How to read a peptide certificate of analysis

Almost every research peptide supplier says its material is third-party tested. Far fewer will hand you the document, and fewer still publish it against a lot number you can match to the vial in your hand. When you do get one, it is worth knowing how to read it — because the layout is conventional enough that the gaps tell you as much as the figures.

The header is the part people skip

Before any result, check four things: the name of the testing laboratory, the date of the test, the lot or batch identifier, and the compound and mass the lab was told it was analysing. If the laboratory is not named, the document is an internal QC sheet wearing a costume. If the lot identifier does not appear on the physical vial label, the certificate cannot be tied to what you received, which makes it decorative.

The test date matters more than people expect. A certificate dated two years before your vial was filled is describing different material. One lot, one fill run, one certificate — that relationship is the entire value of the document.

Purity by RP-HPLC

This is the headline number and the one every supplier quotes. Reverse-phase high-performance liquid chromatography separates the sample by hydrophobicity; the detector, usually at 214 or 220 nm, measures the peptide bond. Purity is reported as the target peak's area as a percentage of total peak area.

Two things constrain it. First, it is relative: it tells you the proportion of UV-absorbing material that is your peptide, not the absolute mass of peptide in the vial. Salts, residual water and counter-ions do not absorb and do not appear. Second, the wavelength matters — a figure quoted at 214 nm and one at 220 nm are not the same measurement, so a certificate that omits the wavelength has omitted something.

Our specification is ≥ 99% by RP-HPLC at 220 nm. Material that will not reach it is rejected rather than moved to a cheaper listing.

Identity by mass spectrometry

Purity without identity is close to meaningless: a sample can be 99% pure of the wrong molecule, and a truncated or deletion sequence that co-elutes will happily count toward a high purity figure. Electrospray ionisation mass spectrometry gives the observed mass, which should match the theoretical mass for the sequence within the instrument's tolerance.

Read the two numbers side by side. A difference of roughly 18 Da suggests a water loss; around 71 Da suggests a missing alanine; a large mismatch means the vial does not contain what the label says. A certificate that reports identity only as "conforms", with no observed mass, is asking you to take the lab's word for the comparison.

Bacterial endotoxin

Lipopolysaccharide from gram-negative bacteria, picked up during synthesis, handling or filling. Measured by Limulus amebocyte lysate assay, usually against USP <85>, and reported in endotoxin units per milligram.

This is the test most often missing from a certificate, and the omission is rarely accidental — it costs money and it is the one most likely to fail. It matters for any assay with an immune or inflammatory readout, where endotoxin at the wrong level does not merely add noise but produces a coherent, plausible, entirely artefactual result. If you work with cells and the certificate has no endotoxin line, you are running blind on the variable most likely to explain a surprising finding.

Heavy metals and water content

Heavy metals by ICP-MS catch catalyst and reagent carry-over from synthesis. Water content by Karl Fischer titration measures residual moisture after lyophilization, and it is quietly the most commercially interesting line on the page: a lot at 12% water contains 12% less peptide than the label mass implies. Price per milligram calculated off the label, without reading this line, is not price per milligram of peptide.

Appearance

A visual check for a white lyophilized cake. It reads as filler and is not — discolouration, collapse or oiling indicates a problem with the lyophilization cycle or with storage since, and it is grounds to quarantine a lot whatever the other results say.

The three questions worth asking

• Can I look up this certificate myself, from the lot number on the vial, without an account? If verification requires emailing the supplier, verification is a favour rather than a property of the document.
• Does the certificate name the laboratory that produced it? "Third-party tested" with no third party named is a claim about a claim.
• Is the full panel here, or only purity? A single-line certificate is a marketing asset. A six-line panel is a record.

Every lot we sell is searchable at rensaresearch.com/verify before it is listed for sale, and the certificate stays online after the lot sells out. That is the only claim this business makes, so it is the one we make checkable.