Enhanced Sensitivity for the Analysis of Perfluoroethercarboxylic Acids Using LC-ESI-MS/MS: Effects of Probe...

Perfluoroethercarboxylic acids (PFECAs) have recently emerged as replacements for toxic per- and polyfluorinated alkyl substances (PFAS) including perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). Compared with other PFAS, many PFECAs including hexafluoropropylene oxide dimer acid (HFPO-DA, trade name GenX) exhibit poor sensitivity during analysis using liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) and are therefore often difficult to quantify. This study examined changes in ESI probe position, mobile phase additive, and capillary voltage with the goal of enhancing PFECA sensitivity. In addition, the relative contributions of existing mechanistic theories for PFAS ionization during ESI are discussed. Results indicated that the LC-ESI-MS/MS sensitivity for 9 PFECAs can be improved significantly by altering the ESI probe position. At the optimal probe position, lowering the capillary voltage from 2.0 to 0.5 kV universally enhanced the LC-ESI-MS/MS sensitivity for PFAS analysis. For most analytes, the use of ammonium bicarbonate rather than ammonium acetate as a mobile phase additive also enhanced the analytical response. These effects have not been previously reported and suggest that many laboratories may be conducting analyses of PFECAs under suboptimal conditions. Using the strategies outlined in this study, PFECAs can be more easily incorporated into comprehensive methods for PFAS analysis. Here, we describe analytical parameters that enhance the sensitivity for some PFECAs by up to 36-fold while maintaining high sensitivity for legacy PFAS. This work not only highlights solutions to mitigate inadequate PFECA sensitivity but also provides insight into the mechanisms underlying PFAS ionization efficiency during LC-ESI-MS/MS.