Monoisotopic vs average mass: reading peptide mass data
Last reviewed 2026-10-10 · Sable Peptides · Research use only
Sable research method explainer. Updated 2026-10-10. Research use only.
Short answer
A peptide has more than one useful mass. The average molecular weight, which PubChem and the Sable /ref pages show as 'molecular weight', is weighted by the natural abundance of every isotope. The monoisotopic mass uses only the most abundant isotope of each element (12C, 1H, 14N, 16O, 32S). For small peptides a mass spectrometer resolves the monoisotopic peak. For large molecules the isotope pattern spreads out, and the average mass or the most abundant peak is often more useful. When checking a certificate of analysis, make sure you compare like with like.
A worked example
Oxytocin's PubChem formula is C43H66N12O12S2. PubChem gives its molecular weight (average) as 1007.2 g/mol and its monoisotopic mass as 1006.4365 Da. A high-resolution spectrum of oxytocin shows its first isotope peak near 1006.44, not at 1007.2.
Semaglutide, C187H291N45O59, has an average molecular weight of 4114 g/mol and a monoisotopic mass of 4111.1154 Da on PubChem. The gap is now about 3 Da, because a molecule with 187 carbons usually contains at least one 13C atom. The monoisotopic peak is no longer the tallest in the cluster.
Why the gap grows with size
About 1.1% of natural carbon is 13C. In a small molecule most copies contain no 13C, so the monoisotopic peak dominates. As the number of atoms rises, the chance that a molecule contains one or more heavy isotopes increases, and the isotope distribution shifts and broadens. Yergey and colleagues described how to calculate these distributions for large molecules (Yergey et al., 1983).
Charge states
Electrospray ionisation usually produces multiply charged ions. A spectrum is plotted as mass-to-charge ratio (m/z), so a peptide carrying three extra protons appears at roughly (M + 3 × 1.007)/3. Instruments and certificates report the deconvoluted neutral mass, but raw spectra need this conversion before they can be compared with a formula.
Checking a certificate against a /ref page
- Identify which mass the certificate reports: monoisotopic, average, or an observed m/z with a stated charge.
- Calculate the expected value for the exact structure, including terminal caps, disulfides and any non-natural residues.
- Allow for the salt form. Counter-ions dissociate in the spectrometer and do not appear in the peptide mass, but they do affect weighed amounts (see the explainer on TFA vs acetate).
- Expect small differences between instruments, within their stated accuracy.
At Sable
Sable checks each batch's identity against the expected mass and releases it at ≥99% purity by HPLC. The /ref pages give average molecular weights from PubChem or other cited registries. The certificate for your batch is available for every lot at /coa. Sable Peptides has been operating since 2019 and supplies every compound mentioned here as laboratory research material only. Nothing on this page is advice on use in people or animals, and no administration or medical-use information is provided.
Key terms
- Isotope
- An atom of the same element with a different number of neutrons, such as 13C and 12C.
- Monoisotopic mass
- The mass calculated using the most abundant isotope of each element.
- Average molecular weight
- The abundance-weighted mass that databases report as molecular weight.
- m/z
- Mass-to-charge ratio, the quantity a mass spectrometer measures.
- Deconvolution
- Converting a series of multiply charged ions into a single neutral mass.
Common questions
Which mass does the /ref page show?
The average molecular weight from PubChem or another cited registry.
Why is the monoisotopic mass lower?
It counts only the lightest common isotope of each element, while the average includes heavier isotopes such as 13C.
What is m/z?
Mass-to-charge ratio. A multiply charged ion appears at a fraction of its neutral mass.
Related explainers
Related reference pages
Sources
- Yergey J, Heller D, Hansen G, Cotter RJ, Fenselau C. Isotopic distributions in mass spectra of large molecules. Anal Chem. 1983;55(2):353-356. https://doi.org/10.1021/ac00253a037
- PubChem compound record: Oxytocin (CID 439302), computed properties. https://pubchem.ncbi.nlm.nih.gov/compound/439302
- PubChem compound record: Semaglutide (CID 56843331), computed properties. https://pubchem.ncbi.nlm.nih.gov/compound/56843331
Questions
Which mass does the /ref page show?
The average molecular weight from PubChem or another cited registry.
Why is the monoisotopic mass lower?
It counts only the lightest common isotope of each element, while the average includes heavier isotopes such as 13C.
What is m/z?
Mass-to-charge ratio. A multiply charged ion appears at a fraction of its neutral mass.
Sources
CoA for every lot — request yours and we email it within 2 hours.
Research use only. Not for human or veterinary use. No health claims.