How to read a certificate of analysis

A certificate of analysis is a laboratory’s record of what it found when it tested a specific batch of material. It is evidence about that batch and nothing else. Read that sentence twice, because almost every way a certificate misleads comes from ignoring the second half of it.

What a certificate establishes

Two questions, answered by two different instruments.

Identity — is this the sequence it claims to be?

Answered by mass spectrometry. The report gives an expected molecular mass, calculated from the sequence, and an observed mass measured from the sample. Those two numbers should agree closely. A discrepancy of one or two mass units can indicate a substitution or an incomplete modification; a large discrepancy means the material is not what the label says.

Mass spectrometry confirms mass, not sequence. Two peptides with the same composition in a different order have identical mass. In practice this is a minor concern from a competent synthesiser, but it is the reason identity testing is a check rather than a proof.

Purity — how much of the sample is that sequence?

Answered by high-performance liquid chromatography. The sample is separated into its components and the report gives a chromatogram: a trace with a large peak for the target and smaller ones for everything else. Purity is expressed as the area of the target peak as a percentage of total peak area.

Two things to understand about that percentage. First, it is relative: it describes the proportion of detected material, not the proportion of the vial’s contents. Residual water, counter-ions and salts from synthesis are typically not represented, which is why net peptide content can be meaningfully lower than the purity figure implies. Second, 98% pure means 2% is something else, and the certificate rarely says what. Those impurities are usually truncated or deleted sequences from synthesis, which are structurally similar to the target and are not necessarily inert.

A purity figure quoted with no method and no chromatogram is a number someone typed.

The lot number is the whole point

Synthesis is a batch process. Two runs of the same sequence by the same manufacturer produce different material with different impurity profiles. A certificate is therefore tied to one lot, and it says nothing about any other.

This makes the lot number the single most important field on the document. A certificate whose lot number you cannot match to the vial in your hand is not evidence about your material. It is evidence about someone else’s.

Who issued it

There is a real difference between a report from an independent testing laboratory and one produced in-house by the party selling the material. Both can be honest. Only one can be checked without trusting the seller.

The strongest form is a report hosted on the laboratory’s own domain, reachable by a verification link that resolves to their servers. That is a document the seller cannot edit. A PDF hosted by the seller is a copy of a document, and a copy is an assertion.

Red flags

• No lot number. The report cannot be connected to anything.
• An image with no underlying data. A screenshot of a chromatogram is not a chromatogram.
• A purity percentage with no chromatogram or method. Unverifiable by construction.
• No date of analysis. Stability is time-dependent; an undated report has no shelf life.
• A report covering a different lot of the same compound. Common, and it certifies nothing about what you received.
• The supplier’s certificate for the supplier’s batch. Their document, about their material, before it was repackaged.

Our position

Every product page states its certificate status plainly, including when no certificate exists yet. Coverage across the catalogue is published on the certificates page as a real count rather than a claim. We would rather show a gap than describe testing that has not happened — a laboratory name attached to a test nobody ran is worse than no claim at all.