Epa 1613 lcs

Standard solutions of PCDDs/Fs and PCBs congeners, namely, EPA-1613 STOCK (Native Stock Solution), EPA-1613 LCS (¹³C₁₂-labeled compound stock solution), EPA-1613 ISS (internal standard solution), and EPA-1613 CVS (calibration and verification solutions, CS1–CS4) for PCDDs/Fs, and WP-STOCK, WP-LCS, WP-ISS, and WP-CVS (CS1–CS6) for dl-PCBs, were purchased from Wellington Laboratories Inc. (Guelph, ON, Canada). The organic solvents n-hexane and DCM and ethyl ether for pesticide residue analysis (all of analytical grade) were purchased from J.T. Baker (Phillipsburg, NJ, USA). Anhydrous sodium sulfate (Na₂SO₄) from FUJI-FILM Wako Chemicals USA Corporation (Richmond, VA, USA) and 95% sulfuric acid from Merck (Darmstadt, Germany) were used. The multilayer silica gel column packed in a glass tube (O.D. 6.35 mm × length 35 cm) was purchased from Supelco (Bellefonte, PA, USA). This column was composed of 3 g 10% Ag-NO₃/silica gel, 0.9 g silica gel, 22% H₂SO₄/silica gel, 4.5 g 44% H₂SO₄/silica gel, 0.9 g silica gel, 3 g 2% KOH/silica gel, and 0.9 g silica gel. Florisil with a mesh size of 60–100 mesh purchased from Merck was used.

¹³C-labeled surrogate standards for PCDD/Fs (EPA 1613-LCS) and PCBs (EPA 68A-LCS) and ¹³C-labeled injection standards for PCDD/Fs (EPA 1613-IS) and PCBs (EPA 68A-IS) were all purchased from Wellington Laboratories (Guelph, Canada). Pesticide-grade dichloromethane and n-hexane were purchased from J.T. Baker (Phillipsburg, USA); nonane was purchased from Sigma-Aldrich (St. Louis, USA). Silica gel (0.063–0.100 mm) was purchased from Merck (Darmstadt, Germany) and baked at 550°C for 12 h before use. Anhydrous sodium sulfate (baked at 660°C for 6 h), concentrated sulfuric acid and sodium hydroxide were of reagent grade (purity>99.8%) and were purchased from Beijing Chemistry Company (Beijing, China). Acid Silica gel was made by evenly mixing 70 g of activated Silica gel impregnated with 30 g of concentrated sulfuric acid, and basic Silica gel was made by mixing 100 g of activated Silica gel and 30 mL of sodium hydroxide solution (1 mol L^-1). Activated carbon was purchased from Supelco (Bellefonte, USA).
PUF disks (diameter: 14 cm; thickness: 1.35 cm) were pre-cleaned by accelerated solvent extraction (ASE350, Dionex, USA) using acetone and a mixture of dichloromethane:hexane (v/v, 1:1) in sequence. Then, they were wrapped with aluminum foil and preserved in sealed plastic bags until use. The other information of reagents and materials were provided in S1 File.

High-purity reagents dichloromethane, n-hexane, and toluene and HPLC grade acetone were supplied by Merck (Taufkirchen, Germany). 1 µm PTFE filters were supplied by Pall Corporation (South Wagner Road, USA). Columns for sample preparation such as CLDS-ABN-STD (silica), CLDA-BAS-011 (alumina), and CLDC-CCE-034 (carbon) were supplied by Fluid Management System (Watertown, MA, USA) and glass wool for filtration from Merck (Taufkirchen, Germany). Nonane puriss standard for GC was supplied by Merck (Taufkirchen, Germany). The 17 high-purity certified commercial standards of 2,3,7,8-chloro-subsituted PCDD and PCDF and the 12 dl-PCBs congeners, as well as surrogates (EPA1613-LCS and WP-LCS) and internal standards (EPA1613-ISS and WP-ISS), were purchased from Wellington Laboratories (Guelph, ON, Canada).