How to Read a Certificate of Analysis
What each section of a COA tells you, which numbers matter, and the three most common ways a report gets misread.
How do you read a Certificate of Analysis?
Start with the header, not the results. Confirm the batch or lot number matches the material you hold, check the date of receipt, and note who submitted the sample. Then read purity as an area percentage, identity as a mass match in parts per million, and content as absolute milligrams. Purity and content are different measurements and are frequently confused.
A certificate is read from the top down for a reason. The header establishes what the document is about; the results establish what was found. Reading the results first is how people end up confidently quoting a purity figure for a batch the laboratory never received.
Check the header before the results
Three fields decide whether the rest of the document is relevant to you at all.
- 1
Batch or lot number
Must match the number on the vial in your hand. A certificate for batch B240612 says nothing about batch B240710, however similar the products look.
- 2
Date of receipt
Establishes when the laboratory took possession. A result describes the material as received on that date, not as it exists after months of uncontrolled storage.
- 3
Submitting party
Names who sent the sample. A certificate issued to a distributor covers the unit that distributor submitted, which may or may not represent what you were shipped.
Purity is an area percentage, not a promise
Chromatographic purity is reported as the area of the main peak expressed as a percentage of total integrated peak area, at a stated detection wavelength.
A result of 99.2% means the main peak accounted for 99.2% of what the detector saw. It does not mean the vial is 99.2% peptide by mass, and it does not mean 0.8% of the contents are impurities.
The distinction is commercially significant. A sample can be 99% pure by HPLC and contain far less peptide than the label claims, because the remaining mass is water, acetate or trifluoroacetate counter-ions, or mannitol, none of which absorb strongly at the detection wavelength or elute inside the integration window.
Identity is a mass match with a tolerance
Identity confirmation compares the observed monoisotopic mass against the theoretical mass of the claimed sequence. A properly reported identity result states the observed mass, the theoretical mass, and the difference in parts per million.
A mass match confirms the molecular formula is consistent with the claim. It does not, on its own, exclude every isomer or every sequence rearrangement of identical composition. Two peptides with the same amino acids in a different order have the same mass.
When sequence certainty genuinely matters, the right assay is peptide mapping with tandem mass spectrometry, not intact mass confirmation.
The three misreadings
The first and most consequential is treating purity as content. Covered above, and responsible for more supplier disputes than every other error combined.
The second is reading 'not detected' as 'zero'. Every method has a limit of quantitation. Not detected means below that limit, and a certificate should state what the limit was. A residual solvent reported as not detected at a 50 ppm limit is a different claim from one not detected at 2 ppm.
The third is assuming a passing sterility result still applies after the vial has been opened. Sterility describes a sealed unit. It is destroyed by the first non-aseptic entry, and no certificate can speak to what happened after the seal was broken.
Frequently asked questions
- What purity percentage is good for a research peptide?
- Most research applications treat 95% or higher by RP-HPLC as acceptable and 98% or higher as good, but the threshold depends entirely on the application. What matters more than the headline figure is whether the certificate states the detection wavelength, the gradient conditions and the limit of quantitation, and whether the chromatogram shows a symmetrical main peak without shoulders. A 99% figure from an undisclosed method is worth less than a 96% figure with full supporting data.
- Why does my peptide show high purity but low content?
- Because purity and net peptide content measure different things. Purity is the proportion of the detected material that was the target compound. Content is the absolute mass of active peptide in the vial. Lyophilised peptide typically carries residual water, trifluoroacetate or acetate counter-ions from purification, and sometimes bulking agents such as mannitol. That mass is real but does not register as impurity on a chromatogram, so a vial can be simultaneously 99% pure and 25% short on its label claim.
- Does a Certificate of Analysis prove a peptide is safe?
- No. A Certificate of Analysis records composition, not safety. It states what a laboratory measured in a specific sample using stated methods. It is not a safety assessment, an efficacy assessment, a regulatory clearance or an authorisation for human or veterinary use. A sample can be highly pure, correctly identified, sterile and endotoxin-free while remaining entirely unsuitable for administration to any living thing.
About the author
REPLACE: Analytical Lead
Head of Analytical Chemistry · PhD Analytical Chemistry
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Purity Versus Net Peptide Content
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Peak shape, baseline drift and shoulders: what the raw trace shows that a purity percentage cannot.
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What 'Not Detected' Means on a Lab Report
Why 'not detected' never means zero, and what a certificate must state for the result to mean anything.