HPLC.

High-performance liquid chromatography, used to assess purity.

High-performance liquid chromatography separates the components of a mixture by pushing it under pressure through a packed column, where different molecules travel at different speeds depending on how strongly they interact with the packing material. A detector at the far end records what comes out and when, producing a trace of peaks. For peptides it is the standard method for estimating purity: the target peptide appears as a main peak, and purity is commonly reported as that peak's area as a percentage of all peak area. Understanding what that number does and does not mean is worthwhile. HPLC separates by physical behaviour, not by identity, so it tells you how many distinct things are present and in what proportion, but it does not tell you what the main peak is. Confirming identity requires mass spectrometry, which is why the two are normally reported together. Substances that do not absorb at the detector's wavelength can be invisible in the trace, and impurities that happen to travel at the same speed as the target can hide underneath the main peak. And chemical purity is silent on sterility, endotoxin, and correct handling. A high purity figure is a real but narrow piece of information.