Monoisotopic vs average molecular mass
Monoisotopic vs average molecular mass
Last updated: 2026-10-10 · Research use only. Not for human or veterinary use.
Short answer
Monoisotopic mass uses only the most abundant isotope of each element (12C, 1H, 14N, 16O, 32S). Average mass weights every isotope by natural abundance and is what molecular weight on a label usually means. High-resolution spectra are compared with monoisotopic mass; low-resolution or deconvoluted broad peaks with average mass. [1]
At a glance
| Term | Definition / specification |
|---|---|
| Monoisotopic mass | Sum of lightest stable isotopes |
| Average mass | Abundance-weighted isotopic mean (= MW) |
| Gap between the two | Grows with size; typical literature figure about 0.6 Da per 1,000 Da [3] |
| Use monoisotopic for | High-resolution MS with resolved isotope peaks |
| Use average for | Formula weight, weighing, low-resolution MS |
| Salt forms | Formula weight of a salt includes counter-ions |
Isotope envelope
Larger peptides show a cluster of peaks spaced by about 1 Da/z; for big molecules the monoisotopic peak becomes small and must be inferred.
Practical tip
When a COA lists a molecular weight, check whether it refers to the free base or a salt form.
FAQ
Why does my measured mass differ by ~1 Da?
The instrument may have picked a 13C isotope peak rather than the monoisotopic peak.
Which mass does PubChem list?
PubChem lists both molecular weight (average) and monoisotopic/exact mass.
Related reference pages
- Mass-spectrometry confirmation of peptide identity
- Net peptide content vs gross weight
- Reference index
Sources
- Valkenborg D et al. A model-based method for the prediction of the isotopic distribution of peptides. J Am Soc Mass Spectrom. 2008. PMID: 18325782
- Mitra I et al. Improved mass defect model for theoretical tryptic peptides. Anal Chem. 2012. PMID: 22401145 · PMC3312599
- Ramjit HG et al. The significance of monoisotopic and carbon-13 isobars for the identification of a 19-component dodecapeptide library by positive ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. Rapid Commun Mass Spectrom. 2000. PMID: 10920357
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