By BioLabs Research · Last reviewed: October 10, 2026
A purity number on a product page is not a lot measurement. A certificate of analysis (COA) is lot paper: it records what was measured for one batch, by which method, on which date. This guide explains the fields a buyer should expect on a peptide COA, what each one can and cannot tell you, and the warning signs that a certificate is not doing its job.
What a COA should identify before any result
Every result on a certificate depends on four header fields. Without them the numbers below cannot be tied to a vial:
- Lot number, matching the label on the vial or carton;
- Product name and sequence, so the certificate describes a specific molecule;
- Test date, so the result can be placed in time;
- Testing laboratory, named and identifiable.
If a certificate is described as externally tested or accredited, check that the named laboratory is separate from the manufacturer and the seller, and look up any cited accreditation in the accreditation body's public directory.
HPLC purity: what area percent means
Reversed-phase HPLC with UV detection is the usual purity method for synthetic peptides. Detection is commonly set in the 210–220 nm range, where the peptide bond absorbs. The result is reported as area percent: the main peak's share of all UV-absorbing signal under that method.
A useful HPLC section shows the chromatogram itself, the column, the mobile phase or method ID, the detection wavelength and the integration approach. A purity figure without those details cannot be compared with anything.
Area percent has limits. It does not see water, counter-ions or residual salts, so it is not the same as net peptide content. See HPLC area percent vs net peptide content. And an impurity that co-elutes with the main peak is counted as product.
Mass spectrometry: identity, not purity
Mass spectrometry answers a different question: is the main component the molecule on the label? The certificate should report the observed mass next to the theoretical mass for the stated sequence, with the ionization method (typically electrospray or MALDI). HPLC alone cannot confirm identity, because two different peptides can elute at similar times.
Impurities that matter
Reviews of peptide impurities list the common by-products of synthesis and storage: deletion and insertion sequences, truncations, incomplete removal of protecting groups, oxidation, deamidation and changes in stereochemistry (D'Hondt et al., 2014). Some of these differ from the target by only a few daltons.
They are not just a paperwork issue. A study of commercial synthetic peptides found impurities that changed downstream assay results (Currier et al., 2008). That is one reason a certificate with a chromatogram and an assigned mass spectrum is worth more than a single purity percentage.
Counter-ions, water and residual solvents
Peptides purified by reversed-phase HPLC often carry trifluoroacetate (TFA) as a counter-ion. Residual TFA has been reported to affect some cell-based assays (Cornish et al., 1999). A useful certificate states the counter-ion and, where relevant, the water content, often measured by Karl Fischer titration. Together these explain the gap between gross weight and net peptide content.
Endotoxin, for cell-culture work
If material is meant for cell culture, ask whether an endotoxin result is on file for that lot, reported per milligram with the method named. United States Pharmacopeia chapter <85> describes gel-clot, photometric and kinetic methods for bacterial endotoxins testing, and FDA guidance discusses how those methods are applied (FDA, 2026). An endotoxin line without a method or units cannot be compared across lots.
Red flags on a peptide COA
- No laboratory name. The certificate cannot be traced to whoever ran the test.
- No lot number. There is no way to know which batch was tested.
- No method. A purity figure without column and wavelength is an orphan number.
- No observed mass. Identity is asserted, not shown.
- Identical certificates across lots. Matching values and dates on different batches suggest a template, not a test.
- Missing dates. Without a test date, there is no way to place the result in time.
Common misreadings
- Reading purity as content. A 98% area figure does not mean 98% of the powder's weight is peptide; water and counter-ions are not in that number.
- Reading one certificate as a product property. Results belong to the lot that was tested, not to every vial sold under the same name.
- Reading a mass match as purity. A correct observed mass shows the main component is right. It says nothing about how much of the sample it makes up.
Checklist: what to verify
- Lot number matches the vial label.
- Sequence or structure matches the order.
- HPLC chromatogram shown, with column, wavelength and method.
- Observed mass reported next to the theoretical mass.
- Counter-ion and water content stated where net content matters.
- Endotoxin result with method and units, for cell-culture use.
- Test date and testing laboratory named.
Getting lot documents from BioLabs Research
Lot documentation for catalog listings is available on request. Use the request on the product page or contact the desk with the product name and the lot number from the vial. Hosted certificates are listed in our COA library, and our guide on how to read lot documents walks through each field in order.
Related reading
References
- D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2–30. https://pubmed.ncbi.nlm.nih.gov/25044089/
- Currier JR, Galley LM, Wenschuh H, et al. Peptide impurities in commercial synthetic peptides and their implications for vaccine trial assessment. Clin Vaccine Immunol. 2008;15(2):267–276. https://pubmed.ncbi.nlm.nih.gov/18077621/
- Cornish J, Callon KE, Lin CQ, et al. Trifluoroacetate, a contaminant in purified proteins, inhibits proliferation of osteoblasts and chondrocytes. Am J Physiol. 1999;277(5):E779–E783. https://pubmed.ncbi.nlm.nih.gov/10567002/
- US Food and Drug Administration. Pyrogen and Endotoxins Testing: Questions and Answers. Guidance for Industry. March 2026. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/pyrogen-and-endotoxins-testing-questions-and-answers
For research use only. Not for human or veterinary use.