Evaluation of trifluoroacetic acid as an ion-pair reagent in the separation of small ionizable molecules by reversed-phase liquid chromatography

This paper describes the results of a study on trifluoroacetic acid (TFA) as an effective ion-pair reagent in the separation of small ionizable compounds by reversed-phase liquid chromatography (RPLC). Retention times were obtained when the amount of TFA in the mobile phase was gradually increased in the separation of a series of basic compounds by gradient elution. For the purpose of comparison, phosphoric acid was also used in the mobile phase. We conclude that TFA can be used as not only a mobile phase pH stabilizer but also sometimes as an effective ion-pair reagent to control both retention and selectivity in the separation of small ionizable solutes. The number of charges (or pK(a) values) on the solutes is not the only factor responsible for the effect of TFA. The other factor comprises the mutual accessibility of TFA and the positively charged functional groups and subsequent non-coulombic interaction. Both factors equally control the ion-pair effect. The results of the study should be applicable to other fluorinated acids, such as heptafluorobutyric acid, for the control of retention and selectivity in RPLC. (C) 1999 Elsevier Science B.V. All rights reserved.