How to read a peptide certificate of analysis
What HPLC, mass spectrometry and endotoxin figures on a peptide COA actually establish — and the common ways a certificate can look reassuring while proving very little.
A certificate of analysis is the only document standing between a labeled vial and an unlabeled one. It is also, in practice, the document buyers scan for a large percentage figure and then file away. That is a shame, because a COA answers two quite different questions, and most of the interesting information sits in the second one.
The two questions are: is this the compound it claims to be, and how much of what is in the vial is that compound. Identity and purity. A certificate that answers only one of them is not doing its job.
Identity: mass spectrometry
Mass spectrometry determines the molecular weight of what is in the sample. Because every peptide sequence has a characteriztic mass, an observed mass that matches the theoretical mass of the target sequence is strong evidence that you are looking at the right molecule.
This is why the molecular weight on a product page matters. If a certificate reports an observed mass of 4731.3 g/mol for a compound whose theoretical mass is 4731.3 g/mol, identity is corroborated. If the reported mass is materially different, something is wrong — a truncated sequence, a different compound, or a transcription error on the certificate itself.
Two peptides can share a molecular weight while differing in sequence order, and a mass measurement alone will not distinguish them. Mass spectrometry establishes that the mass is right, not that every residue sits in the right position. That is why sequence confirmation is a separate line item on a thorough certificate, established by peptide mapping rather than by intact mass alone [1].
Purity: HPLC
High-performance liquid chromatography separates the components of a sample by how strongly each interacts with a stationary phase as a solvent carries it through. Components emerge at different times, and a detector records each as a peak. The area under the target peak, expressed as a percentage of total peak area, is the purity figure.
That definition contains the caveat that most people miss. HPLC purity is a percentage of what the detector saw — not of what is in the vial. Anything that does not absorb at the detection wavelength, or does not elute at all, is invisible to the calculation. There is a further assumption underneath it — that each peak is a single species — and establishing that is its own analytical problem [2].
What that excludes in practice
- —Residual water. Lyophilized peptides are hygroscopic and retain moisture that contributes mass but no chromatographic peak.
- —Counter-ions. Synthesis and purification commonly leave acetate or trifluoroacetate associated with the peptide. These can be a meaningful share of vial mass and do not appear in the purity figure.
- —Residual solvents and inorganic salts, which likewise do not register at peptide detection wavelengths.
None of this makes an HPLC figure dishonest. It makes it specific. A 99% purity result means the target compound accounts for 99% of the detected chromatographic area — a genuinely useful statement, provided you read it as the statement it is rather than a claim about vial contents by mass.
Reading the chromatogram, not just the number
A certificate that includes the chromatogram is telling you considerably more than one that reports a bare percentage. Things worth looking at:
| Feature | What it suggests |
|---|---|
| A single sharp, symmetrical main peak | Clean separation, well-resolved product |
| Peak tailing or fronting | Column or method issues; the integration may be less reliable than the number implies |
| A cluster of small peaks near the main peak | Closely related impurities — often deletion or truncation sequences from synthesis |
| A flat baseline with no detail | Possible over-processing of the image, or a gradient that did not resolve much |
| No chromatogram at all | You are being asked to accept a number without the evidence behind it |
Endotoxin
Endotoxins are lipopolysaccharides from the outer membrane of Gram-negative bacteria. They are heat-stable, survive many sterilization approaches, and interfere with a wide range of cell-based assays. For in-vitro work with cultured cells, endotoxin contamination is a common and under-diagnosed source of confounded results. Detection is by the limulus amebocyte lysate assay or by a recombinant Factor C method, which are actively compared against one another in the analytical literature [3].
Endotoxin testing is reported in endotoxin units per milligram (EU/mg). Its relevance depends entirely on the experiment: it matters a great deal for cell culture work and considerably less for a compound being used as an analytical reference standard.
The batch number is the whole point
A certificate that cannot be tied to the specific lot in your hand is decorative. Testing is performed on a batch; a different batch is a different sample with a different result. If a supplier publishes one certificate and continues to present it as material moves through several production runs, that certificate has stopped describing anything you actually received.
This is the single most useful question to ask a supplier: not "do you have a COA", but "can you send me the certificate for the lot number printed on this vial". The answers to those two questions are frequently different.
A certificate produced by the facility that synthesized the material is a self-assessment. It may be entirely accurate, but it is not verification. An independent laboratory analyzing a sample with no stake in the result is a materially different claim, and it is worth knowing which one you are holding.
A short checklist
- 01Does the certificate name a batch or lot number, and does it match the vial?
- 02Is there a chromatogram, or only a percentage?
- 03Does the observed mass match the theoretical mass for the sequence?
- 04Is sequence identity confirmed separately, or inferred from mass alone?
- 05Who performed the analysis — the synthesizer, or an independent laboratory?
- 06Is the test date consistent with the production date of the batch?
All compounds referenced here are supplied strictly for laboratory and research use. Nothing in this article describes or implies suitability for human or animal use, and no dosing, administration or protocol guidance is provided.
Primary sources for the analytical claims above, linked so you can read them directly rather than take our word for it.
- [1]Peptide Mapping for Sequence Confirmation of Therapeutic Proteins and Recombinant Vaccine Antigens by High-Resolution Mass Spectrometry: Software Limitations, Pitfalls, and Lessons LearnedDobrowolski M, Urbaniak M, Pietrucha T · Int J Mol Sci · 2025 · PMID 41155256
- [2]A strategy for assessing peak purity of pharmaceutical peptides in reversed-phase chromatography methods using two-dimensional liquid chromatography coupled to mass spectrometry. Part I: Selection of columns and mobile phasesPetersson P, Buckenmaier S, Euerby MR et al. · J Chromatogr A · 2023 · PMID 36841023
- [3]Study of LAL and Recombinant Cascade Reagents Methods for Bacterial Endotoxin Testing in Pharmaceutical ProductsThota P, Kumar P, Teotia AK et al. · PDA J Pharm Sci Technol · 2026 · PMID 41844320