A Certificate of Analysis — the COA — is the single most useful document a peptide supplier can give you, and one of the least understood. It is the lab report for a specific batch of material: not marketing, not the datasheet for the molecule in general, but the measured result for the vials that came from one production run.
For research use, the label on a vial is a claim; the COA is the evidence. This guide walks through what each section means and how to sanity-check one before you commit to a purchase.
Why the batch matters
Two vials of the same peptide, from the same supplier, can come from different synthesis batches with different purity. A COA is only meaningful when it is tied to the lot number printed on the vial you actually received. A certificate that doesn’t name a batch — or names a batch that doesn’t match your vial — tells you nothing about your material.
The first check, then, is trivial but essential: does the lot on the COA match the lot on the vial?
HPLC purity — the number everyone quotes
High-Performance Liquid Chromatography (HPLC) separates the contents of a sample by how quickly each component moves through a column. The output is a chromatogram: a plot with peaks, where the target peptide should be one dominant peak and impurities show up as smaller ones.
Purity is reported as the area of the main peak as a percentage of all peaks — e.g. “99.2% by HPLC.” Two things matter here:
- The figure itself. For research peptides, 98%+ is the usual expectation.
- The chromatogram behind it. A purity number with no trace is just an assertion. A real COA shows the chromatogram so you can see whether that 99% is one clean peak or a main peak flanked by several sizeable impurities.
Mass spectrometry — identity, not just purity
HPLC tells you how pure the sample is; it does not, on its own, tell you the peak is the right molecule. That is what mass spectrometry (MS) is for.
MS measures the molecular weight of the compound. The COA should state the expected (theoretical) mass for the peptide and the observed mass from the instrument. If a peptide’s calculated monoisotopic mass is 1419.5 Da and the COA reports an observed mass matching that, the identity is confirmed. A mismatch — or a missing MS section entirely — means purity numbers are being reported for a molecule whose identity was never verified.
Net peptide content — the number few people check
This is the section most buyers skip. A lyophilised peptide vial does not contain 100% peptide by weight. The powder also contains bound water, counter-ions from synthesis (commonly acetate or trifluoroacetate/TFA), and residual salts. Net peptide content is the fraction of the powder’s mass that is actually peptide.
It changes what “5 mg” means:
| COA line | Example value | What it tells you |
|---|---|---|
| Gross / labelled mass | 5 mg | The powder mass in the vial |
| Net peptide content | 82% | Fraction that is actually peptide |
| Effective peptide mass | 4.1 mg | Gross × net content |
| Counter-ion | Acetate | Which salt form was produced |
Two vials both labelled “5 mg” can differ by 15–20% in actual peptide once net content is accounted for. For any research where quantity is part of the record, this is the number that makes results reproducible.
The other tests you may see
Depending on the material and the supplier, a COA can also report:
- Water content (often by Karl Fischer titration) — residual moisture in the lyophilised powder.
- Counter-ion / residual solvent — acetate vs TFA salt form, and any leftover synthesis solvents.
- Appearance — a plain description (e.g. “white to off-white powder”) that should match what you received.
Not every test applies to every product, and their absence isn’t automatically a problem. The two that should essentially always be present are HPLC and MS.
A quick red-flag checklist
Before you buy, a COA is worth a second look if any of these are true:
- No batch/lot number, or one that doesn’t match the vial.
- A headline purity figure with no chromatogram behind it.
- No mass-spectrometry section — purity without confirmed identity.
- The document is undated or reused across obviously different products.
- Net peptide content is omitted on a quantity-sensitive product.
None of these on its own proves anything is wrong, but each one is a reason to ask the supplier for the underlying data. A supplier that tests properly will have it and will share it.
The bottom line
Read a COA in four moves: confirm the batch matches, read the HPLC purity and look at the trace, check that MS confirms identity, and — for anything quantity-sensitive — read the net peptide content. A supplier that publishes complete, per-batch certificates is giving you the tools to verify their work, which is exactly the point.