Analytical method
LC-MS identity testing for peptides: what a mass match proves
By Vitality Chems · Published · updated · vendor-authored explanatory note
Short answer
An LC-MS identity result compares the mass the instrument observed with the mass expected for the named peptide. A match within the lab's stated tolerance confirms the molecule has the right elemental composition; it does not prove the amino acids are in the right order or have the right chirality, and it says nothing about how much peptide is in the vial. Extra peaks at predictable offsets are usually explainable: +21.982 Da from H to Na, +37.956 Da from H to K, +15.995 Da for an oxidized methionine and +0.984 Da for a deamidated asparagine or glutamine.
Catalog materials discussed on this page
| Observed offset from [M+H]+ | Usual explanation | Source of the value |
|---|---|---|
| +21.982 | Sodium adduct [M+Na]+ instead of [M+H]+ | NIST atomic masses (Na-23 minus H, both as cations) |
| +37.956 | Potassium adduct [M+K]+ | NIST atomic masses (K-39 minus H) |
| +17.027 | Ammonium adduct [M+NH4]+ | NIST atomic masses |
| +15.995 | One oxidized residue, usually methionine | Unimod accession 35 |
| +0.984 | Deamidation of Asn or Gln (often hidden in the isotope pattern) | Unimod accession 7 |
| About minus one residue mass | Deletion or truncation impurity from synthesis | Residue masses from the sequence |
What the instrument actually measures
Electrospray LC-MS reports mass-to-charge ratios (m/z), not masses. A peptide of monoisotopic mass M carrying z extra protons appears at (M + z × 1.00728) / z. Small peptides mostly show the singly and doubly protonated ions; larger ones spread over several charge states. Our expected m/z calculator does this arithmetic for catalog compounds.
A COA should state which ion was observed, the expected value, and either the difference or the tolerance the lab accepts. A bare 'identity: conforms' with no numbers cannot be checked.
Reading extra peaks
Peaks near the main ion are not automatically impurities. Sodium and potassium are everywhere in glassware and solvents, so [M+Na]+ and [M+K]+ peaks at the offsets in the table are common and reflect the same molecule.
Some offsets do indicate a different molecule. A +15.995 Da peak in a methionine-containing peptide is usually the oxidized form, which is also a separate HPLC peak. Missing-residue masses point to deletion sequences from solid-phase synthesis. These should line up with impurity peaks on the HPLC chromatogram.
Blind spots
Mass alone cannot see changes that keep the elemental composition the same. A peptide with two residues swapped, or with an L-amino acid replaced by its D-form, has the identical mass. Ruling those out takes MS/MS fragmentation, comparison against a reference standard's retention time, or chiral analysis.
Mass also says nothing about quantity. Net peptide content, water and counter-ions (such as TFA or acetate) are separate measurements; see our notes on net peptide content and salt forms.
- Same mass, different order: sequence scrambles are invisible to a single mass value
- Same mass, different chirality: D/L isomers need chiral methods or a reference standard
- Mass match, wrong amount: identity does not measure how much peptide is present
A worked example
Semax (Met-Glu-His-Phe-Pro-Gly-Pro) has a PubChem monoisotopic mass of 813.348 Da, so its [M+H]+ ion is expected at 814.355 and [M+2H]2+ at 407.681. A peak at 836.337 is the sodium adduct of the same molecule. A peak at 830.350 is Semax with its methionine oxidized, a real impurity that should also appear on the HPLC trace.
Related on Vitality Chems
- Verify a lot — enter the lot number on your vial to open its report.
- Expected m/z calculator — charge-state values for catalog compounds.
- HPLC vs LC-MS — why purity and identity are separate tests.
- Selank vs Semax — two heptapeptides told apart by mass.
- HPLC purity vs net peptide content — why identity and purity are not quantity.
- TFA vs acetate salts — counter-ions and their mass.
- Peptide COA checklist — ten checks for any certificate of analysis.
- Semax — lot records and COA status.
Frequently asked
What does LC-MS identity testing prove for a peptide?
That a molecule with the expected mass, and so the expected elemental composition, is present. It does not prove sequence order, chirality, purity or quantity.
Why is there a peak 22 Da above my peptide's mass?
That is usually the sodium adduct [M+Na]+, which sits 21.982 Da above [M+H]+. Potassium adducts sit 37.956 Da above. Both are the same molecule.
What does a +16 Da peak mean?
Usually one oxidized residue, most often methionine (+15.995 Da). It is a real impurity and should match a separate peak on the HPLC chromatogram.
Can LC-MS tell D- and L-amino acid versions apart?
Not from mass alone, because isomers have identical masses. Chiral methods or comparison with a reference standard are needed.
References
- Unimod accession 35: Oxidation, monoisotopic delta 15.994915.Mass shift for oxidation.
- Unimod accession 7: Deamidated, monoisotopic delta 0.984016.Mass shift for deamidation of Asn and Gln.
- NIST Atomic Weights and Isotopic Compositions: sodium.Relative atomic mass of Na-23 (22.98976928) used for the sodium adduct offset.
- NIST Atomic Weights and Isotopic Compositions: potassium.Relative atomic mass of K-39 (38.96370649) used for the potassium adduct offset.
- NIST CODATA: proton mass in atomic mass units.Proton mass used for [M+zH]z+ values.
- PubChem CID 9811102: Semax (National Library of Medicine).Monoisotopic mass used in the worked example.
