How to Read a Certificate of Analysis
Identity by mass spec, purity by HPLC — and the details a headline percentage never tells you.
A Certificate of Analysis (COA) is the document that turns “trust us” into evidence. For research peptides it is the single most useful piece of paper you will handle, and yet it is easy to glance at the purity number and miss the rest. Here is how to read one properly.
Identity: is it actually the right compound?
Before purity means anything, the COA should confirm identity — that the vial contains the peptide it claims to. This is usually done by mass spectrometry: the measured molecular mass is compared against the theoretical mass for that sequence. A match within the expected tolerance tells you the molecule is what the label says.
Purity: the HPLC trace
Purity is typically measured by reversed-phase HPLC. The result is reported as a percentage — the area of the main peak relative to everything else detected. A single dominant, sharp peak with little else is what you want. Be wary of a headline purity figure with no chromatogram behind it; the trace itself tells you whether that number is a clean main peak or a generous interpretation of a messy one.
What else to look for
- Lot / batch number — it should match the vial you received. A COA for a different lot is not a COA for your product.
- Test date and method — recent, and naming the actual instruments and conditions used.
- Appearance and physical checks — often a white-to-off-white lyophilized solid.
- Sequence, molecular formula and molecular weight — the reference data the identity test is checked against.
Reading purity honestly
No synthetic peptide is 100.00% pure, and any COA claiming so deserves scepticism. High-quality research material is commonly reported in the high-90s by HPLC. What matters as much as the number is that the document is specific to your lot, names its methods, and shows its working.
At Peptide Labs a COA is available for the lot you receive — just ask, with your product or order number, and we will send it.
This article is reference material for researchers evaluating supplier documentation.