Buyer Guide• 6 min read
Reading a Peptide COA: What Purity & HPLC Numbers Actually Mean
A Certificate of Analysis (COA) is the document that proves a peptide is what the vendor says it is. A real COA is a third-party lab report — not a marketing PDF. This guide walks through every section so you can spot a legitimate document at a glance.
What a real COA contains
- Laboratory name, address, and accreditation (often ISO/IEC 17025)
- Sample ID and lot/batch number tied to the vial you received
- Date sample was received and date of analysis
- Test methods used — typically HPLC for purity and ESI-MS or MALDI-TOF for mass confirmation
- Quantitative results — purity percentage, observed mass vs theoretical mass, water content (Karl Fischer), acetate or TFA counter-ion content
- Analyst signature
Purity: what '99 %+' actually means
HPLC purity is the percentage of the target peak's area under the curve relative to total area. 99 % means 1 % of the mass is impurities — typically truncated sequences, deletion peptides, or oxidation products. Research-grade peptides should report ≥ 98 % HPLC purity; ≥ 99 % is the higher standard most reputable US suppliers ship.
Mass spec — confirming identity
Purity tells you how clean the sample is; mass spec tells you what's actually in it. The observed mass should match the theoretical monoisotopic or average mass within ± 1 Dalton. If the observed mass is significantly off, the peptide is the wrong sequence — purity is irrelevant.
Red flags
- No third-party lab name (the vendor's own QC isn't independent)
- Missing chromatogram image — just a percentage with no peak
- Lot number on the COA doesn't match the vial label
- PDF dated long before your purchase — could be a generic template reused across batches
- Purity quoted as 'pharmaceutical grade' without a numeric HPLC percentage
How Zerilyx handles COAs
Every Zerilyx lot ships with a signed third-party HPLC and mass-spec COA tied to the specific batch on your shelf. View samples on the COA page.