Isotopologue

An isotopologue is a chemical species that differs from another only in the isotopic composition of one or more atoms, while retaining the same chemical connectivity. In other words, the molecule has the same arrangement of atoms, but some atoms are replaced by their isotopes. For example, carbon monoxide can exist as 12CO, 13CO, or C18O, and water as H2O, HDO, or D2O. These substitutions change the molecule’s mass and certain physical properties but not its basic bonding pattern.

Isotopologues are widely used in spectroscopy, mass spectrometry, environmental tracing, and astrochemistry. Because isotopes have different masses, isotopologues show distinct spectral lines and mass-to-charge ratios, enabling precise measurements of abundances, reaction pathways, and isotopic ratios. Natural abundances of isotopes (for example, 13C around 1.1% and 18O around 0.2%) influence the observed intensities of different isotopologues, providing information about processes from atmospheric chemistry to stellar nucleosynthesis. In astronomy, isotopologues such as 12CO and 13CO are observed in interstellar clouds to infer molecular gas masses and isotopic chemistry in galaxies.

Relation to related terms: isotopologues refer to species that differ only in isotopic composition, whereas isotopomers are isotopologues that differ in the positions of the isotopes within the molecule while keeping the same isotopic counts. Thus, isotopologues form a broader category that includes isotopomers.