The Term „Stable Isotope Dilution Assay“ has a historic background and first has been used by Rittenberg and Foster (1940). It is based on an element as an entity showing a natural isotope pattern. By adding an artificially enriched isotope this pattern is being changed, i.e. the isotope distribution has been „diluted“. This principle is also applicable for molecules and isotopologues.

Definitions for isotope-labelled molecules

are „isotopic isomers“, i.e. these isomers do not differ in their isotope sum formula but in the position of the isotopes in the molecule.

are „isotopic homologues“, i.e. the homologues differ in their isotope composition.