A Certificate of Analysis (CoA) looks like a compact little table, but every field is there for a reason. Reading the sections apart, and asking of each field “what exactly does this establish?”, is how a certificate yields what it has to offer without being overread. This walkthrough goes through the fixed sections, with what typically appears and how to read that value.
The header: substance, batch and date
At the top sits the identification: the name of the substance, the batch or lot number and the production or test date. These three anchor the whole document. The batch number should reappear on the vial label; if it does not match, this certificate does not belong to this vial. The date places the measurements in time: a value measured on the test date is a statement about that moment, not about the months of storage afterward.
Identity: the measured mass
The identity section states the molecular mass, usually confirmed by mass spectrometry, alongside the theoretical mass that follows from the sequence. The question here is simple: do the measured and expected mass match? If so, that is strong evidence the correct molecule was made. What this section does not say is how much peptide is present or whether it is biologically active; that is not what the mass is for.
Purity: the HPLC percentage
The purity section gives a percentage, most often by HPLC, with the method noted. “98%” means: of everything the detector saw with this method, 98% belongs to the main component. It is a ratio (how small the related impurities are), not a quantity. Two things help when reading it: is the method stated (without it, a percentage is hard to weigh), and is it purity or content? Those two are often confused, and they are different numbers.
Content: how much peptide really
The content section, where present, answers the question purity leaves open: how much of the weighed powder is actually peptide? The rest is bound water, salts and counterions. A peptide can be 98% pure and at the same time have a net content of, say, 80%; that 18% is weighed-in non-peptide mass, not a contaminating peptide. The reference method is amino acid analysis. If no content is stated, that is an open question, not a proven 100%; absence of an entry is not evidence of absence of the problem.
Water and residual solvents
Two sections appear where the research calls for them. The water content (by Karl Fischer titration) shows how much of the weight is bound moisture and speaks to stability. The residual solvents (by gas chromatography) show traces of solvents from synthesis and purification. These fields are not on every certificate; their presence is informative, their absence not a guarantee that there was nothing to report.
The counterion form: the detail that explains the weight
A subtle but important field is the counterion form. Synthetic peptides usually emerge from purification as a trifluoroacetate salt (TFA salt); those counterions weigh on the balance without being peptide. If the certificate states the counterion form, it is easier to understand why the weighed mass and the peptide content diverge. If it is missing, that difference remains partly unexplained.
Three questions to read with
To read a certificate quickly but carefully, keep three questions in mind. Is this figure purity or content? By which method was it determined? And is the counterion form stated, since it affects the weighed mass? Anyone who can answer these three knows what is really in the vial, and what the number on the label does and does not mean. A good certificate makes that reading easy by answering the questions in advance rather than leaving them open.