Purity, net content, net peptide content and salt forms
Why a powder can read 99% by HPLC and still be far from 99% peptide.
A purity percentage is meaningful only with its method. HPLC area percent, net content, and net peptide content are related but different measurements. A powder may show 99% by HPLC and still contain water, counterions, residual solvent, or another salt form that affects the total mass.
Salt forms matter because the reported molecular unit can include a counterion. Compare like with like, record the stated form, and avoid turning one method’s percentage into a different quantity the certificate does not report.