A certificate of analysis is the single most cited document in the research-peptide market, and one of the least understood. Vendors wave it around as proof of quality. Buyers file it away without reading past the percentage at the top. Both are missing the point. A COA is a data sheet, and like any data sheet it rewards people who know which numbers matter and which are decoration.
Here is how to actually read one.
HPLC purity is a relative number, not an absolute one
The headline figure on almost every peptide COA is HPLC purity, usually expressed as something like "≥99%." High-performance liquid chromatography separates the contents of a sample by how the molecules interact with a column, and the purity figure tells you what fraction of the detected material is your target peptide versus everything else that showed up.
The catch: HPLC only reports what it can see. Salts, residual solvents, and water of hydration often do not register on a standard UV detector. So a "99% pure" peptide can still be a meaningful fraction acetate salt and bound water by actual mass. That is not fraud, it is just the limit of the method. It is also why HPLC purity and net peptide content are two different numbers, and why a serious COA reports both.
Mass spec is the identity check
Purity tells you how clean the sample is. It does not tell you that the clean part is the right molecule. That is what mass spectrometry is for. The COA should list an observed molecular mass that matches the theoretical mass of the peptide's sequence.
If a COA shows a beautiful purity number but no mass-spec confirmation, you have a clean sample of something unverified. For a multi-component blend, the mass-spec section matters even more, because it is the only way to confirm that each component is present and intact rather than degraded.
Net peptide content is the number nobody quotes
Net peptide content is the percentage of the vial's contents that is actually peptide, after you subtract counter-ions, water, and other non-peptide mass. It is frequently lower than the HPLC purity figure, and it is the number that determines how much active compound is genuinely in the vial.
A COA that reports HPLC purity, mass-spec identity, and net peptide content together is giving you the full picture. One that reports only the first is giving you a marketing number.
What a complete COA includes
When you evaluate a research compound, look for all of the following on the documentation:
- Product identity — name, sequence, and molecular formula
- HPLC chromatogram — not just the percentage, the actual trace
- Mass-spec data — observed mass versus theoretical mass
- Net peptide content — the real active fraction
- Lot or batch number — so the document maps to your specific vial
- Test date and method references
A percentage with no chromatogram, no mass spec, and no lot number is a claim, not a certificate.
Why this matters for research
In a research setting, the entire value of a compound depends on knowing what is in the vial. Irreproducible results often trace back not to the experiment but to the material: a degraded peptide, a mislabeled mass, or a net content far below the assumed amount. Reading the COA properly is the cheapest quality-control step available, and it happens before the compound ever leaves the freezer.
This article is educational and intended for the laboratory research community. Trulogic Labs products are sold for laboratory and research use only and are not for human consumption. See our Quality & Testing page for our documentation standards.