How to Read a Certificate of Analysis: What It Proves and What It Doesn't

A certificate answers two questions, not one

A useful certificate of analysis establishes two separate facts, and conflating them is the most common error buyers make. The first is identity: is the molecule in this vial the molecule named on the label? The second is purity: what fraction of the material is that molecule, and what is the rest?

Identity is normally established by mass spectrometry. The instrument measures molecular mass to within a fraction of a dalton, and the observed mass is compared against the theoretical mass for the intended sequence. A match is strong evidence of identity; a mismatch is conclusive evidence against it.

Purity is normally established by high-performance liquid chromatography. The sample is separated across a column, and the detector output is integrated to give the proportion of total signal attributable to the main peak. A stated '99% purity' almost always means 99% of integrated peak area at a specified wavelength.

Mass spectrometry tells you what it is. HPLC tells you how much of it is that. A certificate showing only one is answering half the question.

The fields that matter

Batch or lot number is the single most important field, because it is what connects the certificate to the physical vial. A certificate with no batch number cannot be tied to anything and is decorative.

Date of analysis matters because peptide purity is a property of the material at a point in time. A certificate from three years ago describes material that may no longer exist.

Testing laboratory should be named. An in-house certificate is not worthless — the supplier may have excellent instrumentation — but it is a self-assessment, and it should be labelled as one rather than implied to be independent.

Molecular weight should be stated and should match published values for the compound. This is where a surprising number of certificates fall apart, and it is the easiest field for a buyer to check independently.

Four ways a true certificate still misleads

First, the batch mismatch. The certificate is real, the analysis was performed, and the vial you received is from a different lot. Check that the batch number on the certificate matches the batch on the vial.

Second, the wavelength choice. HPLC purity is reported at a detection wavelength. At 220 nm the peptide bond absorbs strongly and most impurities are visible. At 280 nm only aromatic residues absorb well, and a certificate reported there can look cleaner than the material is. A certificate that does not state its wavelength is withholding relevant information.

Third, net peptide content. Lyophilized peptide is not pure peptide by mass — it contains residual water and counterions, commonly trifluoroacetate from purification. A vial can be 99% pure by HPLC and still contain meaningfully less peptide by mass than the label implies. Net peptide content is a separate measurement and is often absent.

Fourth, the single certificate reused across a catalog. If every product page links to a certificate with the same batch number, the certificates are illustrative rather than real.

A worked example: reading one real certificate

Take Janoshik Analytical task #200337, the certificate behind our Retatrutide batch CS-re10-0625. Read top to bottom, each field answers a specific question.

Task number and verification key (#200337, key AKC4Q31UXR3S). These are what make the document checkable. Enter the key at janoshik.com/verification and the laboratory returns its own copy of the report. A certificate with no key, or a key that returns nothing, is a picture of a record rather than the record.

Sample and batch. The sample line describes the vial as the laboratory received it, and the batch code is what ties the report to a lot. Match that code against the label on the vial in your hand; if they differ, the certificate describes other material.

Client. The party that commissioned the test, here the manufacturer. That is normal and worth knowing: an independent laboratory performed the analysis, but someone chose which vial to send. A run of certificates across many batches is the answer to that, not any single report.

Tests requested. This one reads "Common GLP-1 peptide blind test (Semaglutide, Tirzepatide and Retatrutide)". A blind test means the laboratory determined which of the three compounds was present instead of confirming a name it was handed, which is a stronger identity result than a simple match.

Dates. Ordered 26 June 2026, received 3 July, analysed 13 July. The analysis date is the one that matters: it says when this material measured as it did.

Purity: 99.637%. The main-peak share of the HPLC chromatogram for this batch, exactly as printed. It is a measurement, not a specification or a target.

Content: 11.76 mg, measured in a vial labelled 10 mg. Content is the peptide mass the laboratory found in the vial it received. A reading somewhat above the label is common because vials are filled with a margin; a reading well below it is the one to question. It describes that vial, and several retail sizes can be filled from the same certified lot.

One certificate describes one batch. Whether the process holds is a question only a series can answer, which is why every Retatrutide certificate we hold is published together, charted by date, and downloadable.

Retatrutide certificate record →·Look up a batch code →·All certificates →

What to do with this

Ask for the certificate before purchase, not after. A supplier confident in their material will send it. The response to that request is itself the most informative test you can run.

Check the molecular weight against published literature. It takes thirty seconds and it catches both honest labelling errors and the TB-500 fragment problem, where two suppliers legitimately ship molecules differing more than fivefold in mass under the same product name.

Match the batch number when the package arrives. If it does not match, the certificate described someone else's material.