Peptide content
In short Peptide content states what proportion of the mass in the vial is actually peptide. Salts, counterions and residual moisture do not count towards it and push the figure below the purity.
An example: a vial with ten milligrams weighed out, 99% purity and 80% peptide content contains roughly eight milligrams of peptide. The remaining two milligrams are counterion and bound water.
For most laboratory applications the nominal amount on the vial is the working figure. If you need exact concentrations, use the peptide content from the certificate instead, where it is stated.
The difference from purity is the most important point: purity compares the target peptide with the other peptides in the same sample, while peptide content compares the peptide with the total mass weighed out. Ninety-nine per cent purity with eighty per cent peptide content is not a contradiction but the normal case.
It is determined by amino acid analysis or from the nitrogen content. Both take more work than a chromatography run, which is why the figure does not appear on every report.
Within one lot the gap to the nominal amount does not matter, because it is the same in every preparation. When you change to another lot or another manufacturer, it is not. A worked example is in the article what is really in the vial.