Understanding a Certificate of Analysis

A Certificate of Analysis is the document that tells you what a specific batch actually is. Knowing how to read one is the single most useful skill in assessing a research compound.

What a COA is

A Certificate of Analysis (COA) is the laboratory report for one batch of one product. It records which tests were performed, by what method, and what the results were. It is tied to a batch number, so a COA for batch A tells you nothing about batch B. A supplier that publishes a single generic COA for a product, rather than one per batch, is not giving you batch-level assurance.

Identity

Identity confirms the compound is the sequence it claims to be. The standard method is mass spectrometry: the measured molecular mass is compared with the mass calculated from the sequence, and a match within instrument tolerance confirms identity. For some compounds, amino acid analysis or sequencing may be added.

Purity

Purity is usually reported from high-performance liquid chromatography (HPLC) as an area percentage — the proportion of the total detected signal attributable to the main peak at a given wavelength, commonly 214 or 220 nm. A result of ≥99% means the target peptide makes up at least 99% of the peptide-related material detected. It does not mean the vial is 99% peptide by weight; see net peptide content below.

Purity versus net peptide content

Lyophilised peptides contain bound water and counter-ions — commonly trifluoroacetate from the purification step — as well as the peptide itself. The net peptide content is the fraction of the gross weight that is actually peptide, and it is often well below 100%, commonly in the region of 70–90%. Purity and net content answer different questions. If your experiment depends on an exact molar quantity, you need the net content, which is determined by amino acid analysis or elemental analysis rather than HPLC.

Why independent testing matters

A COA produced by the manufacturer or the seller is a statement about their own product. A COA produced by an independent laboratory is a check on it. Both have value, but only the second gives you assurance that does not depend on trusting the party with a commercial interest in the result. Look for the testing laboratory's name on the certificate.

What to check

• The batch number on the certificate matches the batch number on the vial
• The tests performed, their methods and the date
• Identity confirmed by mass spectrometry against the calculated mass
• Purity by HPLC, with the method and wavelength stated
• The name of the laboratory that carried out the testing

For laboratory research use only — not for human or veterinary use. This article is general laboratory reference information and is not advice on the use of any product.