Guide

How to Read a Peptide Certificate of Analysis (COA)

A Certificate of Analysis is the independent lab report that states what a research material actually is, how pure it is, and what else is in the vial. It is the single most important document in this market — and the one our entire supplier ranking is built on. This guide explains every section, in plain terms.

Why a COA matters

Peptides are not commodities. Two vials carrying the same compound name can differ substantially in purity, salt form, moisture content and synthesis byproducts. In a research setting those differences are not cosmetic — an impure or misidentified material can invalidate an entire experimental series. Independent third-party testing exists to close that gap: when a laboratory with no financial interest in the sale analyzes a batch and publishes the result, researchers get data instead of a label claim. A COA that names its lab, its methods and its batch is the baseline of a trustworthy supply chain.

The two non-negotiables: HPLC-UV purity and LC-MS identity

Nearly every credible COA rests on two analytical pillars. If either is missing, the document is incomplete.

1 · HPLC-UV — how pure is it?

High-performance liquid chromatography with UV detection (HPLC-UV) separates the components of a sample and measures how much of the total signal comes from the target peptide versus everything else.

• Purity by peak area % — the headline number. Reputable research-grade material typically reports ≥95%, with many batches at 98–99%+.
• The chromatogram itself — a good COA includes the actual trace. One dominant peak with minimal shoulders is what you want; multiple significant peaks mean impurities, truncated sequences or degradation products.
• Column and gradient conditions — documented so the result is reproducible, not just asserted.

HPLC-UV tells you how much of the sample is one thing. It cannot confirm that the thing is the right peptide.

2 · LC-MS — is it the right molecule?

Liquid chromatography–mass spectrometry (LC-MS) identifies the molecule by mass: the chromatograph separates components, and the mass spectrometer measures the mass-to-charge ratio of what comes out.

• Observed vs. theoretical molecular weight — the COA should state the peptide's expected mass and the mass actually measured; they should agree within the instrument's stated tolerance.
• Charge-state data — peptides typically appear as multiply charged ions; the report may show the deconvoluted mass.
• Method identification — instrument type and ionization mode should be stated.

Together, the two tests answer the only two questions that matter: how pure is it, and is it what it claims to be? A supplier publishing both, per batch, from a named independent lab is doing the minimum a researcher should accept — it is also the first criterion in our scoring methodology.

Net peptide content vs. gross weight

A vial labeled "10 mg" contains 10 mg of lyophilized powder — and that powder is not 100% peptide. It also contains counter-ions from the salt form (commonly acetate or trifluoroacetate), residual moisture, and trace residual solvents. Net peptide content — the actual mass of peptide — is routinely 70–90% of gross weight. A COA that reports net content (often by nitrogen or amino acid analysis) is giving you the honest number; a supplier who only ever quotes gross weight may be shipping less material than the label suggests. Researchers doing quantitative work should calculate from net content.

What else a thorough COA checks

• Appearance — normally a white to off-white lyophilized powder or cake. Discoloration or a collapsed, gummy residue can indicate degradation or moisture intrusion.
• Endotoxins (LAL test) — bacterial endotoxins, measured in Endotoxin Units (EU). High levels can confound cell-based assays; a reported value, or a stated limit the batch passed, marks a serious testing program.
• Residual solvents — acetonitrile, DMF, ether and others from synthesis and purification, reported against accepted limits.
• Moisture content (Karl Fischer) — excess water accelerates degradation; reported as a percentage.
• Amino acid analysis — confirms the amino acid composition matches the expected sequence; also used to determine net content.
• Microbial / sterility testing — more common for materials destined for sensitive cell-culture work.

Not every COA includes all of these — but the more that appear with measured values, rather than a bare "pass," the stronger the document.

Lot matching: the step most buyers skip

A COA certifies one batch, identified by its lot or batch number. Before relying on any certificate, match that number to the label on the vial in your hand — they must be identical. Then check the test date against the batch's age: recent material with a recent certificate is the ideal pairing. Suppliers who post the current lot's COA on the product page (and keep prior lots visible) make this check possible before purchase; suppliers who publish one undated sample certificate make it impossible. This is exactly why "COA access" carries 25% of the score in our supplier ranking.

Red flags on a COA

1. No named testing laboratory. No lab identity, no accreditation reference, no contact — the document is unverifiable.
2. No batch or lot number. A certificate that can't be tied to your vial certifies nothing about your material.
3. No methods. "Purity: 99%" with no HPLC conditions, column or detection method is a claim, not a result.
4. No chromatogram or spectrum. Numbers without the underlying traces cannot be independently assessed.
5. Purity without identity. A 99% pure unknown is still an unknown.
6. Template reuse. Identical chromatograms, values or dates across different compounds or batches suggest a copied document.
7. No date. Testing is a snapshot in time; an undated COA has no validity window.
8. Self-testing only. In-house results are better than nothing, but an independent lab removes the conflict of interest.

Frequently asked questions

What does ≥98% purity actually mean?

That ≥98% of the HPLC-UV chromatographic peak area is attributable to the target peptide; the remainder is impurities such as truncated sequences or synthesis byproducts. Higher is generally better for reproducibility — but purity alone never confirms identity. That requires mass spectrometry.

Why are both HPLC and mass spec needed?

HPLC-UV measures how much of the sample is a single component; LC-MS confirms what that component is. Either alone leaves a gap: purity without identity can be a pure wrong molecule, and identity without purity can be the right molecule carrying contaminants.

Should a COA come from a third-party lab?

Yes. Independent testing removes the conflict of interest in supplier self-testing, and the lab should be named so its independence can be checked.

How often should a batch be retested?

A COA reflects the batch at the time of testing. Peptides degrade with age and poor storage, so weigh the certificate's date and the material's storage history for critical work.

References: USP General Chapter <621> (chromatography); USP General Chapter <85> (bacterial endotoxins test); Snyder, Kirkland & Dolan, Introduction to Modern Liquid Chromatography, 3rd ed.; Gross, Mass Spectrometry: A Textbook. Guide last updated October 2026. All materials discussed are for laboratory research use only.