How to Actually Read a Peptide Certificate of Analysis

Almost every research peptide now arrives with a certificate of analysis. Almost none of them are read properly. The document is treated as a badge — a PDF that exists, therefore the material is fine — when in practice a certificate is a set of claims that can be checked, and occasionally a set of claims that quietly contradict each other.

This is a walkthrough of the fields that decide whether a certificate carries information, in the order we read them.

1. The identifiers, before anything else

A certificate that cannot be tied to the vial in front of you is decorative. Three fields do that work: the batch or lot number, the date of analysis, and the sample identity as received by the laboratory.

If the lot number on the document does not match the lot printed on the vial, you have a certificate for a different batch of material — not evidence about yours. This is the single most common failure we see, and it is not always deception; batch documentation drifts when a supplier repackages.

The date matters for a second reason. A certificate produced at manufacture describes the material as it left synthesis. It says nothing about eighteen months in a warehouse of unknown temperature.

2. HPLC purity: read the method, not just the number

The headline figure is almost always chromatographic purity: >99% by HPLC. The number on its own is close to meaningless without the method underneath it.

What we look for:

• The gradient and column. A short, steep gradient can co-elute impurities into the main peak. A shallow gradient on a C18 column with a stated run time separates them.
• The detection wavelength. 214 nm detects the peptide bond and is the standard for peptide purity. 280 nm only sees aromatic residues, and can flatter a sample that lacks them.
• The actual chromatogram. A purity claim without an attached trace cannot be verified. With the trace, you can see whether the baseline is clean, whether there is a shoulder on the main peak, and whether the integration was generous.

Purity by HPLC is a relative measurement: it is the main peak as a percentage of total detected peak area. Anything that does not absorb at the detection wavelength — salts, water, residual counter-ions — is invisible to it. Which is why the next field exists.

3. Peptide content is not purity

This is the distinction that changes dosing arithmetic, and it is the one most often missed.

Purity tells you what fraction of the peptide material is the peptide you wanted. Peptide content (sometimes net peptide content) tells you what fraction of the powder in the vial is peptide at all. The remainder is typically trifluoroacetate counter-ions, residual water and salt from lyophilisation.

A vial that is 99% pure and 78% peptide content is entirely normal for a TFA salt. It also means a nominal 10 mg vial contains roughly 7.8 mg of peptide. If a protocol is built on the label weight, it is systematically off by a fifth — and the certificate said so on page one.

Purity answers "is it the right molecule?". Content answers "how much of it is there?". You need both.

4. Mass spectrometry: identity, not quantity

Mass spectrometry confirms that the molecular weight of the main species matches the theoretical mass of the intended sequence. Look for the observed mass, the theoretical mass and the ionisation mode. A deviation of a few tenths of a dalton is instrument noise; a deviation of 18 suggests hydrolysis, and a deviation matching one residue suggests a deletion sequence.

What MS does not do is quantify. A clean mass spectrum alongside a poor chromatogram means the right molecule is present in a mixture, not that the mixture is clean.

5. The fields nobody prints, and why that matters

The absence of a test is information. For lyophilised peptides intended for laboratory use, the tests that are quietly omitted most often are:

• Water content (Karl Fischer) — affects both mass accuracy and storage stability.
• Residual solvents — acetonitrile and TFA from purification.
• Bacterial endotoxin and sterility — routinely absent, and routinely assumed to be present by readers who did not check.

A certificate that reports only purity and mass is not falsified. It is simply narrower than most readers assume it is.

6. Who signed it

Third-party analysis by a named, independent laboratory is verifiable: the lab exists, the report has a reference number, and it can be requested again. In-house analysis is not automatically worse, but it is unaudited. Suppliers that publish per-batch third-party reports and let you retrieve them by lot number make the check take thirty seconds instead of an email exchange. The Zeus Peptides research peptide catalogue with batch COAs is one European example: purity method, lot number and peptide content are listed per product rather than summarised as a marketing claim.

A short checklist

1. Lot number on the certificate matches the vial.
2. Date of analysis is recent relative to purchase.
3. Chromatogram attached, 214 nm, gradient stated.
4. Peptide content reported separately from purity.
5. Observed mass within a fraction of a dalton of theoretical.
6. Analysing laboratory named and reachable.

Six checks, two minutes. It will not make a bad vial good, but it reliably tells you when a document has been asked to do more work than it can.