The recovered signal was then compared to additional unspiked samples (n=3) which were processed using SPE, but reconstituted with a solution of ~50 g L -1 of all analytes in injection solvent (theoretical 100 % recovery concentration). The latter solutions were also used to evaluate matrix effect by comparing them to a standard of equivalent concentration prepared directly in injection solvent (n=3), with no SPE. The equations used for recovery and matrix effect are in Table . Method recovery was also compared to that of an existing SPE procedure for acids , modified to match the concentration factor of the proposed method. Developed for PMOCs analysis, this procedure closely follows the manufacturer's instruction for generic acid compounds analysis without including a washing step. Oasis WAX SPE cartridges for PFAS analysis (150 mg, 6 mL reservoir) were purchased from Waters. The cartridges were conditioned with 5 mL of 2% formic acid in methanol followed by 5 mL of ultrapure water. 10 mL of sample were loaded, and the cartridges were eluted with 5% ammonia in methanol. The eluates were then evaporated to dryness at 38 C, reconstituted with 100 L of ultrapure water:methanol 1:1, centrifuged at 2000 rpm for 5