What a COA is for
A certificate of analysis is the primary quality document for a research peptide. It answers two questions: is the material in this vial the compound it claims to be, and how much of what is in the vial is that compound rather than something else. Without a batch-specific COA, neither question has an answer, and any result generated with the material is difficult to defend.
The elements to check
A usable COA identifies the specific lot and reports analytical results for that lot. Look for:
- Product name and sequence. The full amino acid sequence should be stated, not just a trade name.
- Lot or batch number. This must match the number on the vial you received.
- Date of analysis. An undated COA tells you nothing about when the material was tested.
- HPLC purity result, with the chromatogram.
- Mass spectrometry result, confirming observed versus theoretical molecular weight.
- Peptide content, distinct from purity.
- Testing laboratory identity. Independent third-party testing is materially more informative than in-house-only results.
- Appearance and solubility notes.
Reading the HPLC chromatogram
High-performance liquid chromatography separates the components of a sample; the resulting chart plots detector response against retention time. The target peptide appears as the dominant peak.
Purity is calculated as the area of the main peak as a percentage of total peak area. What to look at:
- The main peak percentage. For research peptides, 98 percent or above is a reasonable expectation. Below 95 percent, ask what the remaining material is.
- The shape of the main peak. A sharp, symmetrical peak is what you want. Broad or shouldered peaks can indicate co-eluting related substances.
- Impurity peaks. Small peaks near the main peak often represent sequence-related impurities such as deletion sequences. Peaks far from the main peak may represent unrelated contaminants.
- The baseline. A noisy or drifting baseline makes integration less reliable.
Reading the mass spectrometry data
Mass spectrometry answers the identity question. The report gives an observed molecular weight, which should match the theoretical weight calculated from the stated sequence, typically within a fraction of a Dalton for the monoisotopic mass.
A mismatch is significant. A difference of around 18 Da can indicate a hydrolysis product; a difference of 16 Da often points to oxidation. If observed and theoretical masses do not agree and the discrepancy is unexplained, the identity of the material is not established regardless of what the HPLC purity says — purity and identity are separate questions.
Purity is not peptide content
This distinction is the one most often missed. Purity describes what fraction of the peptide material is the target sequence. Peptide content describes what fraction of the vial's total mass is peptide at all, with the remainder being residual water, salts from synthesis, and counter-ions such as trifluoroacetate.
A vial can be 99 percent pure and still be only 80 percent peptide by mass. For a nominal 5 mg vial, that is approximately 4 mg of actual peptide. Work that depends on accurate concentration must use peptide content, not the nominal label weight.
Warning signs
Treat the following as reasons to ask questions:
- A COA with no lot number, or a lot number that does not match the vial
- No chromatogram or spectrum image — results stated as bare numbers only
- The same document reused across multiple different lots
- No testing laboratory named
- Purity claims stated without any supporting analytical method
- A supplier unwilling to provide the COA before purchase
A supplier that tests properly has no reason to withhold the documentation. Being able to review a COA before ordering is a reasonable expectation.
Research use only. This content is for educational and informational purposes related to scientific research. It is not medical advice and does not describe dosing or administration for human use. Products are sold for laboratory research by qualified professionals only.