents.