A Certificate of Analysis (COA) is the lab report for a specific production batch of a peptide. It is the single most important document for judging quality, and a credible supplier ties each COA to the exact batch you receive — not a generic sample from years ago.
This guide walks through every field on a typical peptide COA and tells you what “good” looks like.
The 30-second check
If you only have half a minute, verify three things:
- Purity is ≥99% by HPLC, shown as a single dominant peak.
- Identity is confirmed by mass spectrometry (MS), with the observed mass matching the theoretical mass.
- The batch/lot number on the COA matches the number printed on your vial.
If all three line up, the certificate is doing its job. Below is what each line actually means.
Field-by-field
Batch / lot number
A unique code identifying the production run. It should match your vial label and the COA published by the supplier. A mismatch — or a “one COA for every batch” document — is a red flag.
Purity (HPLC)
High-Performance Liquid Chromatography separates the sample into its components and reports the percentage of the target peptide versus impurities. Look for ≥99% and a chromatogram with one tall, sharp peak and minimal “shoulders” or extra peaks.
Identity (mass spectrometry)
HPLC tells you *how pure*; MS tells you *what it is*. Mass spectrometry measures the molecular mass and confirms the molecule is the peptide named on the label. The report should state that the observed mass equals the theoretical (expected) mass.
Net peptide content
A lyophilized vial is not 100% peptide — it also contains bound salt and residual water. Net peptide content (often 80–95%) states how much of the vial mass is actual peptide. A transparent COA reports it rather than hiding it.
Counter-ion (acetate vs. TFA)
Peptides are supplied as a salt. The counter-ion is usually acetate or trifluoroacetate (TFA). Acetate is generally preferred for many applications; either way, a good COA discloses which one is present, because it affects net peptide content and some assays.
Water content (Karl Fischer)
Karl Fischer titration measures residual moisture in the freeze-dried powder. Lower is better for long-term stability; the figure should be reported, typically as a low single-digit percentage.
Endotoxin / appearance / other parameters
Depending on the product, the COA may also list endotoxin levels, appearance (e.g., white lyophilized powder), and the analytical methods used. More disclosure is better.
How to verify a COA is real
- Cross-check the batch number against the vial and the supplier’s published certificate.
- Look for the lab’s name and methods, not just a headline number.
- Prefer third-party, accredited labs over in-house-only figures, which can’t be independently confirmed.
Key takeaways
- A COA is batch-specific; treat generic or missing COAs with suspicion.
- HPLC = purity; MS = identity. You want both.
- Net peptide content, counter-ion and water content separate a transparent supplier from a vague one.
FAQ
What’s the difference between purity and net peptide content? Purity (HPLC) is the percentage of the *target peptide among peptide-related species*. Net peptide content is the percentage of the *vial mass* that is actually peptide (the rest is salt and water). Both matter.
Is a higher purity number always better? A higher, independently verified HPLC purity is better — but only if identity (MS) is also confirmed and the COA is tied to your batch. A purity number with no identity test or no batch link is incomplete.
Why does the counter-ion matter? It changes how much of the powder is peptide and can interfere with certain assays. Acetate is commonly preferred; the key is that the COA discloses it.
Browse verified research peptides
Every batch we ship is independently tested by Janoshik Analytical — HPLC for purity, mass spectrometry for identity — with a batch-matched Certificate of Analysis you can verify yourself. Browse the catalog → · See the latest COAs →
*For laboratory and research use only. Not for human or veterinary use, consumption, or administration. Nothing in this article is medical advice or a claim that any compound treats, cures, or prevents any condition.*