April 15, 2026·6 min read·primer, quality, fundamentals
Reading a peptide Certificate of Analysis — what to check
Anatomy of a COA, red flags, and how to verify identity and purity.
Anatomy of a COA, red flags, and how to verify identity and purity.
A Certificate of Analysis (COA) is the document that comes with your peptide — per-batch proof that the batch met quality specifications. It's issued by the manufacturer's QA lab or by an independent testing lab. If you know how to read it, a COA tells you whether you can trust what's in the vial. A COA audit is essential before reconstituting or using any peptide, and it's the primary way you verify what research-grade quality actually means in practice.
A COA is a quality snapshot of a specific batch on a specific date. It certifies that the peptide was tested and the results met specification. It is not a guarantee that the batch will remain stable forever, nor is it a guarantee that every vial in the batch is identical (though it should be, in principle).
A reputable vendor ships a lot-specific COA with every order. A COA that's undated, un-numbered, or generic (applies to all batches of a compound) is worthless.
Here's what to look for, section by section.
Header and batch identification: The COA should state the peptide name, the lot number (or batch number), the synthesis date, and the expiry date. Example: Lot #2026-Q2-1847, synthesized 2026-03-15, expiry 2028-03-15 (typical 2-year expiry for lyophilized peptides at –20°C).
Sequence and identity: The full amino-acid sequence, the molecular formula, and the theoretical (calculated) molecular weight. Example: Sequence: Gly-Phe-Asp-Asp-Ala-Gly-Thr-Phe-Thr-Ser-Asp-Leu-Ser-Lys-Gln-Met-Glu-Glu-Glu-Ala-Val-Arg-Leu-Ala-Gln-Asp-Val-Leu-Leu-Ser-Ala. Molecular Formula: C₁₅₉H₂₁₅N₃₇O₄₁S. Theoretical Mass: 3,566.42 Da.
Appearance: Should state "white to off-white lyophilized powder." If it says "yellow powder" or "crystalline solid," something is wrong. Any deviation from white/off-white should be a red flag.
HPLC purity: The percent purity by HPLC area (expressed as %). Research-grade should be ≥98%. The COA should show the actual chromatogram — a graph of the HPLC run — so you can see the target peak and any minor peaks (impurities). A number alone, without the chromatogram, is insufficient. HPLC is the standard tool used in how peptides are made to purify the crude synthetic product.
Mass spectrometry data: The measured molecular weight by MALDI or ESI mass spectrometry. It should match the theoretical mass within ±1 Da. Example: "Measured M+H: 3,567.45 Da (theoretical 3,567.42 Da)."
Water content (Karl Fischer titration): Percentage of water in the lyophilized powder. Typical range is 2–10%. Values above 10% suggest incomplete lyophilization; values near 0% are suspicious (usually indicates the machine was miscalibrated or the test was skipped). Karl Fischer titration is the industry standard for measuring water in lyophilized peptides.
Acetate or TFA counter-ion content: The peptide is often synthesized as an acetate salt or a trifluoroacetate (TFA) salt — these are the counterions left over from the cleavage step. The COA may state "as TFA salt" or declare the residual TFA content (e.g., "≤5% w/w"). This affects the actual mass of the peptide per milligram and is important if you're doing precise dosing.
Endotoxin (LAL assay): Bacterial endotoxin in EU/mg. Research-grade typically less than 10 EU/mg. This is critical if the peptide will be used systemically or with immunocompromised organisms.
Sterility (if applicable): Some COAs include a sterility test (usually for reconstituted solutions or pharmaceutical batches). Research-grade lyophilized peptides often skip this; the desiccated state is assumed to prevent microbial growth.
Solubility note: A brief statement about expected solubility (e.g., "soluble in water, 0.9% saline, and bacteriostatic water"). Some peptides are hydrophobic and require acidified water or organic solvents to reconstitute.
Missing mass-spec data. If the COA lists purity and appearance but no measured molecular weight, the identity was not confirmed. This is a significant gap. A purity number alone doesn't guarantee the peptide is what it claims to be.
Suspiciously round purity numbers. "99.0% purity" or "exactly 100%" are red flags. Real HPLC results have decimal variation — 98.7%, 99.3%, 99.1%. A number that's exactly 99 or 100 suggests the lab either rounded for simplicity or faked the data.
No chromatogram attached. The COA should include the actual HPLC chromatogram — a line graph showing the target peak and any minor impurities. Without it, you can't see the impurity profile or spot-check the purity number yourself.
Lot dates older than 1–2 years. If the COA is dated 2024 and you're buying in 2026, the batch was synthesized two years ago. Lyophilized peptides are stable, but older stock suggests slow inventory turnover or that the vendor is clearing old inventory.
Identity-only COAs. A COA that states "Identity confirmed by mass spec" but provides no further testing (no purity, no endotoxin, no water content) is incomplete. This often indicates the vendor did minimal QA.
Illegible or unsigned. A COA with no signature, no lab name, or no date is not verifiable. It could be a template the vendor printed without actually running the tests.
The most credible COA is issued by an independent laboratory, not the manufacturer's in-house QA. Independent labs have no incentive to inflate purity numbers. They cost more (typically $50–150 per batch for peptide testing), so reputable vendors offer it as an option or include it for premium orders.
If your vendor offers independent COA verification, pay for it. It's worth the cost for peace of mind.
Ask the vendor for the full COA before paying. If they say "it's in the box" or "we'll send it after delivery," request it upfront anyway. A vendor confident in their product will provide documentation before purchase.
If the COA arrives with missing sections (e.g., no mass-spec data, no chromatogram), ask the vendor to provide the missing information or request a refund. You have no basis to trust the product without complete documentation.