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Reading a Certificate of Analysis (COA)
What a peptide COA contains, how to read the HPLC and mass-spec results, and why the batch number matters.
A Certificate of Analysis (COA) is the document that backs a supplier's purity claim. For research peptides it is the single most important trust signal, and a serious supplier provides one for every batch.
Purity by HPLC
The COA reports purity as a percentage derived from the HPLC chromatogram, for example 99 percent or higher. The chromatogram shows a main peak for the target peptide alongside any minor impurity peaks. A tall, clean main peak with little else indicates high purity.
Identity by mass spectrometry
The MS section confirms the peptide is what the label says by matching the measured molecular weight to the expected value. Purity without identity is not enough, so look for both.
Batch and lot numbers
A COA is tied to a specific batch. Cross-reference the lot number printed on the vial against the COA so you know the certificate actually describes the material in your hand, not a different production run.