Term: isotopically labelled https://doi.org/10.1351/goldbook.I03336 Definition: Describes a mixture of an isotopically unmodified compound with one or more analogous isotopically substituted compound(s). Related Terms: 1) isotopically unmodified (http://doi.org/10.1351/goldbook.I03339). 2) isotopically substituted (http://doi.org/10.1351/goldbook.I03338). Source: Blue Book, p. 514 (https://www.acdlabs.com/iupac/nomenclature/) Citation: 'isotopically labelled' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.I03336 License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.