Fluorinated carboxylic acids as "ion repelling agents" in reversed-phase chromatography

Fluorinated carboxylic acids have been in use as ion-pairing reagents for over three decades. It has been observed that ion-pairing reagents not only increase the retention of oppositely charged analytes on reversed-phase HPLC columns but also decrease the retention of similarly charged analytes; these latter effects, however, have not been thoroughly investigated for the fluorinated carboxylic acids, and the application of these reagents has been rather restricted to their ion-pairing capacity to separate basic analytes. In the present study, we report a systematic investigation about the effects of three fluorinated carboxylic acids (trifluoroacetic acid (TFA), pentafluoropropionic acid (PFPA), and heptafluorobutyric acid (HFBA)) on the retention and selectivity of the separation of halogenated carboxylic acids and sulfonic acids by reversed-phase chromatography with an inductively coupled plasma mass spectrometry detector (ICPMS). Several eluents were tested and compared at different concentrations (0-100 mM) and pH values, including sulfate, nitrate, phosphate, oxalate, TFA, PFPA, and HFBA. The fluorinated carboxylic acids resulted in a consistent decrease in the retention factors (up to ca. 9-fold with HFBA) in a concentration dependent manner, which plateaued at around 50 mM. Significant improvement of the peak symmetry of the chromatographed acids was also observed. We highlight the advantages of incorporating the fluorinated carboxylic acids in modifying the selectivity and retention of organic acids in reversed phase chromatography in general, and particularly when employing chromatographic detectors with limited compatibility with organic mobile phases such as the ICPMS.