HPLC Purity: What "≥99%" Actually Means

When a peptide COA reports ≥99% purity by HPLC, it specifically means: under defined chromatographic conditions, the target peptide peak accounts for ≥99% of the total UV absorbance area at the specified wavelength.

This is not a statement about the absolute mass of peptide in the vial — that depends on counterion and water content. It is a statement about the chromatographic purity of the peptide fraction itself.

The remaining ≤1% typically contains: deletion sequences (peptides missing one or more residues from incomplete coupling), oxidation products (especially for methionine- and tryptophan-containing sequences), and racemized residues from extended deprotection.

For most in vitro research, ≥99% chromatographic purity paired with MS-confirmed mass is sufficient. For receptor-binding studies at extremely low concentrations, even sub-1% impurities can produce confounding signal — at that level, consider preparative HPLC re-purification before use.