Formic acid is a common additive component of mobile phases when using reversedphase liquid chromatography separations. One of the most important uses of formic acid in liquid chromatography is as a buffer component of aqueous mobile phases (formic acid-sodium formate or formic acid-ammonium formate solutions) when pH (values around its pka, 3.75) must be controlled to achieve separation of target compounds with acid-basic properties. However, in some cases, the presence of only a low concentration of formic acid in the mobile phase is also known to improve peak shapes of the resulting separation. When liquid chromatography-mass spectrometry techniques are used, formic acid is also a common component of the reversed-phase mobile phases because it provides protons for the LC-MS analysis in positive ionization mode, by producing [M+H]+ ions. Moreover, it has the advantage of not being a strong ion-pair agent, unlike trifluoroacetic acid, and it does not suppress MS ionization of, for instance, polypeptides. However, the use of formic acid could sometimes produce ion suppression when LC-MS methods are developed, especially when negative ionization mode is used, for instance for the analysis of bisphenol A and related compounds. In these cases its use must be prevented. This chapter will focus on the description of the most frequent applications and uses of formic acid in liquid chromatography-mass spectrometry methods, not only focusing on its use as an additive in the mobile-phases but also as a component for the sample treatment procedures that will finally affect the method performance. Advantages and drawbacks of its uses in LC-MS will be discussed by means of relevant applications. Coverage of all kind of applications is beyond the scope of the present contribution, so we will focus on the most relevant LC-MS application fields such as bio-analytical, food and environmental analysis. © 2014 Nova Science Publishers, Inc.
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