How to read a peptide certificate of analysis (COA)
A certificate of analysis (COA) is a lab's written result for one sample: what was tested, which methods were used and what was found. For research peptides it is the main way to check that a vial contains the right compound, in the right amount, without much else in it. This guide explains the fields you will meet on a typical report, what the numbers do and do not tell you, and how to check that a report is genuine.
The fields on a typical report
| Field | What it tells you |
|---|---|
| Lab and task number | Who did the analysis and the lab's own reference for this job. |
| Client | Who sent the sample to the lab. |
| Sample | The product and labelled amount as declared by the client, for example "Tirzepatide 30mg". |
| Manufacturer and batch | Where the material comes from and which production lot. When the batch is not stated, the result applies to the sample tested and cannot be tied to a specific lot. |
| Dates | When the test was ordered, when the sample arrived and when it was analysed. |
| Results | The measured amount and purity, sometimes with identity confirmation. |
| Verification key | A code or QR link to check the report on the lab's own website. |
Purity
Purity is usually measured by HPLC (high-performance liquid chromatography). The sample is pushed through a column that separates its components, and a UV detector, typically set around 210–220 nm where the peptide bond absorbs light, records each one as a peak. Purity is the area of the main peak as a share of the total area of all peaks. A result of 99.5% means that 99.5% of what the detector saw at that wavelength is the target peptide.
The other 0.5% is usually related peptides: chains missing an amino acid, chains that were not fully deprotected during synthesis, or oxidised forms. For synthetic research peptides, a purity above 98% is generally considered high, and above 99% very high.
What purity does not include: water, salts and counterions such as trifluoroacetate (TFA) or acetate do not absorb UV light at those wavelengths, so they are invisible to this measurement. A sample can be 99% pure by HPLC and still contain a fair amount of moisture or salt by weight.
Measured content
For a finished vial, labs often report how many milligrams of the compound they found, by comparing the sample's peak against a reference standard of known amount. This answers a different question from purity: not "is it clean?" but "is the vial filled as labelled?". A result at or slightly above the labelled amount means the vial is not under-filled. Small differences of a few percent either way are normal for weighing and analysis.
Purity vs net peptide content
On reports for bulk powder you may also see net peptide content: the share of the total weight that is peptide rather than water and counterions. It is measured separately, for example by amino acid analysis or elemental nitrogen analysis, and for freeze-dried peptides it is often well below 100%. The two numbers combine: 10 mg of powder with 80% peptide content and 98% purity holds about 10 × 0.80 × 0.98 ≈ 7.8 mg of the target peptide. A vial result in milligrams already takes this into account.
Identity
Purity only says that one compound dominates the sample; it does not by itself prove which compound it is. Identity is confirmed by mass spectrometry (MS, often coupled to the HPLC as LC-MS), which measures the molecular mass and compares it with the expected value for that sequence, or by matching the sample against a reference standard of the same peptide. When a lab reports an amount of a named compound measured against its standard, the identity has been checked as part of that measurement.
What a standard report does not cover
- Endotoxins: bacterial residues measured with a separate test (LAL). Not part of a purity analysis.
- Sterility: a microbiological test, also separate.
- Heavy metals and residual solvents: need dedicated methods.
- Other batches: a report describes the sample tested on that date. It is good evidence about a supplier's material, not a guarantee for every vial ever produced.
How to check a report is genuine
- Look for the lab's name and a verification key or QR code. Independent labs offer an online check; Janoshik, for example, at janoshik.com/verify.
- Enter the key on the lab's own website, not through a link supplied by someone else, and compare the result with the PDF: compound, amounts, purity, dates and task number should all match.
- Check that the sample on the report matches the product: the same compound and the same labelled amount.
- Look at the date. An old report is still useful, but a supplier with ongoing testing will publish new ones over time.
Warning signs
- No lab name, or a "lab" that cannot be found independently.
- No way to verify the report, or a key the lab's site does not recognise.
- Suspiciously round figures, such as exactly 100.00% purity.
- A report for a different compound, dose or company than the one being sold.
- Visible editing: mismatched fonts, misaligned numbers, a blurred lab logo.
Our reports
We publish our independent lab reports, unchanged and with their verification keys, on the lab testing page. You can check each one with the lab yourself using the steps above.
References
- Bachem. Quality control of amino acids and peptides: a guide. Knowledge center article, 2025.
- Iris Biotech. Net content and purity, two key parameters in peptide synthesis. Technical blog.
- GenScript. Examining the components of your peptide sample with AccuPep QC. Technical note.
- Snyder LR, Kirkland JJ, Dolan JW. Introduction to Modern Liquid Chromatography. 3rd ed. Wiley, 2010.