How to read a certificate of analysis without being misled

What a certificate actually certifies

A certificate of analysis is a record of measurements taken on a specific batch by a specific method on a specific date. It is not a statement about the compound in general, and it is not a guarantee about the vial in your hand unless the batch number on the vial matches the batch number on the certificate. That match is the first thing to check and the most commonly skipped.

A certificate that carries no batch number, no analysis date and no method description is a marketing document with a laboratory typeface. Treat it as one.

Why the purity figure is the weakest number on the page

Purity by HPLC is area-normalised: it reports the target peak as a percentage of total detected peak area. Everything in that sentence is a limitation. Impurities that do not absorb at the detection wavelength are invisible. Impurities that co-elute with the target are counted as target. Anything that does not come off the column at all is simply absent from the calculation.

This is why a 99% figure from an undescribed method tells you very little. Ask what column, what gradient, what detection wavelength, and what the run time was. A short isocratic run on a compound that elutes near the solvent front can report excellent purity while resolving almost nothing.

For peptides, add a second question: is this HPLC purity or net peptide content? They are different numbers and the gap between them is usually 15–30%. A vial labelled 10 mg of a TFA-salt peptide at 98% HPLC purity may contain 7.5 mg of peptide, the balance being counter-ion and residual water. Quantitative work that ignores this is wrong before it starts.

Identity confirmation is separate from purity

Purity tells you how much of the sample is one thing. Identity tells you what that thing is. A certificate can report 99.5% purity for entirely the wrong compound, and this happens more often than the industry likes to acknowledge.

For peptides, identity means a mass spectrum with the observed mass stated, not merely 'MS: conforms'. Compare the observed value against the sequence yourself. For small molecules, identity means NMR — mass alone cannot distinguish structural isomers sharing a formula, and isomeric impurities are exactly what a synthesis is likely to produce.

Watch for the specific traps: C-terminal amidation differs from the free acid by one dalton and low-resolution MS will not resolve it; methionine oxidation adds sixteen; and compounds separated by a single functional group — Melanotan 2 and PT-141 differ by eighteen — are trivially swapped in a supply chain and non-trivially distinguished without checking.

The absences that should stop an order

No batch number, or a batch number that does not match the vial. No analysis date, or a date that predates the batch. No named method. A spectrum reproduced as a low-resolution image with no axis labels. A supplier who cannot produce the underlying chromatogram when asked.

For recombinant proteins, add: no endotoxin figure, and no bioassay or ELISA-derived activity result. Chromatographic purity says nothing about whether a protein is correctly folded, and a denatured preparation can be spectroscopically perfect and biologically inert.

For hygroscopic solids, add: no water content by Karl Fischer. Without it, you do not know what fraction of the weighed mass is compound.

A working checklist

Match the batch number to the vial. Confirm the analysis date is after the manufacture date. Read the method description and decide whether it could resolve the impurities you would expect from that synthesis. Check the observed mass against the structure yourself rather than accepting 'conforms'. Establish whether the purity figure is chromatographic or content-based. File the certificate with the experimental records — not in a supplier folder, but with the data it underwrites.

None of this takes more than a few minutes per batch, and it is the difference between a result you can defend and a result you cannot explain.

References and authoritative sources

Chemical identity, regulatory status and anti-doping classification should be confirmed against primary sources rather than against this page. The records below are the ones to check first.

1. PubChem compound record (US National Library of Medicine)
2. PubMed indexed literature

These are database and regulatory sources. Citations to specific primary papers are being added to this page as part of ongoing scientific review; until they appear, treat the mechanism and literature sections above as a summary to verify rather than a citation of record.