# How to Read an HPLC Peptide Purity Report
An HPLC purity report looks intimidating the first time. It is a small set of numbers and one chart, and once you know what each is for, it becomes routine.
The chromatogram
The chart is signal (usually UV absorbance at 214 nm or 220 nm) plotted against time. Each peak is a compound eluting off the column. A well-resolved chromatogram for a research peptide has:
- A large main peak — the peptide of interest.
- Small related substance peaks — synthesis byproducts, deletion sequences, oxidation products.
- An injection front near time zero and possibly a solvent peak — ignore these for purity math.
The purity number
Purity is reported as area percent: the main peak's integrated area divided by the total integrated area of all peaks in the analysis window, times 100. A 98.5% purity means 1.5% of the UV-absorbing material in the sample is not the target peptide.
Two nuances:
- Area % is not mass %. Different impurities have different UV extinction coefficients.
- The analysis window matters. Peaks eluting outside the integration window are not counted. Reports should disclose the integration window.
The related substances table
Below the chromatogram, a good COA lists each significant impurity peak by retention time and area %. If the table just says "Total impurities: 1.5%" without breaking peaks out, the report is hiding information. Individual impurities matter — a single 0.7% oxidation peak has different implications than seven 0.2% peaks.
