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What MS confirms on a peptide COA that HPLC purity cannot
MS on a peptide COA confirms the observed mass matches the intended molecule. HPLC purity is a peak-area share; it proves neither identity nor mass.
Mass spectrometry on a peptide COA confirms that the molecular weight observed in the sample matches the intended molecule, which is identity. HPLC purity cannot: it reports the main peak’s share of UV detector response, so it says nothing about which molecule the peak is or how much of the vial’s mass is peptide.
The question of what mass spectrometry confirms on a peptide COA that HPLC purity does not has one method-determined answer, and it does not date; this is an evergreen explainer, not a news item. This desk read two contract laboratories’ own method descriptions on 20 September 2026: Creative Proteomics’ peptide purity analysis service page and AltaBioscience’s article on peptide purification and product analysis, both undated.
The three assay lines, method by method
| COA line | What the instrument measures, as the two labs describe it | What the reported figure establishes | What it leaves open |
|---|---|---|---|
| RP-HPLC purity | UV absorbance of eluting peptide species on a C18 column, read at 210-220 nm (Creative Proteomics) or 215 nm (AltaBioscience); main-peak area as a share of total peak area | The target peak’s share of peptide-related detector response under that method | Which molecule sits under the peak; the mass fraction of peptide in the solid, since water and counterion are not in the figure |
| MS identity (MALDI-TOF or ESI-MS) | The molecular weight of the ions produced from the sample | Whether the observed mass matches the theoretical mass of the intended molecule | How much peptide is present, and how pure it is |
| Amino acid analysis | The amount of each amino acid after acid hydrolysis, separated by ion exchange and detected with ninhydrin | Composition, and by summing the residues the net peptide content of the dried solid | The order the residues were joined in, since the chain is broken before it is measured |
What HPLC purity on a peptide COA measures
Creative Proteomics describes its RP-HPLC line as quantitative purity by peak-area normalization on a C18 column, with UV detection at 210-220 nm. AltaBioscience reads its output at 215 nm because, in its words, that wavelength is optimum for detecting the peptide bond.
The impurities it resolves are the ones Creative Proteomics lists: deleted and truncated sequences, incomplete deprotection and cleavage by-products. AltaBioscience adds the sentence vendor guides tend to skip: the purity value does not include any water and trifluoroacetate salt present in the dried material. This site has already worked through what that exclusion means for a 10 mg label mass and how the TFA and acetate counterion lines differ.
What mass spectrometry confirms that HPLC purity does not
Creative Proteomics assigns MALDI-TOF and ESI-MS to one job: molecular weight confirmation. AltaBioscience calls its MALDI-TOF instrument the ideal method to ascertain that the target peptide has been synthesised. Neither description gives the instrument any role in quantity.
So the MS line answers the question the HPLC line cannot: is the material consistent with the intended molecule? The mass spectrum compares an observed mass with the calculated mass of the intended sequence; a match is the certificate’s identity evidence. Neither lab page states a numeric acceptance window for the match, so this desk prints none.
What MS does not do is count. Creative Proteomics lists its three methods as orthogonal and reserves composition to amino acid analysis. AltaBioscience is blunter: all dried peptides contain a variable amount of water plus a fixed amount of counter-ion, and quantitative amino acid analysis is the only method which enables the net peptide content to be determined. Its typical net-content values, for its own product range, run from 70 to 90%; that is one manufacturer’s statement, not a market figure.
Why “99.4% of the powder” is a claim the HPLC line does not make
One vendor explainer holding the results for this query, from Onyx Biolabs, states in the portion read while the query was scoped that 99.4% pure means 99.4% of the powder is the target peptide. On the method descriptions above, the line does not say that: the figure is a share of detector response among peptide species, and water and counterion sit outside it. Apex Laboratory’s own reading guide, a research-peptide vendor rather than a laboratory authority, says the same: a reported 99.2% area does not by itself establish molecular identity, peptide content or supplied amount. The Onyx explainer, in the portion read, states no detection wavelength and names no third orthogonal method.
Which line answers which question
A buyer asks two questions of a COA. Is it the right molecule? That is the MS line, and only the MS line. How much of the vial is it? That is net peptide content, which on the two lab descriptions comes from amino acid analysis. The HPLC purity line answers a narrower third question: of the peptide species the detector saw, what share was the main peak. As this desk has argued of vendor-published certificates generally, the document is evidence of what it records and nothing more.
Questions readers ask
What is the difference between HPLC and mass spectrometry on a peptide COA?
HPLC separates the sample and reports the main peak’s share of UV detector response, which is the purity percentage. Mass spectrometry measures the molecular weight of what is in the sample and reports whether it matches the intended molecule. One is a relative-abundance figure under a stated method; the other is an identity check. Neither reports milligrams of peptide in the vial.
Does 99% HPLC purity mean the peptide is the right molecule?
No. A 99% HPLC figure means the assigned main peak accounted for 99% of the included detector-response area. It does not name the molecule under that peak. Identity comes from the MS line, where an observed mass is compared with the theoretical mass of the intended sequence. A certificate with no MS data has not documented identity.
What does observed mass vs theoretical mass mean on a COA?
Theoretical mass is the molecular weight calculated from the intended sequence. Observed mass is what the spectrometer measured in the delivered sample. A match is the evidence that the target peptide was made; a mismatch points to a different or modified molecule. The certificate should name the instrument and say how the theoretical mass was calculated, or the comparison cannot be checked.
None of this bears on what any peptide does. The materials on these certificates are supplied for laboratory research use only, and the only question this desk asks of a COA is what each of its lines documents.