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A line-by-line guide to interpreting COAs — purity percentages, mass spec data, net peptide content, and the red flags to watch for.
A Certificate of Analysis is the document that accompanies a peptide lot describing what laboratory testing found in that specific batch. It is the primary tool researchers use to verify that what is on the label matches what is in the vial.
A good COA is specific, dated, lot-numbered, and independently verifiable. A poor COA is vague, generic, or describes a product different from the one you actually purchased. Knowing the difference matters.
A complete peptide COA typically includes the following sections. Each carries different information value.
Product name and chemical identifier. The peptide name, sometimes with CAS number, amino acid sequence, and molecular formula. This is the basic identity claim. The sequence should be present — if a COA does not list the amino acid sequence, that is unusual and worth questioning.
Lot number. Identifies the specific synthesis batch the COA describes. A COA without a lot number is meaningless, because synthesis batches vary. Always verify the lot number on the COA matches the lot number on the vial label.
Manufacturing date and analysis date. Tells you when the material was made and when the analysis was performed. Older lots are not necessarily lower quality, but you should know what you are buying.
HPLC purity percentage. The standard quality metric, expressed as a percentage of the main peak's area relative to the total chromatogram area. Research-grade peptides should be 98.0% or higher. Values below 95% indicate a substandard product.
HPLC chromatogram. The actual graph from the analysis. A reliable COA includes this image rather than just stating a number. The chromatogram should show:
Pay attention to the gradient method specified (typically an acetonitrile/water gradient with TFA modifier). Different gradients can give different purity values for the same sample — the method matters.
Mass spectrometry data. Confirms the molecular weight matches the expected sequence. The COA should report:
The measured and theoretical values should match within instrument tolerance (typically ±1 Da for standard instruments, much tighter for high-resolution MS). A discrepancy is a red flag — it suggests the sample is not what the label claims.
Peptide content or net peptide content. This is one of the most commonly overlooked numbers. A lyophilized peptide vial contains the peptide itself plus residual TFA salts (from cleavage chemistry) and bound water.
A 10 mg vial of peptide rarely contains 10 mg of pure peptide. Typical net peptide content runs 70-90% of the labeled mass, depending on the sequence and the form (TFA salt versus acetate salt versus net peptide).
For dose calculations, net peptide content is the operative number. If a research protocol calls for a specific peptide mass and your COA reports 75% net peptide content in a 10 mg vial, you actually have 7.5 mg of peptide to work with — not 10.
Water content (by Karl Fischer titration). Reports how much residual water is in the lyophilized product. Typically 3-8% for well-prepared material.
Counterion content (TFA, acetate, hydrochloride). The salt form affects solubility, stability, and biological compatibility in some research contexts.
Endotoxin testing. Important for certain research applications. Not always included on standard COAs.
Microbial testing. Less common but appears on COAs for higher-grade material.
A few patterns that suggest a COA is unreliable or the underlying product is substandard:
Generic COAs. A document that could apply to any lot of the product, with no specific lot number, date, or batch-specific data, provides no real verification. Some suppliers reuse the same COA across many lots.
Missing chromatogram or mass spec images. A claim of "98.5% purity by HPLC" without showing the actual chromatogram should be treated skeptically. The image is what verifies the claim.
Identity test failures or partial data. If HPLC purity is reported but mass spec is missing, the document tells you the sample is mostly one thing but cannot confirm what that one thing is.
Purity reported without the analytical method. Different HPLC methods (different columns, gradients, detection wavelengths) can produce different purity values. A credible COA specifies the method used.
No third-party verification. In-house COAs from the manufacturer are common and not inherently invalid, but third-party analysis from an independent laboratory carries more weight. The principle is straightforward: a third party has no financial incentive to overstate purity.
Implausible numbers. A claim of 99.9% purity on a long peptide is unusual. For shorter sequences it is achievable; for longer ones with many synthesis cycles, the realistic maximum is typically lower.
For higher-stakes research, requesting an independent COA from a laboratory you select — rather than accepting the supplier's documentation — adds verification value. Several specialty laboratories accept peptide samples for analysis at modest cost.
This approach is particularly relevant when:
A reliable peptide COA, at minimum, includes:
If most of these are present and the values are consistent, the document is doing its job. If most are absent, the document is paperwork rather than verification.
NoteThis article is intended for informational and educational purposes only. It does not constitute medical advice.
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