Lab calculator
Expected m/z for peptide LC-MS identity checks: charge-state calculator
Published 2026-10-29 · updated 2026-10-29 · vendor-authored explanatory note
Short answer
In electrospray LC-MS a peptide of neutral mass M usually appears as several protonated ions, [M+zH]z+, at m/z = (M + z × 1.00728) ÷ z. Larger peptides show higher charge states. To check identity on a COA, compute the expected m/z for each charge state and confirm the spectrum's main peaks fall at those values within the instrument's stated accuracy.
Expected m/z calculator
Average-mass values for [M+zH]z+ ions. Use the monoisotopic mass instead when comparing against high-resolution spectra.
| Charge state | Ion | Expected m/z |
|---|---|---|
| 1+ | [M+1H]1+ | 4,115.01 |
| 2+ | [M+2H]2+ | 2,058.01 |
| 3+ | [M+3H]3+ | 1,372.34 |
| 4+ | [M+4H]4+ | 1,029.51 |
| 5+ | [M+5H]5+ | 823.81 |
| 6+ | [M+6H]6+ | 686.67 |
M = 4114 g/mol, PubChem computed average MW for Semaglutide (CID 56843331).
The formula
Each added proton contributes 1.007276 Da of mass and one positive charge. For charge state z: m/z = (M + z × 1.007276) ÷ z.
Worked example: semaglutide's PubChem average molecular weight is 4114 g/mol, so [M+3H]3+ is expected near (4114 + 3.022) ÷ 3 ≈ 1372.3 and [M+4H]4+ near 1029.5.
Average versus monoisotopic mass
PubChem's molecular weight is the isotope-weighted average mass. Low-resolution instruments and deconvoluted spectra are usually compared against average mass. High-resolution instruments resolve the isotope pattern, so the monoisotopic peak should be compared against the monoisotopic mass (listed on each PubChem compound page as exact or monoisotopic mass).
For peptides above a few thousand daltons the monoisotopic peak becomes small, and the most abundant isotope peak is often reported instead.
What to look for on a COA mass spectrum
A useful identity report shows the spectrum or a peak table, states the observed mass or m/z values, the expected values, and the ionization mode. A certificate that only states 'conforms' without values cannot be checked independently.
Adducts and modifications appear as satellite peaks: a sodium adduct (Na+ in place of H+) adds 21.98 Da to the ion's mass, so its m/z shifts by 21.98 ÷ z; oxidation of a methionine adds 15.99 Da. Their size relative to the main peak is informative.
Related on Vitality Chems
- HPLC vs LC-MS for research peptides — what each method proves.
- Peptide molecular weight and CAS table — masses for every listed compound.
- Semaglutide lot documentation — the worked example compound.
- Peptide COA checklist — the other nine checks.
Frequently asked
How do I calculate m/z for a peptide charge state?
Add z × 1.007276 to the neutral mass M and divide by z. For z = 2, a 1419.5 Da peptide is expected near (1419.5 + 2.015) ÷ 2 ≈ 710.8.
Which charge states should I expect?
Small peptides (under about 1,500 Da) mainly show 1+ and 2+ ions; peptides of 3,000 to 5,000 Da commonly show 3+ to 5+. The distribution depends on the sequence's basic residues and the instrument conditions.
Does a matching mass prove purity?
No. Mass spectrometry confirms identity. Purity is measured separately, usually by HPLC with UV detection, and both should be on a complete COA.
Where do the calculator's masses come from?
PubChem's computed average molecular weights, retrieved 2026-10-29, for compounds whose CAS number matches the catalog. You can also enter any neutral mass.
References
- PubChem (National Library of Medicine)Molecular weights used by the calculator.
- NIST: CODATA proton massProton mass in atomic mass units (1.007276).
