Trifluoroacetate (TFA) salt

Peptides carry charged groups, so they are isolated as salts with a counter-ion balancing the charge. Trifluoroacetic acid is used in the purification step of solid-phase synthesis, and unless it is deliberately exchanged the peptide is delivered as a trifluoroacetate salt. Many suppliers perform a salt exchange to acetate, which is the form most pharmaceutical peptides use, and a certificate of analysis should say which salt is present and at what level.

The distinction matters in two ways. Residual TFA can affect cell-based assays and is a known irritant, which is why acetate is preferred for injectable-grade material. And the counter-ion adds weight: a vial labelled 5 mg of peptide as a TFA salt may contain 5 mg of salt, of which perhaps 80 to 90% is peptide, unless the label specifies net peptide content. The "peptide content" line on a good certificate resolves the ambiguity.

Neither salt changes the peptide's identity or how it is reconstituted. The COA guide explains where to find the salt form and content figures on a certificate.