A Liquid–Liquid Extraction Method for Measuring Ultrashort PFCAs and Longer-Chain PFAS in Water

Natural waters can contain diverse per- and polyfluoroalkyl substances (PFAS) with contrasting chemistries, presenting challenges for using a single extraction for analysis. Prior work has reported large increases in environmental concentrations of trifluoroacetic acid (TFA), a highly polar ultrashort-chain perfluorocarboxylic acid (PFCA). However, standard solid-phase extraction (SPE) methods often have poor recoveries for ultrashort-chain PFAS. Here we present a novel two-stage liquid–liquid extraction (LLE) that facilitates analysis of long-chain PFAS and ultrashort PFCAs such as TFA in a single analytic workflow. The method leverages acidic extraction into methyl tert-butyl ether (MTBE) followed by basified back-transfer (pH-programmed partitioning) and evaporative transfer of MTBE-retained PFAS to the aqueous residue. Recoveries are facilitated by pH-programmed partitioning for ultrashort PFCAs and evaporative transfer for longer-chain targets. Ultrashort perfluoroalkyl sulfonic acids have low pKa, which restricts their extraction using the LLE method and some neutral precursors experience matrix-induced ionization suppression. Overall, absolute recoveries for 53/61 targeted PFAS analyzed fell within ±30% and concentration recoveries were within ±13% for all analytes with matched extracted internal standard (EIS). Method detection limits (MDLs) were 6.67 ng L–1 for TFA, approximately an order of magnitude lower than concentrations reported in most natural waters, and 0.03–0.22 ng L–1 for the six federally regulated PFAS in the United States. Overall, the LLE method enables sensitive, reproducible quantification for ultrashort PFCAs and diverse longer-chain PFAS.