In peptide research, the quality of your starting material sets a ceiling on the quality of your results. This guide explains how peptide purity is measured, what a certificate of analysis (COA) contains, and how to read one so you can evaluate research material with confidence.
A COA is a document that reports the results of the analytical tests performed on a specific batch of material. For a research peptide, it answers two questions: is this the molecule it claims to be? (identity) and how pure is it? (purity). A COA is tied to a particular lot or batch, so the numbers reflect the actual material supplied, not a generic specification.
Peptide purity is most commonly determined by high-performance liquid chromatography (HPLC). The sample passes through a column that separates a mixture's components by their chemical properties; the main peptide and impurities elute at different times, producing a chromatogram of peaks. The area of the main peak relative to the total gives the purity percentage. "99% by HPLC" means the target peptide accounts for 99% of the measured material. A clean chromatogram with one dominant, sharp peak is what researchers look for.
Purity tells you how much of the sample is a single compound, but not which compound. That is the job of mass spectrometry (MS), which measures the peptide's molecular weight precisely. Comparing the measured mass against the theoretical mass calculated from the intended sequence confirms the molecule's identity. HPLC and MS together, purity plus identity, form the core of a credible COA.
The product name and amino acid sequence, plus the batch or lot number tying results to the material you receive.
HPLC purity (%), the proportion that is the target peptide, and the mass spec result confirming molecular identity against the theoretical mass.
Test date and testing lab, plus the chromatogram and spectrum, the raw evidence behind the reported numbers.
Purity is quantified by HPLC and identity confirmed by mass spectrometry, so every batch is verified before it reaches the lab.
A supplier can report its own in-house testing, and reputable ones do. But independent, third-party lab testing carries additional weight because the lab has no stake in the result. When a COA comes from an accredited independent lab, or when a supplier makes third-party results available, researchers can place more confidence in the figures. For widely copied peptides, this independent verification is one of the clearest signals separating quality material from unreliable sources.
Impurities, incorrect sequences, or degraded material introduce uncontrolled variables. Two "identical" studies using peptides of different real-world purity can produce different results for reasons unrelated to the biology. Insisting on documented purity is a direct investment in reproducible, interpretable research. Combined with correct reconstitution and storage, verified purity is the foundation of reliable peptide work.
How semaglutide, tirzepatide, and retatrutide differ.
Research use only. The information on this page is provided for educational purposes and refers to laboratory research material only. Research peptides are not for human or veterinary use and are not intended to diagnose, treat, cure, or prevent any disease. Nothing here is medical advice.
