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 pK_a, 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.