Definition
A chromatogram is the graphical output of a chromatographic separation: detector response, usually UV absorbance, on the vertical axis against elution time on the horizontal axis. Each component appears as a peak. For peptide purity, software integrates the area under each peak, and purity equals the main peak's area as a percentage of the total integrated area. A useful chromatogram shows the full gradient, labelled peaks, retention times, the integration table and the method conditions. Shoulders or poorly resolved peaks can hide impurities, and a blank run helps distinguish system artefacts from sample peaks. A certificate of analysis that reports purity without attaching the chromatogram gives researchers no way to check the figure.