A certificate of analysis (CoA) is the document that tells you what is actually in a research peptide vial. Reputable suppliers provide one for every batch. Knowing how to read it properly is not optional if you care about the quality of your research data — it is the difference between knowing what you have and guessing.
What a Certificate of Analysis Contains
A well-prepared CoA should include the peptide identity, sequence, molecular weight, purity percentage, and the analytical methods used to determine those values. Most credible CoAs are produced by third-party testing laboratories rather than the supplier themselves — this distinction matters because a supplier testing their own product has an obvious commercial incentive to report favourable results.
Purity: The Number Everyone Looks at First
Purity is reported as a percentage and almost always refers to the proportion of the desired peptide present in the total sample, measured by HPLC (high-performance liquid chromatography). A purity of 98% means that 98% of what is in the vial by area under the HPLC curve is your target compound. The other 2% is impurities — which could be related peptide fragments, incomplete synthesis products, or residual reagents from the manufacturing process.
For most research purposes, 95%+ purity is considered acceptable. Purity below 90% is a concern and affects how you should interpret your results. A 98% pure compound and a 90% pure compound of the same peptide are not interchangeable at the same stated mass — the effective dose differs by about 8%, which accumulates into a significant confound across a long research protocol.
The HPLC Chromatogram
A CoA worth trusting will include the actual HPLC chromatogram — the graph showing the peaks that correspond to the compounds detected in the sample. Your target peptide should show up as a single dominant peak. A cluster of similar-height peaks indicates poor synthesis and multiple compounds of similar retention time. A small clean peak next to a large main peak might be a closely related fragment. This visual is far more informative than a single percentage number.
Mass Spectrometry Confirmation
Purity by HPLC tells you what percentage of the sample is the dominant compound — but it does not confirm that the dominant compound is actually what it is supposed to be. A peptide could theoretically be 99% pure and 100% the wrong sequence. Mass spectrometry (MS) closes this gap by confirming the molecular weight of the detected compound, which, combined with the known theoretical weight of the target peptide, gives strong evidence of correct identity.
Look for MS data on the CoA alongside HPLC. If a supplier can only provide HPLC purity without mass spectrometry identity confirmation, that is a yellow flag — not necessarily a dealbreaker, but worth noting.
Molecular Weight: Checking the Numbers
The theoretical molecular weight of a peptide is calculable from its amino acid sequence. Your CoA should report the observed molecular weight from mass spectrometry. These two numbers should match within a very small margin (typically ±1 Da for small to medium peptides). A significant discrepancy suggests a sequencing error, a modification, or a completely different compound.
Batch Number and Date
Always check that the batch number on the CoA matches the one printed on your vial or order documentation. CoAs are batch-specific — a certificate from a previous batch tells you nothing about the quality of a different batch, even from the same supplier. Also note the testing date. A CoA from three years ago for a product being sold today is not reassuring.
What to Do If Something Looks Off
If the purity is lower than claimed, if the mass spectrometry data is missing, or if the chromatogram shows multiple significant peaks, contact the supplier before using the compound. A legitimate supplier will engage with your questions and, if necessary, offer replacement product. One that deflects or dismisses technical questions about their CoA data is not a supplier worth using for serious research.