The COA guide
A Certificate of Analysis, line by line.
A research-supplier COA isn't a complicated document. But it's often unfamiliar territory. Here's what each section means.
Identity confirmation
The supplier's testimony that what's in the vial matches the label. Two numbers matter:
- Theoretical molecular weight — calculated from the amino acid sequence
- Observed molecular weight — measured by mass spectrometry (MALDI-TOF or ESI-MS)
Match within ~1 Da is expected. Larger drift suggests synthesis error or contamination.
Purity (HPLC area %)
Usually reported at 220 nm (peptide-bond absorbance). The number is the target peak's area divided by total peak area. It's an objective integration — but only if you can see the chromatogram.
A ">99% pure" claim with no chromatogram attached is less useful than a slightly lower number with the chromatogram visible. Always look for the chromatogram.
Counter-ion identity
The non-peptide ion that accompanies the peptide in dry form. The two common options:
- TFA (trifluoroacetate) — the standard from preparative HPLC purification
- Acetate — preferred when TFA interferes with downstream assays
Counter-ion choice affects solubility and assay compatibility. For receptor-binding work, acetate is typically preferred.
Peptide content (%)
The percentage of the gross vial mass that is actual peptide. Typically 70-90%. The rest is counter-ion plus residual water.
For concentration-sensitive work, the peptide content figure is what you use to calculate molar concentration — not the gross vial mass.
Endotoxin (EU/mg)
Bacterial endotoxin (LPS) measured via the LAL or recombinant Factor C assay. Critical for:
- Cell culture work (especially primary cells or immune-cell assays)
- Animal models with inflammation-related endpoints
- Any in vivo research
Threshold: <1 EU/mg for most research use. Cosmetic-grade material typically omits this test.
Bioburden (CFU/g)
Total viable microbial contamination. Plated on agar, counted as colony-forming units per gram. Reference threshold: <100 CFU/g.
Lot number
The audit trail. The lot number on the COA should match the lot number on the physical vial label. Suppliers who can't or don't track lots cannot meaningfully test each batch.
Test date
When the tests on this COA were performed. For research-grade material, expect the test date to be within 6-12 months of your purchase. Significantly older COAs raise the question of whether the material has degraded since testing.