Where it comes from
Trifluoroacetic acid is used to cleave peptide from resin, to remove protecting groups, and as a mobile-phase additive in reversed-phase chromatography because it produces sharp peaks. By the end of the process it is present as the counter-ion paired with basic sites on the peptide.
Why it persists
TFA binds to positively charged side chains and is not removed by simple drying. A peptide with several basic residues carries correspondingly more of it, and it can account for a noticeable fraction of the total mass.
Salt exchange
Where TFA is undesirable it can be exchanged for another counter-ion, commonly acetate, by repeated lyophilization from a solution of the replacement acid or by ion exchange. This is an additional processing step and is stated on documentation when performed.
The salt form is therefore part of the product specification rather than a detail, and it affects both the mass balance and, in some experimental systems, the result.
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.