Comprehensive analytical approaches to close the fluorine balance. ; to be discussed in the following content). Therefore, it was critical to identify a solvent that would ensure the effective extraction of all target PFAS from the milled samples (slurry-like samples for AFFF treatment). To determine this, we spiked twelve isotopelabeled PFAS (fluorocarbon number, n=3-9; each PFAS has a concentration of 1000 μg/mL) into the BN powder. The mixture was ultrasonically extracted using various solvents, including 0.3% methanolic ammonium hydroxide (MAH; a solvent recommended by EPA Draft Method 1633 ), Milli-Q water, 25% methanol (MeOH), 50% MeOH, and acetonitrile with 0.2% formic acid (details are provided in Text S1). MeOH (50%) was the best-performing solvent, with recoveries of PFAS with various structures from 80-110% (Figure ).