How to Read a Peptide Certificate of Analysis

A peptide certificate of analysis (COA) is a report from an analysing laboratory that records which tests were run on a sample of one batch, by which method, and what each test found. For a synthetic peptide the core lines are identity by mass spectrometry and purity by reversed-phase HPLC, often joined by appearance, net peptide content, counter-ion content and water content. Before reading any number, check three things: that the certificate names the batch you hold, that it was issued to the company supplying it, and that every result states its method.

This guide walks through a generic certificate field by field. The example entries are illustrative only. They are not taken from any real certificate and they describe no product we sell.

What a certificate of analysis is, and what it is not

A certificate reports measured results for a sample drawn from one batch, analysed on a stated date. It is evidence about that sample, and it connects to your material only through the batch number.

Two other documents are easy to confuse with it:

• A specification states the limits a material is meant to meet, such as a minimum purity or an appearance. It is a target that applies to the product in general, not a measurement.
• A technical data sheet describes the compound itself: sequence, molecular formula, molecular weight, storage. It carries no batch results.

A good certificate puts specification and result side by side: one column saying what was required, the next saying what was found. A result without a method, or a specification without a result, tells you much less.

The header: who, what, which batch and when

The top of a certificate identifies the document, the material and the parties. The example references below are invented.

The "issued to" line matters more than it looks. A laboratory issues a certificate to the client who submitted the sample. If that client is a different company from your supplier, the document describes someone else's sample.

Identity: the mass spectrometry line

The identity line compares the mass measured by a mass spectrometer with the mass calculated from the peptide's molecular formula. An illustrative entry might read:

Three details decide whether the line means anything:

• Which mass convention. Average mass and monoisotopic mass differ by about 1 Da for a peptide of this size and by about 3 Da for a peptide near 5 kDa. The certificate should say which one it quotes.
• Which ion. A figure 1 Da above the expected neutral mass is often the singly protonated ion, [M+H]⁺, reported without conversion rather than a mismatch.
• What tolerance. The acceptance window depends on the instrument's resolution and mass accuracy. A high-resolution measurement is usually judged in parts per million; a low-resolution one within a tolerance stated in daltons.

A consistent offset can point to a specific problem: about +16 Da for an oxidised residue, about −18 Da for the loss of water, or the mass of one residue for a deletion sequence. Mass spectrometry and peptide identity explains ionisation, charge states and these offsets in detail.

Purity: the HPLC line

The purity line reports a reversed-phase HPLC analysis. An illustrative entry:

We have left the figure as XX.X because this example is not a real certificate. On a genuine certificate, that number is what you compare with the specification.

When you read the line, check that the detection wavelength is stated. The peptide bond absorbs strongly below about 220 nm, so detection between 210 and 220 nm sees every peptide. At 280 nm only aromatic residues absorb, and a peptide without tryptophan or tyrosine would barely register. Check also that the chromatogram is attached, ideally with a table of the integrated peaks, their retention times and their areas. A purity figure with no trace behind it cannot be read back against the data.

Remember what the figure is: the main peak's share of the UV-absorbing material that the method separates. It is not the fraction of the powder that is peptide, and it does not confirm identity. HPLC and peptide purity covers columns, gradients, integration and the method's blind spots.

Content lines: appearance, net peptide content, counter-ion and water

A fuller certificate describes the solid as well as the molecule. Illustrative entries:

Synthetic peptides are usually isolated as salts. Basic groups such as the N-terminal amine and the side chains of lysine, arginine and histidine carry positive charges, and these are balanced by anions left from synthesis and purification, most often trifluoroacetate or acetate. Freeze-dried solids also hold some water. Net peptide content is what remains once those are accounted for. It counts peptide-related impurities along with the target sequence, so it measures something different from HPLC purity and cannot stand in for it.

The content figures matter whenever a mass of powder is converted into an amount of peptide. In the illustrative example, 10.0 mg of material at 81.5% net peptide content contains 8.15 mg of peptide. The molarity calculator works from the molecular weight you enter, and peptide purity vs net peptide content explains how the two measures relate.

Checks before you rely on a certificate

Work through these in order. The first three decide whether the document applies to your material at all.

1. Issued to the seller. The client named on the certificate should be the company that supplied you.
2. Batch match. The batch or lot number must be identical to the one on the vial or order documents. A certificate with no batch number cannot be tied to a specific batch.
3. Method for every result. Mass spectrometry should name the technique and the mass convention; HPLC should give the column type and detection wavelength; water should name Karl Fischer titration or another stated method.
4. Data attached. A chromatogram with its peak table and a mass spectrum let you read the result back against the measurement.
5. Internal consistency. The expected mass should match the compound's molecular formula, which you can check against a public record such as PubChem. The salt form should match the product description, and the dates should run in order.
6. A laboratory you can contact. A named laboratory and a report number let you ask the laboratory to confirm that the report is genuine.

A certificate that passes all six checks documents one sample from one batch. It does not describe other batches of the same product.

How certificates are published at British Peptide Labs

On certificates, we state three things:

• Certificate status shown on every product page.
• Certificates of analysis published in the COA Library where available.
• Batch certificate available on request.

We publish only certificates issued to British Peptide Labs or AODAEMON LTD. Each is described as the document states it: the issuer, the date, the methods and the batch, where one is given. The purity stated for our research compounds is ≥99% (HPLC, supplier specification). That figure is our supplier's specification for the material, not a certificate result, and it is always shown with that basis. The quality page sets out what each product page documents, and the glossary defines the terms used on certificates.