How to Read a Peptide Certificate of Analysis (COA)

A Certificate of Analysis is the single most important document attached to a research peptide. It is the laboratory's record of what a specific batch actually contains — its identity, its purity, and how much active peptide is in the vial. For a researcher, the ability to read a COA is the difference between working with a characterized reference material and working with an unknown. This guide explains, section by section, what a peptide COA reports and how to tell an informative certificate from a decorative one.

What a COA is — and what it is not

A COA is generated when an analytical laboratory tests a sample from a production batch and documents the results. A meaningful COA is tied to a specific lot or batch number, carries a report date, and names the analytical methods used. It describes that batch and no other.

A COA is not a marketing claim, a general product description, or a statement of safety or efficacy for any use in humans or animals. It reports measured chemical properties of a research material. A supplier that publishes "≥99% purity" as a blanket slogan without a batch-specific document behind it has told you nothing verifiable.

The sections that matter

1. Batch / Lot number and sample identity

Every credible COA opens with a batch or lot number and the sample name. This is the thread that ties the paper to the vial in front of you: the lot on the certificate should match the lot printed on the vial label. If a supplier prints batch codes on their labels and publishes the matching COA, you can independently confirm the two agree. If the certificate carries no batch number, it cannot be matched to anything and its value drops sharply.

2. HPLC purity

High-Performance Liquid Chromatography (HPLC) is the standard method for measuring peptide purity. The instrument separates the components of the sample, and the result is reported as the percentage of the total that corresponds to the target peptide — commonly expressed as a figure such as ≥98% or ≥99%. On the chromatogram, this appears as one dominant peak; smaller peaks represent related impurities or synthesis by-products.

Purity is reported relative to detectable material. A well-run COA states the method (for example, RP-HPLC) and shows or references the chromatogram rather than simply asserting a number.

3. Mass spectrometry (identity)

HPLC tells you how pure the sample is; it does not, on its own, prove what the compound is. Mass spectrometry (MS) confirms identity by measuring the molecular weight of the peptide and comparing it to the theoretical weight of the intended sequence. When the observed mass matches the expected mass, the identity of the compound is corroborated. A COA that pairs HPLC purity with an MS identity confirmation is substantially more informative than one reporting purity alone.

4. Net peptide content

This is the section most often overlooked. A lyophilized (freeze-dried) peptide vial does not contain 100% peptide by weight — the remainder is typically residual water, counter-ions, and salts left from synthesis. Net peptide content reports how much of the vial's mass is actually peptide. A vial can be 99% pure (of the peptide present, 99% is the target) while its net peptide content is lower, because purity and content measure two different things. For a researcher calculating quantities, net peptide content is the figure that reflects how much active material is present.

5. Appearance, method, and testing lab

Supporting fields round out the picture: the physical appearance (for peptides, usually a white lyophilized powder), the analytical methods used, and — importantly — who performed the analysis. A COA produced by an independent, third-party analytical laboratory carries more weight than one generated in-house by the seller, because the tester has no stake in the result.

A simplified example

The table below shows the kind of specification-versus-result layout a clear COA uses. The values are illustrative.

How to tell a real COA from a decorative one

• It names a batch. No lot number means the document can't be matched to a physical vial.
• It shows methods, not just numbers. "HPLC ≥99%" with a referenced chromatogram beats a bare percentage.
• It confirms identity, not only purity. Mass spec (or equivalent) should appear alongside the purity figure.
• It reports net peptide content. Certificates that omit this leave out the figure researchers most need.
• It is verifiable. The strongest sign is a supplier who lets you match the COA to the batch code on the vial — and ideally look the certificate up independently.