Purity assay versus content assay: two different numbers

On the subject in the title: Purity assay versus content assay: two different numbers Working notes rather than a conclusion.

I would like to understand what this number means before I repeat it anywhere.

A VendorInvestigate report on a semaglutide lot gives 97.2% purity. The supplier certificate for the same lot states 97.6%. Both documents name a reversed-phase method; neither states the same gradient.

My question is not "who is right". It is: given that those two figures were produced by different methods, what is the largest difference I should expect from method alone, and at what point does a gap stop being explainable that way?

Worth separating two things that the opening post runs together.

Reading old certificates: if a certificate is dated years ago, the lot it describes might be old. That is not automatically a problem but it is worth noting. Fresh testing on the current lot is stronger than old testing on an older lot.

Endotoxin testing is separate: a certificate of purity says nothing about endotoxin. A chemically pure preparation can carry a clinically significant endotoxin load. If endotoxin matters, it needs to be tested and reported.

Amino acid analysis gives content: it hydrolyses the peptide and quantifies the residues. It measures the peptide content independent of chromatographic purity. Very few supplier certificates report it.

Purity and content are different measurements: purity tells you what proportion of the detected material is the intended species. Content tells you how much of the intended species is present in the container. A lyophilised vial can be 99% pure and contain considerably less than the label claims because the remainder is water, counter-ion and excipient.