How to Read a Peptide Certificate of Analysis

A certificate of analysis is a lab report, and a lab report is only worth the provenance of the lab that produced it. A PDF with a vendor's own logo and no lab name, no date, and no batch number is decoration.

Here is what each section of a real COA measures, and the four checks that tell you whether the document in front of you means anything.

The two tests that matter

HPLC — how pure is it?

High-performance liquid chromatography separates the components of a sample and measures their relative proportions. The output is a chromatogram: a baseline with peaks, where the target peptide should dominate and impurities appear as smaller peaks.

A purity figure of 98% means the target peak accounts for 98% of the total peak area. It does not say what the other 2% is — that could be truncated synthesis by-products, residual solvent, or something else entirely.

Mass spectrometry — is it the right molecule?

Mass spec measures molecular weight, answering a different question: is this the compound it claims to be? HPLC can show a beautifully pure sample of the wrong peptide. Purity without identity is not reassurance.

Purity is not the same as content

This is the distinction that catches most people out. Two different numbers describe two different things:

Lyophilised peptide carries counter-ions (commonly acetate or trifluoroacetate) plus residual water. Those contribute mass without being peptide. A vial can be 99% pure by HPLC and still contain notably less than its labelled milligrams of actual peptide — the two figures are not in conflict, they are answering different questions.

Most research-grade COAs report purity and omit net peptide content. That omission is worth noticing, since it is the figure that determines what is really in the vial.

Four checks on the document itself

1. Is there a third-party lab named? An independent laboratory with a name you can look up is the entire point. A certificate carrying only the seller's branding is a self-assessment.

2. Does the batch number match your vial? A COA is specific to one production lot. A generic certificate linked from a product page tells you about some batch, not necessarily yours.

3. Is it dated, and how recent? A certificate from three years ago describes material made three years ago.

4. Are the raw traces included? A real report shows the chromatogram and the mass spec trace, not just a summary table. A table alone is trivially easy to type.

Reading the chromatogram

• One dominant peak is what you want, with a flat, quiet baseline.

• Shoulders on the main peak suggest a closely related impurity — often a truncated sequence that eluted at almost the same time.

• A noisy or drifting baseline makes the integration less trustworthy, and the purity figure is derived from that integration.

• Axes should be labelled. An unlabelled trace cannot be checked against anything.

Where COAs fit in vendor research

A COA is one input, not a verdict. It says something about one batch at one moment, from a lab whose independence you may or may not be able to establish. It sits alongside price per milligram, shipping practices, and the consistency of a vendor's disclosures.

The vendor comparison tracks pricing and listings across suppliers, and how rankings work sets out the methodology behind it.

Frequently asked questions

Is 99% purity better than 98%?

Nominally, though the difference is within the range where methodology and integration choices matter. A verifiable 98% from a named independent lab is worth more than an unverifiable 99.9%.

Can a COA be faked?

A PDF is trivial to edit, which is exactly why the named lab, the batch number, and the raw traces matter more than the headline figure.

What is net peptide content?

The proportion of the vial's mass that is actually peptide, as opposed to counter-ions and residual water. It is usually 70–85% and is usually not reported.