Quality Control
Reading mass spectrometry on a peptide COA
What the MS line on a research peptide Certificate of Analysis confirms, how observed mass is compared with theoretical mass, and the common reasons the two disagree.
3 min read · Updated 2026-09-13 · VaultLabs Research Editorial · Reviewed by VaultLabs compliance review
Research use only. For in vitro research use only. Not for human or animal consumption. Not for human consumption, medical use, or personal application.
What the MS result answers
Mass spectrometry answers “is this the right molecule?” It ionises the sample and measures mass-to-charge ratio, and the COA compares the observed mass against the theoretical mass calculated from the published sequence.
That is a different question from the one HPLC answers. HPLC purity reports how much of the sample is the main peak; MS supports what that peak is. A lot can be 99% pure and still be 99% of the wrong peptide, which is why a COA carrying only a purity number is weaker than one carrying both.
Observed mass versus theoretical mass
Peptides are usually run by electrospray ionisation (ESI), which produces multiply-charged ions, or by MALDI-TOF. ESI spectra are deconvoluted back to a neutral mass before the comparison is made, so the number printed on a COA is typically that deconvoluted result rather than a raw m/z reading.
Expect the observed value to sit close to theoretical rather than exactly on it. What counts as close depends on the instrument and whether the figure is monoisotopic or average mass — two conventions that differ by roughly a Dalton on a peptide of this size, and a frequent source of confusion when a COA does not say which it used.
What a mass mismatch usually means
A shifted mass is informative rather than simply a failure. Increments of about +16 Da suggest oxidation, commonly at methionine or tryptophan; about +42 Da suggests acetylation; a deficit matching one residue suggests a deletion sequence carried through synthesis.
Synthetic peptides are also frequently supplied as trifluoroacetate (TFA) salts from purification. That affects net peptide content against gross vial weight, which is a weighing and concentration issue rather than an identity one — but it is a common reason a vial does not behave as expected despite a clean MS result. Reconstitution basics covers the concentration side.
What MS alone does not establish
Mass is not a unique fingerprint. Isomers and some scrambled sequences share a molecular formula, so an intact-mass measurement consistent with theoretical does not by itself prove residue order. Sequence-level confirmation requires fragmentation data (MS/MS) or an orthogonal method, which many supplier COAs do not include.
MS also says nothing about the endotoxin, residual solvent, or stability profile of the lot, and it describes the vial the sample was drawn from on the date tested.
How this appears on VaultLabs records
Where a manufacturer supplies MS data for a lot, it is reproduced on that batch record and reachable through batch verification and the COA portal. If a record shows purity but no MS, treat identity as unconfirmed by that document rather than assuming it passed.
Acceptance criteria remain the receiving laboratory’s decision. VaultLabs publishes the analytical records it receives and does not interpret them for study design.