A certificate of analysis is the only document that connects a specific batch to a specific measurement. Being able to read one — and knowing what it does not cover — is the most useful practical skill in this area.
A certificate of analysis records what testing was performed on a specific batch of material, by whom, when, and what the results were.
The word specific is doing the work there. Peptide synthesis varies between runs. A certificate describes the batch it was issued for and nothing else — which is why a certificate without a batch or lot number printed on it, matching the one on what you received, is of no value.
High-performance liquid chromatography separates the components of a sample as they pass through a column at different rates. The output is a chromatogram: a trace with peaks, each corresponding to a component.
Purity is reported as the area of the main peak as a percentage of total peak area — so '≥99% by HPLC' means the target peptide accounts for at least 99% of what the detector saw.
Two caveats are worth knowing. The figure depends on the detection method, typically UV absorbance at a specified wavelength, and anything that does not absorb at that wavelength is invisible to it. And purity says nothing about what the other 1% is.
HPLC can tell you a sample is 99% one thing. It cannot tell you that thing is what the label says.
Mass spectrometry answers that by measuring mass-to-charge ratio, which is compared against the calculated mass for the expected sequence. A match within tolerance confirms identity.
The two tests are complementary and neither substitutes for the other: purity without identity could describe a very pure wrong compound.
This is the figure most often misread. Lyophilised peptide is not pure peptide by mass — it also contains counter-ions, residual water and salts from synthesis and purification.
Net peptide content states what proportion of the material's mass is actually peptide. A material can be 99% pure by HPLC and still have a net peptide content well below that, because the two measure different things: purity is about the peptide fraction's composition, net content is about how much of the total mass is peptide at all.
Where net peptide content is determined it should be reported. Where it is not, the certificate should say so rather than leave it implied.
This is where most overreach happens.
A few checks separate a meaningful certificate from a decorative one. Does it carry a batch or lot number that matches the one on what you received? Is it dated? Does it name the laboratory that performed the testing? Does it state the methods used, not just the results?
A certificate that is undated, carries no lot number, names no laboratory, or appears as an unlabelled image with no traceable origin does not provide assurance regardless of what figures it shows.
It is entirely reasonable to ask a supplier for the certificate covering the batch currently in stock before ordering. A supplier that treats that as an awkward request is telling you something.
This article is reference material describing published research. It is not medical advice, not a therapeutic claim, and contains no dosing or administration guidance.