Analytical method interferences for perfluoropentanoic acid (PFPeA) and perfluorobutanoic acid (PFBA) in biological and environmental samples

Abstract

While high-resolution MS (HRMS) can be used for identification and quantification of novel per- and polyfluorinated alkyl substances (PFAS), low-resolution MS/MS is the more commonly used and affordable approach for routine PFAS monitoring. Of note, perfluoropentanoic acid (PFPeA) and perfluorobutanoic acid (PFBA), two of the smaller carboxylic acid containing-PFAS, have only one major MS/MS transition, preventing the use of qualitative transitions for verification on low-resolution instrumentation. Recently our lab has observed widespread chemical interference in the quantitative ion channel for PFPeA (263 → 219) and PFBA (213 → 169) in numerous matrices. PFPeA interference was investigated using HRMS and putatively assigned as a diprotic unsaturated fatty acid (263.1288 Da) in shellfish and a separate interferent (¹³C isotope of 262.1087 Da) in hot cocoa, which had been previously described by the FDA. PFBA interference caused by saturated oxo-fatty acids, previously demonstrated in tissue, was also observed in liquid condensate from a residential air conditioning unit. Therefore, in support of PFAS analysis on low-resolution instrumentation, authors recommend several adjustments to analytical methods including altering liquid chromatography (LC) conditions as well as using matched internal standards to investigate and expressly confirm PFBA and PFPeA detections in both biological and environmental samples.

Keywords: HRMS; Low-resolution; Methods; PFAS; PFBA; PFPeA.

Fig. 1.

(A) Wide window extracted ion chromatogram (EIC) for PFPeA (262.9757 Da), (B) narrow window EIC for PFPeA, (C) narrow window EIC for identified unknown ion 263.1288 m/z, and (D) isotopic spectra and formula prediction of 263.1288 m/z. Starting chromatographic conditions were 75:25. (2 columns).

Fig. 2.

(A) Shellfish extract narrow window chromatogram of 263.1288 Da and suspected diprotic peak at 131.0607 Da with EIC scaled to the largest peak to maintain scale. (B) Narrow window chromatogram of 263.1288 Da and suspected diprotic peak at 131.0607 Da with both EIC peaks scaled to 100%. (C) Isotopic spectra of diprotic mass. (D) Fragmentation pattern of isolated 263.1288 Da (Average of 5 scans, CID at 10.0, isolation width of 1.3 m/z).

Fig. 3.

(A) Calibration standard for PFPeA on a low-resolution instrument, (B) PFPeA spiked oyster sample, and (C) normal oyster sample. Samples shown here were run with a 75:25 L C gradient. Peaks are scaled (%) based on the largest peak in each Extracted Ion Chromatogram (EIC). Height of the tallest peak is listed on the right side of each EIC. NL denotes normalization level for each EIC. IS-PFPeA represents the M3PFPeA isotopically labeled standard from Wellington Laboratory catalogue. (1 or 1.5 columns).