Two different questions
Purity asks: of the peptide-related material present, what fraction is the target? Net peptide content asks: of the total mass in the vial, what fraction is peptide at all?
A batch can be 99% pure and, say, 80% peptide by mass. The remaining mass is counter-ions, residual water and salts.
Where the rest of the mass goes
Synthetic peptides are isolated as salts, usually acetate or trifluoroacetate, and the counter-ion is part of the delivered mass. Lyophilized solids also retain some water, and buffer components from the final step may remain.
For a peptide with many basic residues the counter-ion contribution can be substantial, because each basic site carries one.
Why it matters
Anyone calculating a concentration from the vial's stated mass is implicitly assuming the powder is all peptide. Where net content is materially below 100%, that assumption introduces a systematic error into every calculation downstream.
Net peptide content is measured separately, typically by amino acid analysis or nitrogen determination, and is not derivable from a purity figure.
Research use only. This page is explanatory and does not recommend any use, quantity or procedure. REVIVE LAB supplies reference materials for laboratory research. These are not medicines and are not for human or veterinary consumption. Anyone considering the use of any compound in humans should do so only under qualified medical supervision.