How to Read a Peptide Certificate of Analysis (COA)

A Certificate of Analysis (COA) is the lab report that supposedly documents what is in a vial. For research compounds sold outside the pharmaceutical supply chain, the COA is often the only quality data a buyer ever sees — which makes reading one critically a basic literacy skill, not a formality. The catch is that a COA is only as trustworthy as the lab that produced it and the methods it actually ran. This is a guide to interpreting the document for research use only; none of it is guidance for human use.

Who issued it, and how the tests work

Start with provenance. A COA written by the manufacturer testing its own product is a self-report. A COA from an independent, accredited third-party lab is a check on that self-report. The two most important assays are complementary and neither substitutes for the other.

HPLC (usually reverse-phase) measures purity. As peptide manufacturer Bachem describes it, analytical HPLC detects the compound by UV absorbance at 210–220 nm and reports the main peak’s area as a percentage of total peak area — so “98%” means the target peak is 98% of what the detector saw. Mass spectrometry confirms identity: in the 2023 Pharmaceutical Research review of synthetic-peptide quality control, McCarthy and colleagues note that electrospray LC–MS/MS “was used to confirm the mass of the peptide and to verify the amino acid sequence.” Purity tells you how clean the sample is; mass spec tells you whether it is even the right molecule.

A high purity number is meaningless without an identity result — a vial can be 99% pure and still be 99% the wrong compound.

Beyond purity and identity

A thorough COA carries several more line items, each answering a different question:

Line item	What it tells you	Typical method
Net peptide content	How much is actually peptide vs. salt and water	Amino acid analysis
Water content	Moisture bound in the powder	Karl Fischer titration
Counterion (acetate / TFA)	Which salt form, and residual TFA	HPLC / ion chromatography
Endotoxin / sterility	Bacterial contamination risk in solution	LAL assay

Net peptide content matters because gross powder weight is not peptide weight. Bachem defines it as “the percentage of peptides relative to non-peptidic material (mostly counterions and moisture),” and for basic peptides that form salts it can be well below the gross figure — often only 60–80%. A vial labeled 10 mg may hold noticeably less actual peptide.

Why the details matter

Impurities are not a rounding error. The FDA’s 2021 guidance on synthetic peptide ANDAs recommends that manufacturers identify every peptide impurity present above 0.10% of the active ingredient, precisely because peptide-related impurities can carry immunogenicity risk. A COA that reports a bare purity percentage with no impurity profile, no method, and no batch traceability is skipping the part regulators treat as most important.

Red flags that should lower your confidence in a COA:

• No batch or lot number — the report can’t be tied to your vial.
• No methods listed — “98% pure” with no HPLC conditions is an assertion, not data.
• Manufacturer-only, no third-party — self-reported with nothing independent to check it.
• Compound mismatch — the tested molecular weight doesn’t match the labeled peptide.
• A stale or missing date — a COA describes one batch at one time, not the vial in front of you.

The takeaway

Read a COA as a document with an author and a chain of custody, not a seal of approval. Confirm who issued it, look for both an HPLC purity result and a mass-spec identity result, check for net content, counterion, and contamination line items, and treat missing batch numbers, missing methods, and manufacturer-only reports as reasons for doubt. A COA that survives this scrutiny is informative; one that doesn’t tells you little. This is research literacy for research-use-only materials, not advice for human consumption.

Sources

• Quality Control of Amino Acids & Peptides: A Guide, Bachem
• Reference Standards to Support Quality of Synthetic Peptide Therapeutics, McCarthy et al., Pharmaceutical Research, 2023 (PMC10338602)
• Assessing the immunogenicity risk of salmon calcitonin peptide impurities (citing FDA 2021 ANDA peptide guidance), PMC11341359