What purity actually measures

Peptide purity on a COA is almost always an HPLC reading — the percentage of the target compound relative to everything else in the sample, measured by High-Performance Liquid Chromatography. A 98% purity peptide means 98% target compound and 2% impurities, degradation products or truncated sequences.

Research-grade peptides typically sit in the 97 to 99% range. Below 95% is usually considered technical-grade and not suitable for sensitive assays. Above 99% is rare and often a claim worth verifying — synthesis at that level of purity is technically demanding and expensive.

Why 2% impurities are not nothing

A 2% impurity profile sounds small. In practice, those impurities are typically structurally related molecules — failed couplings during synthesis, oxidation products, truncated sequences. Structurally related means they can:

  • Bind the same receptor with different affinity, creating signal you did not expect.
  • Compete for the same binding site and shift your dose-response curve.
  • Degrade further over time, producing breakdown products that were not in the original sample.

For a screening assay this might be tolerable. For a binding affinity measurement where you need a clean Kd, a 2% structural impurity can materially change the number you report.

Why batch-to-batch consistency matters more than a single high number

A 99.2% pure batch followed by a 97.8% pure batch looks fine on paper. In the lab, it means your control experiments from six months ago now carry a different impurity profile than your current experiments. Reproducibility drops quietly.

A supplier that maintains a narrower purity band across batches — say 98.5 to 99.2% — is more useful than one advertising an occasional 99.5% result while drifting down to 96% on other runs. Ask for the COA of the specific batch, not an average.

How to verify

A credible COA shows:

  • The direct HPLC purity reading (not "min. X%" but the actual measured value).
  • Reference to the analytical method or column used.
  • Mass spectrometry confirmation of identity — because a high-purity compound of the wrong molecule is still useless.
  • Date of analysis and batch number for traceability.

If the COA reads "≥ 98%" without a measured number, or if there is no batch reference, ask. A supplier serious about research supply will have nothing to hide.