Definition
The percentage of a lyophilised powder's total mass that is peptide, with the rest being counter-ions, water and salts.
Net peptide content is the fraction of the total powder mass in a vial that is peptide. The remainder consists of counter-ions such as trifluoroacetate or acetate bound to basic residues, residual water, and trace salts. Typical values are 70–90%. Content is determined by amino acid analysis, elemental nitrogen (CHN) analysis or quantitative UV absorbance. It is independent of purity. For example, a vial labelled 10 mg with 80% content and 99% purity holds about 7.9 mg of the target peptide. Correcting for content matters whenever an experiment depends on accurate molar concentration, such as receptor binding or concentration–response studies.