Mass spectrometry grade acetonitrile, ammonium acetate, formic acid, and methanol were acquired from Fluka /Honeywell (St. Louis, MO, USA). Sodium hydroxide pellets (>98%) were obtained from Fluka/Honeywell, and ammonium hydroxide (33%) and sodium acetate were procured from Sigma-Aldrich (St. Louis, MO, USA). Hydrochloric acid (37%) was purchased from Carlo Erba Reagents (Cornaredo, MI, Italy). Oasis WAX (Weak Anionic eXchange) 3 cc Vac Cartridges, 60 mg (Waters Corp., Milford, MA, USA) were used for solid-phase extraction (SPE).
Ultrapure water (18.2 MΩ/cm) was obtained using a Human Power I lab water purification system (Human Corp., Seoul, South Korea). PFOS, PFOA, PFNA, and PFHxS standards and the corresponding isotopically labelled standards (IS), sodium perfluoro-1-[1,2,3,4-13 C4]octanesulfonate (M-PFOS), perfluoro-n-[1,2,3,4-13 C4]octanoic acid (M-PFOA), perfluoro-n-[1,2,3,4,5-13 C5]nonanoic acid (M-PFNA), and perfluoro-1hexane[ 18 O2]sulfonate (M-PFHxS) were purchased from Wellington Laboratories (Guelph, ON, Canada). An appropriate amount of each standard was combined and diluted in methanol to obtain a 'PFAS working solution' at a concentration of 50 ng/mL. The 'IS working solution' was prepared using the same procedure.
Ultrapure water (18.2 MΩ/cm) was obtained using a Human Power I lab water purification system (Human Corp., Seoul, South Korea). PFOS, PFOA, PFNA, and PFHxS standards and the corresponding isotopically labelled standards (IS), sodium perfluoro-1-[1,2,3,4-13 C4]octanesulfonate (M-PFOS), perfluoro-n-[1,2,3,4-13 C4]octanoic acid (M-PFOA), perfluoro-n-[1,2,3,4,5-13 C5]nonanoic acid (M-PFNA), and perfluoro-1hexane[ 18 O2]sulfonate (M-PFHxS) were purchased from Wellington Laboratories (Guelph, ON, Canada). An appropriate amount of each standard was combined and diluted in methanol to obtain a 'PFAS working solution' at a concentration of 50 ng/mL. The 'IS working solution' was prepared using the same procedure.