DEMO=0 python3 build.py after wiring real content.The number on a certificate is a summary. The trace is the evidence. Here is what each part of the plot actually means.
Time runs left to right in minutes; absorbance runs bottom to top in milli-absorbance units at a fixed wavelength — for peptides, almost always 214 nm, where the peptide backbone absorbs.
The tall peak is your compound eluting off the column. Its retention time is a weak identity check: it tells you the compound behaves consistently, not that it is the right molecule.
Purity is the area under the main peak divided by the total area of all peaks. That is why a purity figure is only as honest as the integration — how the software decided where one peak ends and the next begins.
A cluster of small peaks before the main one usually means incomplete synthesis or degradation. Peaks after it are often aggregates or oxidation products. A 99% purity figure with a messy shoulder is not a clean batch.
HPLC says how much of the UV-absorbing material is the main species. It cannot tell you what that species is. Only mass spectrometry can.