Assay one
Purity, by HPLC
The sample is separated by high-performance liquid chromatography and the peptide peak is measured against everything else that came off the column. The figure on the certificate is the share of the material that is the peptide itself.
The method is printed on every report. Across the lots on file it is HPLC-UV or HPLC-MS; where MS detection was used it is part of the purity measurement, not a separate identity test. The chromatogram is published alongside the number so the peak can be read rather than taken on trust.