How to read a Certificate of Analysis for a research peptide

A Certificate of Analysis (COA) is the document that connects a specific batch of material to the analytical tests a laboratory performed on it. Read well, it tells you what the laboratory measured, how, and when. Read carelessly, it can give false comfort. This guide walks through the elements that matter.

1. Who issued it, and for what

The first thing to establish is the testing laboratory and the sample it describes. A credible COA names the laboratory, carries a report or certificate number, and identifies the sample by compound name and batch (lot) number. If any of these are missing, the document cannot be reconciled to the material in front of you.

Check that the batch number on the COA matches the label on the material. If the two do not agree, the document does not describe what you are holding, however reassuring its numbers look.

2. Identity versus purity

These are different questions and are answered by different methods.

• Identity asks "is this the molecule it claims to be?" For peptides it is usually answered by mass spectrometry (LC-MS or MALDI-TOF): the observed mass should match the calculated mass of the stated sequence, including any modifications such as N-terminal acetylation, C-terminal amidation or acyl side chains.
• Purity asks "how much of the sample is the main component?" It is usually a chromatographic measurement (reverse-phase HPLC with UV detection) expressed as the area of the main peak as a percentage of all peaks.

A purity figure without an identity result tells you only that the sample is mostly one thing — not what that thing is.

3. The method matters

HPLC purity depends on the column, gradient, detection wavelength and integration settings. Two laboratories can report different purities for the same material. A good COA states the method (for example "RP-HPLC, C18, 220 nm") so that results can be interpreted and, if necessary, reproduced.

Mass spectrometry results should state the technique and the observed versus expected mass. For larger peptides, the observed mass may be reported as a deconvoluted average mass; for smaller ones, as a monoisotopic [M+H]⁺ ion.

4. Purity is not the same as net peptide content

A lyophilised peptide is not 100% peptide by weight. It contains water, counter-ions (commonly acetate or trifluoroacetate) and sometimes residual solvent. Net peptide content — the proportion of the powder that is actually peptide — is measured separately, typically by amino acid analysis or nitrogen determination, and can be substantially lower than chromatographic purity. A 99% HPLC-pure peptide might have 70–85% net peptide content.

If net content is not reported, assume it is unknown. This matters for any quantitative experiment.

5. Dates and validity

A COA describes the sample at the time of testing. Peptides degrade, particularly in solution or with repeated freeze–thaw cycles. Note the test date and consider how the material has been stored since.

6. Red flags

• No laboratory name, or a name that cannot be found.
• No batch number, or a batch number that does not match the label.
• Purity claimed to one or two decimal places with no method stated.
• An identity method that cannot distinguish the claimed molecule from close analogues (for example, UV absorbance alone).
• A document that has clearly been edited, re-branded or assembled from screenshots.
• A single "COA" presented for multiple batches.

7. How Positive Peptides uses this

Every peptide we supply is tested to 99% purity by HPLC with identity confirmed by mass spectrometry, and laboratory documentation for the batch supplied is available with your order on request. We would rather you read that documentation critically, using the checks above, than take any figure — including ours — on trust. Our standards page sets out how we verify the information published on this site.

This article is for scientific information only and does not constitute advice about the suitability of any material for any purpose.