Weigh the contents of a lyophilized vial and you will usually find more mass than the label states. The difference is not extra peptide. It is counter-ions, bound water and sometimes a bulking agent, and it is why "net peptide content" appears on better certificates of analysis.
Where the extra mass comes from
- Counter-ions. Basic residues (lysine, arginine, histidine) and the N-terminus pick up an acid counter-ion during purification, usually trifluoroacetate (TFA, 113 g/mol) or acetate (59 g/mol). A peptide with four basic sites as a TFA salt can carry several hundred daltons of counter-ion.
- Water. Lyophilized peptides retain a few percent of water, and absorb more from the air if a vial is opened cold.
- Excipients. Some manufacturers add mannitol so that very small peptide masses form a visible cake. The excipient is inert but adds weight.
Which number to use
Use the labeled peptide mass. A supplier that reports net peptide content (by amino-acid analysis or UV absorbance) fills vials so that the peptide, not the powder, matches the label. Dividing the labeled mass by the diluent volume gives the concentration you want; weighing the powder and dividing by the volume would overstate it by the counter-ion and water fraction.
Why it matters for comparisons
Two vials both labeled "5 mg" can contain different amounts of peptide if one supplier labels gross weight and the other labels net content. When comparing lots or suppliers, compare net peptide content from the CoA, not label claims. Ask for the CoA of your specific lot; the certificate guide explains what to look for.