How to Read a Certificate of Analysis

A certificate of analysis (COA) is the document that links a specific batch of material to the tests performed on it. It is the single most useful thing a supplier can give you, and also the easiest document to fake — which is why the details matter more than the letterhead.

Batch identity comes first

The top block should name the compound, the batch or lot number, the manufacture date and the quantity per vial. Cross-check the batch number against the vial in your hand. A COA that does not carry a batch number cannot be tied to anything you received.

HPLC purity

Reverse-phase HPLC separates the target peptide from truncated and modified sequences. The report should show the chromatogram itself, not just a number: a single dominant peak with a clean baseline, the retention time, the gradient used and the integration table. A bare statement of "99% purity" with no trace behind it is a claim, not a result.

What impurity peaks tell you

Small peaks eluting close to the main peak are usually deletion or oxidised sequences. Widely separated peaks can indicate residual reagents. Neither is automatically alarming at trace level; an absence of any visible baseline detail usually means the image has been cropped or reused.

Mass spectrometry

MS confirms identity. The observed molecular weight should match the theoretical weight for the sequence within the instrument's tolerance. If the COA states purity but never confirms mass, it has verified how clean the material is without verifying what it is.

Water, counter-ion and appearance

• Water content by Karl Fischer titration — lyophilised material always retains some.
• Counter-ion content, typically acetate or trifluoroacetate from purification.
• Appearance — a white to off-white cake or powder; visible discolouration is a defect.
• Peptide content by mass, when the supplier measures it.

Red flags

1. 1No batch number, no analysis date, or a date that predates the batch.
2. 2The same chromatogram image reused across different compounds.
3. 3Purity quoted without any instrument trace.
4. 4A named testing laboratory with no contact details or report reference.
5. 5A COA that cannot be reproduced on request for the exact lot you received.