Peptide mass checker
15 residues
Formula
C62H98N16O22
Monoisotopic mass
1,418.7042 Da
Average mass
1,419.54 g/mol
Matches PubChem CID 9941957: monoisotopic 1,418.7042 Da, average 1419.5 g/mol. Open PubChem
Ions you may see on a mass spectrum
| Ion | m/z (monoisotopic) |
|---|---|
| [M+1H]1+ | 1,419.7114 |
| [M+2H]2+ | 710.3594 |
| [M+3H]3+ | 473.9087 |
| [M+4H]4+ | 355.6833 |
| [M+5H]5+ | 284.7481 |
How this works
The tool adds up the elemental composition of each residue, adds one water for the free ends, then applies the modifications you tick. Masses come from five element values (carbon, hydrogen, nitrogen, oxygen, sulfur) as tabulated by Unimod from IUPAC data: the monoisotopic mass uses the most abundant isotope of each element, the average mass uses standard atomic weights.
Every preset was checked against its PubChem record: the computed formula matches exactly and the monoisotopic mass agrees to within 0.005 Da. Peptides with lipid side chains or unusual residues, such as tirzepatide, semaglutide and retatrutide, cannot be built from standard residues; their compound pages give the PubChem masses instead.
The ion table shows where the molecule appears on an electrospray spectrum, which usually shows several charge states rather than the neutral mass. A certificate may print the neutral mass, a deconvoluted mass or one of these m/z values, so check which one it states.
Questions
- Which mass should a peptide COA show?
- It depends on the instrument. High-resolution mass spectrometry reports the monoisotopic mass, usually to four decimals, and states the gap as a ppm mass error. Lower-resolution electrospray results are often deconvoluted to an average mass. A good certificate says which one it reports and names the method. If it gives only a single rounded number with no method, ask the lab.
- Why does my COA show a number half the size of the peptide's mass?
- Electrospray ionisation adds protons, so a peptide appears at several m/z values: the mass plus protons, divided by the charge. A 2+ ion sits at roughly half the neutral mass and a 3+ ion at roughly a third. The ion table above lists them so you can match the printed value.
- Can a matching mass prove a peptide is genuine?
- A matching mass is strong evidence that the main component has the right formula, but it cannot separate isomers with the same formula, and it says nothing about purity or amount. It also cannot prove a certificate is authentic: verify the report with the lab that issued it.
- Does this tool work for tirzepatide or semaglutide?
- Not from the sequence alone. Both carry a fatty acid side chain attached through a linker, which is not a standard residue. Their compound pages list the formula and masses from PubChem so you can compare a certificate against those values directly.