Reading a Certificate of Analysis: A Field Guide

A Certificate of Analysis (COA) is the supplier’s testimony about what’s in the vial. Per lot, per peptide. If a supplier won’t show you one, walk away. If they will, here’s how to read it.

The five sections that matter

1. Identity confirmation

Look for mass spectrometry — usually MALDI-TOF or ESI-MS. The COA should list:

• Theoretical molecular weight (calculated from sequence)
• Observed molecular weight (from the mass spec)
• Match within ~1 Da is expected; larger drift suggests a synthesis error or contamination

2. Purity

Almost always reported as HPLC area % at a single wavelength (often 220 nm for peptide bonds).

• >95% is the floor for serious research use
• >98% is the standard for reference work
• >99% is the floor for clinical-grade comparison standards

A purity number without a chromatogram attached is less trustworthy than a slightly lower number with the chromatogram shown.

3. Peptide content vs net mass

The vial may contain 5 mg of material, but only 4.2 mg of that is actual peptide — the rest is counter-ions (acetate, TFA salt) and water. The COA should report:

• Peptide content (%) — usually 70-90%
• Net peptide mass per vial — derived

This matters for any concentration-sensitive comparison work.

4. Counter-ion identity

• TFA salt — trifluoroacetate, the standard from preparative HPLC. Most common.
• Acetate — for compounds where TFA interferes with downstream assays. More expensive.
• HCl salt — common for older peptide chemistry; less common today.

Counter-ion choice affects solubility and assay compatibility — flag this when planning experimental work.

5. Endotoxin / bioburden

Reference-grade material destined for cell-culture or animal-model work should report:

• Endotoxin (EU/mg) — low single digits is typical
• Bioburden (CFU/g) — should be <100, ideally <10

Cosmetic-grade material often doesn’t carry these tests. Research-grade should.

Red flags

• COA dated more than a year before your purchase (peptides degrade)
• “Pass / Fail” with no numerical data
• Same COA reused across multiple lots
• Stock photo of a chromatogram (yes, this happens)
• No batch number on the vial label

What an honest COA looks like

The chromatogram is included. The mass spectrum is included. The lot number on the COA matches the lot number on your vial. The date is recent. The supplier publishes the COA on their site, not as a PDF you have to email-request.

If the supplier hesitates to share any of this, you have your answer.