A Certificate of Analysis is only useful if you know what each section actually confirms. Here is how to read the HPLC trace and mass spectrum line by line.
A Certificate of Analysis (COA) is the single most important document accompanying any research peptide, yet many researchers skim past the numbers without understanding what each section actually verifies. This reference walks through a typical COA section by section so you can judge whether a batch meets your lab's standard before it goes anywhere near an experiment.
Section 1: Identification and batch metadata
Every COA should open with the peptide name, lot or batch number, synthesis date, and net peptide content. The lot number is what ties the document to the specific vial in front of you — if a supplier cannot produce a lot-specific COA on request, treat that as a red flag rather than an oversight.
Section 2: The HPLC purity trace
High-performance liquid chromatography (HPLC) separates the peptide from synthesis by-products such as truncated or deletion sequences. On the chromatogram, look for a single dominant peak that accounts for the reported purity percentage — typically ≥98%. Smaller shoulder peaks represent residual impurities; a cluttered trace with multiple significant peaks close to the main one suggests incomplete purification.
Section 3: LC-MS mass confirmation
Liquid chromatography-mass spectrometry (LC-MS) confirms molecular identity rather than purity. The reported mass should sit within roughly ±0.5 Da of the theoretical monoisotopic or average mass for the sequence. A mass mismatch — even alongside a clean HPLC trace — indicates the material may not be the intended peptide at all, which is why the two tests are always reported together, never substituted for one another.
Section 4: Counterion and salt form
Most synthetic peptides are supplied as a salt, commonly acetate or trifluoroacetate (TFA), left over from the purification solvent system. The COA should disclose which counterion is present and its approximate mass contribution, since this affects the true net peptide content of a vial versus its gross weight.
| COA section | What it confirms | Common warning sign |
|---|---|---|
| Batch metadata | Traceability to a specific production lot | Generic spec sheet with no lot number |
| HPLC trace | Purity, absence of major synthesis by-products | Multiple significant secondary peaks |
| LC-MS spectrum | Correct molecular identity | Mass outside ±0.5 Da tolerance |
| Counterion report | True net peptide content | No mention of salt form at all |
Section 5: Third-party verification
Ideally, the COA is issued by an independent laboratory rather than the manufacturer's own quality-control department. Third-party verification removes the conflict of interest inherent in a supplier grading its own material, and is the standard most serious New Zealand research buyers now expect as default.
A quick reference checklist
- Batch identifier and synthesis date traceable to the lot record
- HPLC purity ≥98% (typically ≥99% for peptides under 30 residues)
- LC-MS confirmed monoisotopic or average mass within ±0.5 Da of theoretical
- Counterion identity and content (acetate or trifluoroacetate) reported
- Independent third-party laboratory verification, not an in-house certificate
Frequently asked questions
What HPLC purity is considered acceptable for research use?
Most research applications expect ≥98% purity, with many single-peak short peptides reporting ≥99%.
Can a peptide pass HPLC purity but fail LC-MS identity?
Yes — a clean chromatogram only tells you the sample is homogeneous, not that it is the correct sequence, which is why mass spectrometry is a separate, mandatory check.
Why does the counterion matter for research calculations?
The counterion adds mass to the vial without contributing active peptide, so ignoring it can lead to overestimating the actual peptide concentration in a reconstituted solution.
