How to Read a Peptide Certificate of Analysis (COA)
A Certificate of Analysis (COA) is a lab report for one specific batch of a product. A good one tells you that the vial contains the right molecule, how pure it is, and which contaminants were tested for. Here is how to read each part, and what a COA does not tell you.
1. Match the COA to your vial
A COA only describes the batch it was run on. Check that:
- The lot or batch number on the COA matches the number printed on your vial. A COA without a lot number cannot be tied to anything you bought.
- The test date is recent relative to the batch, and the compound name and stated amount (for example 10 mg) match the label.
- The testing lab is named, and ideally the report can be verified on the lab's own site or by a QR code. Independent third-party labs are more credible than a vendor's in-house testing.
Red flags: the same COA image reused across many batches or products, missing lab details, cropped or edited reports, and "representative" COAs that are not lot-specific.
2. Identity: mass spectrometry
Mass spectrometry (MS or LC-MS) confirms the vial contains the intended molecule by measuring its molecular weight. The report should list the theoretical mass of the peptide and the observed mass, which should agree closely (typically within about 1 dalton for small peptides). HPLC alone cannot prove identity: a wrong peptide can still produce a single clean peak.
3. Purity: HPLC
High-performance liquid chromatography (HPLC) separates the sample into components and reports the main peak as a percentage of everything detected, for example 99.1%. Keep in mind:
- HPLC purity is relative. It compares the target peptide with related impurities (truncated or modified sequences) that absorb UV light at the measured wavelength.
- It is not the same as net peptide content. Lyophilized peptides also contain water and counter-ions such as acetate or TFA, so a vial with 99% HPLC purity may be only 70-90% peptide by weight.
- Look for the chromatogram itself, not just a number. One dominant peak with small, labelled minor peaks is what you want to see.
4. Quantity per vial
Some labs also measure how much peptide is actually in the vial (quantitative HPLC or mass balance). This catches underfilled vials, which purity testing alone will not. If a COA reports only purity, the stated milligrams are the vendor's claim.
5. Endotoxin (LAL test)
Endotoxins are fragments of the outer membrane of gram-negative bacteria (lipopolysaccharide). They can be present even when no living bacteria remain, they survive filtration and freeze-drying, and they can cause fever and inflammation if introduced into the body. They are measured with the Limulus amebocyte lysate (LAL) test and reported in endotoxin units (EU) per milligram.
Many research-peptide vendors do not test for endotoxin at all, and a high HPLC purity says nothing about it. If endotoxin matters for your research, ask the vendor directly whether they test each lot and for the numeric result. Do not assume a COA covers it unless an endotoxin result is printed on it.
6. Sterility
Sterility testing (for example USP <71>) checks for living microorganisms. Pharmaceutical injectables must pass it; research-grade lyophilized peptides generally are not sold as sterile products and usually are not tested. A missing sterility result is normal for research chemicals, but it means sterility is not established.
7. Heavy metals and residual solvents
Some labs screen for heavy metals such as lead, arsenic, cadmium and mercury (usually by ICP-MS) and for solvents left over from synthesis (usually by gas chromatography). These tests are less common on research-peptide COAs; when present, results should be below stated limits.
Quick checklist
| Check | What good looks like |
|---|---|
| Lot match | COA lot number matches your vial |
| Lab | Named, independent lab; verifiable report |
| Identity (MS) | Observed mass matches theoretical mass |
| Purity (HPLC) | Chromatogram shown; main peak typically 98% or higher |
| Quantity | Measured amount close to the labelled mg |
| Endotoxin (LAL) | Numeric EU/mg result, if tested; ask if absent |
| Sterility | Pass, if tested; research peptides often are not |
This guide explains lab reports. It is not medical advice and does not endorse using research chemicals in humans.
FAQ
What does 99% purity on a peptide COA mean?
It usually means the target peptide made up 99% of the UV-absorbing material detected by HPLC. It does not measure water, salts or counter-ions, and it does not test for endotoxin or sterility.
Does a COA prove a peptide is free of endotoxin?
Only if the COA includes an endotoxin (LAL) result. Many research-peptide COAs report only HPLC purity and mass spec identity, so ask the vendor whether each lot is tested for endotoxin.
Why should the lot number on the COA match my vial?
A COA describes one batch. Without a matching lot number there is no way to know the report applies to the product you have.