Peptides carry positive charges on basic residues (lysine, arginine, histidine) and on a free N-terminus. In the solid state those charges are balanced by negatively charged counter-ions. Which counter-ion is present depends on the last acids the peptide met during purification and isolation.
The counter-ion adds mass, lowering net peptide content, and it can affect solubility and hygroscopicity. A peptide purified with trifluoroacetic acid is isolated as a trifluoroacetate (TFA) salt unless it is exchanged to another form, such as acetate or hydrochloride.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.