Formic or Triflouroacetic Acid?

Additives are commonly used in LCMS. Sometimes to provide proper ionization of the analyte to ensure appropriate sensitivity or as ion pairing agent to reduce ion supression by species, that display better ionization efficiency compared to compound(s) of interest.

Two of the common additives are formic and triflouroacetic acid. Both are (semi-)volatile and have limited precipitation, which is important to maintain operation of the LCMS system.
However, when using TFA in positive mode, it will be tougher to see no ion suppression in negative mode.
Due to thier acidic properties both show ion pairing properties, although TFA has more ion pairing capacity then FA since it is a stronger acid (pKa -0.3 compared to pKa of 3.75) and therefore display more ion suppression then FA.
When it comes to LC performance TFA is more advantageous because it enhances of peak symmetry, whereas FA causes peak tailing.

To summarize the major facts one should remember: 
LC - TFA over FA
ESI-MS - FA over TFA
MALDI MS - TFA over FA