Reading a Certificate of Analysis: Why COAs Matter

May 28, 2026

Peptide Basics

A Certificate of Analysis is the single most important document attached to a research compound, and also the one most frequently skimmed rather than read. This article covers what each section actually verifies, what it does not, and how to catch the difference between a COA that supports a claim and one that merely accompanies it.

Quick summary

A COA is lot-specific. It covers the exact batch it was tested on — nothing more.
Identity (mass spectrometry) and purity (HPLC) are two separate tests answering two separate questions.
High HPLC purity does not rule out incorrect sequence, wrong stereochemistry, or endotoxin contamination — those need separate tests.
“Net peptide content” and gross vial weight are different numbers; a COA that reports only one is telling you less than one that reports both.
Third-party (independent lab) testing carries more weight than in-house testing, for an obvious conflict-of-interest reason.
Always match the lot number on the vial to the lot number on the COA before trusting either.

Why lot-specificity is the whole point

Peptide synthesis is a batch process, and no two batches are chemically identical. Coupling efficiency, purification, and final characterization vary run to run. A COA reports results for one specific lot — not the product generally, not “typical” results, that lot.

This is why the lot number is the load-bearing detail on both the vial and the document. A COA for a different lot of the same product tells you nothing reliable about the vial in your hand, however similar the numbers look. Checking that the two lot numbers actually match is the first and most-skipped step.

Identity: is it the right molecule?

Identity testing, typically by mass spectrometry, confirms that the molecule present has the expected molecular weight. Some COAs use MALDI-TOF or LC-MS, which can additionally provide sequence-level confirmation rather than mass alone.

What identity testing cannot do on its own is distinguish between molecules of identical mass but different structure. A peptide with the correct amino acid composition but scrambled disulfide bonds has the same mass as the correctly folded version — same atoms, different connectivity — and would pass a mass check while potentially being biologically inactive. Similarly, a D-amino acid substituted for an L-amino acid changes stereochemistry, not mass. Identity testing answers “is this the right formula,” not “is this the right molecule doing the right thing.”

Purity: how much of it is actually the target?

Purity is typically assessed by HPLC, which separates a sample by chemical behavior and reports what fraction of the detected signal corresponds to the main peak. A result of “99.2% pure” means 99.2% of what the detector measured falls under that peak.

Two limitations matter here. First, HPLC purity is relative to whatever the detector can see — it typically does not detect compounds that don’t absorb at the wavelength used, so it is not an absolute statement about total contamination. Second, and more relevant to peptides specifically: the main synthesis impurities — deletion sequences missing a single amino acid — are structurally very close to the target molecule. They can co-elute closely or even partially overlap with the main peak, meaning a high purity number does not fully rule out their presence. This is why identity and purity testing are complementary rather than redundant; each catches problems the other can miss.

What a standard COA usually does not cover

Endotoxin (bacterial lipopolysaccharide contamination) requires a separate assay, typically LAL-based, and is not implied by HPLC purity or mass spec identity. Residual solvent testing, heavy metal screening, and microbial testing are likewise separate analyses that some COAs include and others do not. A COA that lists only identity and purity is not incomplete or dishonest — those are the two most fundamental tests — but it is answering a narrower question than a COA that also addresses endotoxin and residual solvents.

Net peptide content versus gross weight

As covered in our peptide basics article, synthetic peptides typically exist as salts — commonly TFA or acetate — and lyophilized material retains bound water. Gross vial weight includes the peptide plus these components; net peptide content is the mass of peptide alone.

A COA that reports net peptide content (sometimes via amino acid analysis) is providing meaningfully more information than one that reports only gross fill weight, particularly for peptides with several basic residues where the counterion mass fraction is larger.

Third-party versus in-house testing

A COA generated by the manufacturer’s own lab and one generated by an independent contract lab are not equally strong evidence, for a straightforward conflict-of-interest reason: the party with a commercial interest in the result is not the ideal party to generate it. This does not mean in-house testing is meaningless, but independent third-party testing is the stronger standard, and it is worth checking which kind of lab produced any given COA.

Frequently asked questions

Does 99% purity mean the compound will be biologically active?

Not necessarily. Purity confirms most of the detected material is chemically similar to the target. It does not rule out disulfide scrambling, incorrect stereochemistry, or other structural issues that share the target’s mass and elution behavior.

Why does the lot number matter so much?

Because synthesis is a batch process and every lot is chemically distinct. A COA only describes the specific lot it was generated for; matching the lot number on the vial to the COA is what makes the document actually apply to your material.

What is the difference between identity and purity testing?

Identity (usually mass spectrometry) confirms the molecule has the correct mass. Purity (usually HPLC) estimates what fraction of the sample matches the main peak. They answer different questions and neither substitutes for the other.

References

US Pharmacopeia. General Chapter <621> Chromatography.
Snyder LR, Kirkland JJ, Dolan JW. Introduction to Modern Liquid Chromatography. 3rd ed. Wiley; 2010.
International Council for Harmonisation. ICH Q6B: Specifications for Biotechnological/Biological Products.

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For laboratory and research use only. Not for human consumption. This article is for informational purposes; it is not medical advice.