HPLC (high-performance liquid chromatography) is a separation technique in which a liquid mobile phase is pumped at high pressure through a packed column of stationary phase. Compounds in a sample partition between the two phases and elute at characteristic retention times that reflect their physicochemical properties.
For peptides, reversed-phase HPLC (RP-HPLC) is the standard method: a hydrophobic C18 column separates residues by their interaction with an acetonitrile-water gradient, typically modified with a low-pH ion-pairing reagent such as trifluoroacetic acid. Purity is reported as the percentage of total peak area attributable to the target species (%HPLC).
HPLC purity is the headline purity metric on every Oxford Research Peptides Certificate of Analysis.