Why is trifluoroacetic acid (TFA) used in reverse-phase chromatography for protein purification?

Trifluoroacetic acid (TFA) is used as an ion paring agent in reversed-phase chromatography to improve peak shape and resolution. The silica used in columns may have metal ion impurities, which can cause peak tailing and loss of resolution. An ion-pair reagent such as TFA added to the mobile phase at a concentration of ~0.1% can “shield” the metal ion and help to maintain good peak shape.