Most synthetic peptides are purified by reverse-phase HPLC using trifluoroacetic acid as an ion-pairing agent. TFA improves peak shape, and a portion of it stays bound to basic residues as the counterion. Unless a supplier has deliberately exchanged it, a peptide with basic residues is very likely a TFA salt.
Why it affects the numbers
The counterion is part of the mass on the balance but not part of the peptide. Together with residual water, it means the peptide fraction of a labelled milligram is always less than one — commonly in the region of 70–90%, depending on sequence and how many basic sites are available to bind.
Any concentration calculated from gross vial mass is therefore an overestimate of true peptide concentration. Where an experiment depends on absolute concentration rather than a relative comparison, the net peptide content from the CoA is the number to use.
Why it affects cell work
Residual TFA has been reported to affect cell viability and proliferation in culture at concentrations that can accompany ordinary peptide dosing. That makes it a genuine confounder: an apparent effect may belong to the counterion rather than the peptide. Acetate is the usual alternative for cell-based work, and salt exchange to acetate is a service many suppliers offer.
For assays where this matters, the control worth running is vehicle plus an equivalent amount of the counterion alone.
Counterion choice is an analytical consideration for laboratory work. Nothing here describes preparation for use in a person.