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What is the TFA salt issue in peptides?
The short answer
Most synthetic peptides are purified using trifluoroacetic acid, and TFA remains bound to the peptide as a counter-ion unless a salt-exchange step removes it. It adds mass without adding peptide and is biologically active in its own right. A vial can be genuinely pure peptide and still carry a substantial TFA fraction that no standard purity figure reveals.
In detail
This is one of the clearest cases where purity and content diverge. Chromatographic purity can read 99% while a meaningful share of the vial's mass is counter-ion.
Quantification against a reference standard already corrects the practical consequence, because it measures peptide rather than powder. Determining the TFA fraction itself is a separate analysis, priced separately across the industry.
Suppliers who have run a salt exchange usually say so, because it costs them money. Silence on the point is not evidence of either answer.
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