Isotopomer vs. Isotopologue: Key Differences in Isotope Chemistry
Isotopomer: molecules with the same isotopic composition but differing in the specific positions of isotopes. Isotopologue: molecules that differ only in the count of a given isotope, regardless of placement.
Chemists swap these terms on Zoom calls because both describe isotope-tagged versions of the same molecule. The subtlety—placement versus count—gets buried in lab chatter and sloppy manuscripts, making the mistake feel harmless until reviewers pounce.
Key Differences
Isotopomers are constitutional isomers of isotopes: 13CH3-CH3 vs CH3–13CH3. Isotopologues sit on a mass ladder: CH4, 13CH4, 13C2H6. Same formula, different masses.
Which One Should You Choose?
Use isotopologue for mass-spec peaks and abundance tables. Use isotopomer when mapping reaction pathways with NMR or isotope scrambling studies.
Examples and Daily Life
Water labeled as H218O is an isotopologue. HOD is an isotopomer of water because the deuterium sits on one specific hydrogen site.
Can a molecule be both?
Yes. 13CH2D-CH3 is an isotopologue of ethane and an isotopomer of 13CH3-CH2D.
Why do journals care about the distinction?
Precise language guides reviewers to the right analytical method and prevents misinterpretation of isotope effects.
Is isotopologue the broader term?
Exactly; every isotopomer is an isotopologue, but not every isotopologue is an isotopomer.