HPLC Explained: What High Performance Liquid Chromatography Measures in Peptides

In reverse phase HPLC the column is packed with particles carrying a hydrophobic coating, most often C18 chains. The sample is pushed through under pressure in a water and acetonitrile mixture, and the share of organic solvent is increased over the run. Molecules that interact more strongly with the coating leave the column later, so a peptide and its closely related impurities, such as truncated or oxidized forms, tend to exit at slightly different times.

A UV detector at the column outlet records absorbance over time, usually near 214 to 220 nm where the peptide bond absorbs. The trace it produces is the chromatogram: time on one axis, signal on the other, one peak for each resolved component.

Software integrates the area under every peak above a threshold. Purity is the main peak's area divided by the total area of all integrated peaks, expressed as a percentage. The figure is relative: it describes how the UV absorbing material in the sample divides between peaks, under that method, on that day.

Because of that, two purity numbers are only comparable when the column, gradient, wavelength and integration settings are comparable. A sharper gradient or a lower integration threshold can change the reported value without any change in the material.

• Identity. A single clean peak shows a uniform sample, not that the molecule is the one on the label.
• Co eluting impurities. Two substances leaving the column together appear as one peak.
• Non UV absorbing content. Water, salts and counterions do not appear, so HPLC purity is not the same as how much peptide is in the vial.
• Absolute quantity. Net peptide content needs a separate measurement such as amino acid analysis or a quantitative reference standard.

A credible HPLC result shows the chromatogram itself, the integrated peak table and the method summary: column type and dimensions, mobile phases, gradient, flow rate and detection wavelength. Pair it with a mass spectrometry identity result for the same lot, as covered in the purity versus identity guide.