A certificate of analysis is a laboratory’s record of what it measured in a specific batch of material. It is not a quality badge, and it is not a guarantee of anything the document doesn’t explicitly state.
That distinction matters, because “third-party tested” appears on nearly every research peptide site and means very different things depending on what was tested, by whom, and whether the result relates to the vial you actually received. This is a guide to reading one properly.
What a complete COA contains
A certificate that lets you verify what you have will state all of the following. Missing items aren’t automatically disqualifying, but each absence narrows what the document tells you.
| Field | Why it matters |
|---|---|
| Product name and sequence | Confirms which peptide was analysed |
| Batch or lot number | Ties the certificate to specific material |
| Date of analysis | Establishes when the figures were true |
| Identity method and result | Confirms the material is what it claims to be |
| Purity figure and method | The headline number, and how it was obtained |
| Appearance | Baseline for comparison on receipt |
| Net peptide content, where determined | Distinguishes peptide from counter-ions and water |
| Storage conditions | The conditions the figures assume |
| Testing laboratory | Who performed the work |
The two that carry the most weight are identity and purity, and they answer different questions. Identity asks is this the right molecule? Purity asks how much of what’s in the vial is that molecule? A sample can be unambiguously identified and still be substantially impure.
Identity: mass spectrometry
Identity is normally confirmed by mass spectrometry, most often ESI-MS or MALDI-TOF.
The principle is straightforward: the instrument measures the mass-to-charge ratio of the molecule, and a peptide’s mass is determined by its amino acid sequence. If the observed mass matches the theoretical mass calculated from the stated sequence, the material is consistent with being that peptide.
What it establishes: the molecular mass matches expectation, usually to within a fraction of a dalton on modern instruments.
What it does not establish: mass alone cannot distinguish between sequences with identical composition in a different order, since they have the same mass. Nor does it quantify how much of the sample that molecule represents. For sequence-level confirmation, tandem MS (MS/MS) fragments the peptide and reads the fragment masses, which does resolve order.
A COA reporting an observed mass close to theoretical tells you the batch contains the intended peptide. It tells you nothing on its own about what else is in there.
Purity: HPLC
Purity is normally determined by reverse-phase high-performance liquid chromatography, RP-HPLC.
The sample is pushed through a column that separates components by hydrophobicity, and a detector — usually UV absorbance at 214 nm, where the peptide bond absorbs — records what comes off and when. The output is a chromatogram: a series of peaks over time.
Purity is reported as the area of the main peak as a percentage of total peak area. A figure of 99% means the target peptide accounts for 99% of the integrated UV-absorbing material detected under those conditions.
That last clause is doing real work, and it’s where the number is more limited than it appears:
It measures what the detector sees. UV at 214 nm detects peptide bonds. It does not detect residual solvents, most inorganic salts, or water. A peptide can be 99% pure by HPLC and still be a substantial fraction non-peptide by mass.
It depends on method conditions. Gradient, column chemistry, temperature and run length all affect whether two similar species resolve into separate peaks or co-elute as one. A related impurity that co-elutes with the main peak is counted as main peak. This is why a purity figure without a stated method is weaker evidence than one with it.
It is relative, not absolute. The figure describes proportion among detected material, not an absolute quantity.
None of this makes HPLC purity a bad measure — it’s the correct standard method, and a well-run 99% figure is meaningful. It means the figure answers a specific question rather than a general one.
Net peptide content: the number most COAs omit
Synthesised peptide is not shipped as pure peptide molecules alone. Lyophilised material typically contains:
- The peptide itself
- Counter-ions — commonly trifluoroacetate from HPLC purification, or acetate where the peptide has been salt-exchanged
- Residual water, which lyophilised material reabsorbs readily
The peptide fraction of total vial mass is commonly in the range of 70–90%, depending on sequence and processing. Net peptide content is determined by amino acid analysis or nitrogen determination.
This is not a defect and it is not an impurity in the ordinary sense — it’s an inherent property of how peptides are made. But it means HPLC purity and net peptide content are different numbers, and a vial labelled 10 mg at 99% purity does not contain 9.9 mg of peptide. It contains 10 mg of material, of which the peptide fraction is whatever the net content figure states.
For work where absolute quantity matters, this is the figure to look for. Many certificates don’t include it.
Sterility and endotoxin: usually not tested
Standard research peptide COAs do not typically include sterility testing or bacterial endotoxin (LAL) results. Those are separate analyses, they cost more, and they aren’t part of routine research-grade release testing.
If a certificate doesn’t mention sterility or endotoxin, the sensible reading is that they weren’t assessed — not that the material passed.
Batch-specific vs generic certificates
This is the distinction that matters most in practice, and the easiest to check.
A batch-specific COA reports analysis of the production lot your vial came from. It carries that lot number, and the lot number appears on your vial.
A generic or representative COA reports analysis of some batch of that peptide at some point. It may be entirely genuine and still tell you nothing about your material, because synthesis quality varies between runs — that’s the reason for testing each batch in the first place.
How to check: find the lot number on your vial and confirm it appears on the certificate. If the certificate has no lot number, or the number doesn’t match, the document is not evidence about what you’re holding.
Every VANTA batch is assayed independently before release and the certificate is searchable by lot number on the Certificates of Analysis page, for every compound in the catalog.
Third-party vs in-house
“Third-party tested” means the analysis was performed by a laboratory independent of the supplier. In-house testing isn’t inherently unreliable, but it lacks the independence that makes the result verifiable by someone with no stake in the outcome.
Useful things to check:
- Is the testing laboratory named? “Third-party tested” without a named lab is an unverifiable claim.
- Is the laboratory accredited? ISO/IEC 17025 is the relevant standard for testing competence.
- Does the certificate come from the lab, or has it been retyped? A document on the testing laboratory’s letterhead with their contact details is stronger evidence than a supplier-branded summary of results.
A short checklist
When you receive material, in order:
- Find the lot number on the vial
- Locate the certificate carrying that lot number
- Confirm the identity method and that observed mass matches theoretical
- Confirm the purity figure and that the method is stated
- Check whether net peptide content is reported
- Check the analysis date and the named testing laboratory
- Compare stated appearance against the vial in front of you
Anything that doesn’t reconcile is worth raising with the supplier before use.
References
- Mant CT, Chen Y, Yan Z, Popa TV, Kovacs JM, Mills JB, Tripet BP, Hodges RS. HPLC analysis and purification of peptides. Methods in Molecular Biology, 2007;386:3–55. doi:10.1007/978-1-59745-430-8_1
- Domon B, Aebersold R. Mass spectrometry and protein analysis. Science, 2006;312(5771):212–217. doi:10.1126/science.1124619
- Rutherfurd SM, Gilani GS. Amino acid analysis. Current Protocols in Protein Science, 2009;Chapter 11:Unit 11.9. doi:10.1002/0471140864.ps1109s58
- Roux S, Zékri E, Rousseau B, Paci M, Mory C, Fournier-Zaluski MC. Structural and biological effects of trifluoroacetate counter-ion in peptide preparations. Journal of Peptide Science, 2008;14(3):354–359. doi:10.1002/psc.951
Related reading
- Understanding peptide purity: HPLC, MS, and what 99% means
- Reconstituting and storing lyophilised peptides
- Certificates of Analysis
All VANTA products are supplied for laboratory research use only. They are not for human or veterinary use, not for use in diagnostic procedures, and have not been evaluated by the U.S. Food and Drug Administration.