TFA is an abbreviation used for trifluoroacetic acid.
This acid is commonly used in HPLC methods because it has some ion-pairing characteristics and is therefore used where peak tailing due to causes such as silanophilic effects is an issue.
It is generally considered to be not compatible with LC-MS except when used in very low concentrations (~0.005%). It is very well suited for UV methods and is generally used as ~0.1% (v/v). It is soluble in both neat DI water and neat acetonitrile so it can be used in A and B solvents as DI water + 0.1% TFA and Acetonitrile + 0.1% TFA.