How to read a certificate of analysis

Start with the batch number, not the percentage

The purity percentage is the number everyone reads first, but it's close to meaningless without a batch or lot number tying it to one specific production run. A COA with no batch number could describe any batch the seller has ever made, including one you will never receive. Check that the number on the COA matches the number printed on the vial, or the seller's own batch lookup, if one exists.

Find out who ran the test

A COA can come from the manufacturer's own bench, from a contract lab the manufacturer hired, or from an independent third-party lab with no financial stake in the result. Only the last one is a genuinely outside check. The lab's name should be printed on the document. If it isn't named, or the document doesn't say who performed the analysis, you're reading a claim, not a finding, whatever the header says.

Check the method, not just the result

A purity figure means one thing after HPLC alone and something narrower after HPLC combined with mass spectrometry, and it means very little if no method is stated at all. Our guide on HPLC purity and mass spectrometry covers what each one actually measures. A serious COA names its method plainly, often alongside the instrument or column used.

Look for a test date

Peptides age, and so does the relevance of a COA. A report from two years ago says nothing about a batch made last week. A dated report at least lets you judge whether the testing is current rather than a document that's been recycled across every batch the company has ever sold.

What a complete COA usually includes

• Product name and the expected sequence or structure
• Batch or lot number
• Test date
• Testing method (HPLC, HPLC-MS, and so on)
• Purity result
• Identity confirmation, showing the measured mass matches what it should be
• For injectable-intent products, often a separate endotoxin or sterility result
• The testing lab's name, and ideally a way to verify the document, such as a public lookup or a QR code

What we do with this

This is the checklist behind the "documents on file" column on our homepage. We score three things on every vendor: whether an outside lab wrote the report, whether the batch number matches, and whether the method is stated. The full rules are on the homepage's method section.