How to Read a Peptide Certificate of Analysis: Identity, Purity, Quantity and the Missing Tests

A certificate of analysis (COA) can be useful, but it is easy to ask it to prove more than it actually does. A COA is best understood as a lot-specific summary of tests, methods, specifications, and results. It is not automatically proof of identity, dose, sterility, endotoxin control, or suitability for any particular use unless those attributes were actually tested with appropriate methods.

Start with the basics: does the report belong to the material?

Before interpreting any result, confirm that the COA clearly identifies the sample or lot. Useful fields include the lot or batch number, sample identifier, date received or tested, laboratory identity, test methods, specifications or acceptance criteria, and the final results.

A technically impressive chromatogram is much less useful if the chain between the reported sample and the actual lot is unclear.

Identity: is the material actually what it claims to be?

Identity and purity are different questions. FDA chemistry guidance describes identity, strength, quality, and purity as separate attributes. For investigational drug substances, FDA gives examples such as spectroscopic methods for identity and HPLC chromatograms for purity or impurity profiling.

For peptides, mass spectrometry can provide strong evidence that the molecular mass is consistent with the intended compound. Chromatography can add information about whether the sample behaves as expected under a specific method. The exact identity strategy depends on the molecule and the analytical method.

The key question is not simply whether “HPLC” or “MS” appears on the page. It is whether the method used is fit for the attribute being claimed.

Purity: what fraction of detected material appears to be the main component?

Chromatographic purity usually addresses relative composition under the conditions of the method. A high purity percentage can be meaningful, but it does not automatically tell you the total amount of peptide present in a vial.

For example, a chromatogram may show that nearly all of the detector response comes from one main peak. That can support a purity claim, but it is a different measurement from determining absolute peptide content.

Quantity or assay: how much material is actually present?

A quantitative assay requires a method designed to measure amount or concentration. Modern analytical-validation guidance emphasizes characteristics such as specificity or selectivity, accuracy, precision, and a defined reportable range. Reference standards or other well-characterized materials are generally central to reliable quantitative work.

If a COA claims a specific amount, look for the method used to determine that amount—not merely a chromatographic purity percentage.

Endotoxin and sterility are separate tests

FDA chemistry guidance for injectable investigational products specifically calls out sterility, pyrogen or endotoxin levels, and particulate matter as distinct quality attributes. A report that contains only identity and HPLC purity does not establish those additional properties.

Likewise, a low endotoxin result does not demonstrate sterility. Endotoxin testing addresses bacterial endotoxins; sterility testing addresses viable microbial contamination. They answer different questions.

Why a supplier COA should not be treated as unquestionable

In regulated drug manufacturing, FDA has repeatedly warned firms against relying blindly on a supplier’s COA without establishing the reliability of the supplier’s analyses. The precise regulatory duties in those warning letters apply to regulated manufacturers, but the general analytical lesson is useful to any reader: documentation is stronger when the laboratory, method, sample identity, and testing relationship are independently verifiable.

A practical COA reading checklist

Lot match: Does the COA identify the same lot or sample you are evaluating?
Laboratory: Is the testing laboratory clearly identified?
Methods: Are the actual analytical methods named?
Identity: Is there a test designed to support molecular identity?
Purity: Is the reported purity tied to a chromatographic method and chromatogram?
Quantity: If an amount is claimed, is there a quantitative assay rather than purity alone?
Microbiology: If sterility or endotoxin claims are being made, are those tests separately reported?
Specifications: Does the report show acceptance criteria, not just a number?
Date and status: Is the test date appropriate for the lot and claimed shelf life?

Bottom line

A good COA is a map of what was tested. It should not be read as proof of attributes that were never measured. Identity, chromatographic purity, quantitative content, sterility, endotoxin, particulate matter, and stability are separate questions that may require separate methods.