How to read a peptide certificate of analysis

A certificate of analysis (COA) is the single most important document a peptide supplier can give you. It is also the easiest thing in the world to fake convincingly, which is why knowing how to read one properly matters.

Start with the lot number

Before you look at any figure, check that the lot number on the certificate matches the lot number printed on your vial label. If they do not match, the document tells you nothing about the material in your hand. A supplier who publishes one certificate across many batches is publishing marketing, not analytics.

Purity is a chromatographic measurement

The purity figure is almost always an HPLC-UV result, reported as the area percentage of the target peak against total peak area. Two things follow from that:

• It measures chromatographic purity, not absolute mass purity. Salt, water and counter-ions do not show up.
• The detection wavelength matters. Peptides are usually monitored at 214nm because the peptide bond absorbs there.

A stated 99% at 214nm means 99% of the UV-absorbing material eluting from the column was your target compound. It does not mean the vial is 99% peptide by weight — that is net peptide content, a separate figure.

Identity needs mass spectrometry

Purity alone tells you the sample is clean. It does not tell you the sample is what you ordered. Mass spectrometry, usually ESI-MS or MALDI-TOF, confirms molecular weight against the theoretical value for the sequence. A certificate that reports purity but not identity is incomplete.

The figures suppliers quietly omit

• Net peptide content — what proportion of the vial mass is actually peptide.
• Residual moisture — measured by Karl Fischer or loss on drying.
• Counter-ion / salt form — TFA versus acetate can matter in cell-based assays.
• Endotoxin — relevant if the material touches anything sensitive to pyrogens.

Ask for the chromatogram

The summary table is a transcription. The chromatogram is the evidence. A legitimate supplier will send it. Look for a single sharp symmetrical peak, a stable baseline, and a legible axis. Broad shoulders, unlabelled axes or a cropped image are all reasons to walk away.

Our position

We publish the lot-matched certificate on every product page before you buy, and we will send the raw analytical package on request. If a supplier will not do both of those things, ask yourself why.