Concentrated sulfuric acid h2so4

Graphite (> 100 mesh, 99.95%) and ammonium persulphate ((NH₄)₂S₂O₈, 99%) were both purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd., China. Ferroferric oxide (Fe₃O₄, 99.5%) was provided by Shanghai Macklin Biochemical Co., Ltd., China. Pyromellitic dianhydride (PMDA, 99%) and 4, 4′-oxydianiline (ODA, 99%) were both supplied by Beijing J&K Scientific Co., Ltd., China. Dimethylacetamide (DMAc, 99%) was purchased from Guangdong Guanghua Technique Co., Ltd., China. Concentrated sulfuric acid (H₂SO₄, 95–98%), potassium permanganate (KMnO₄) and hydrogen peroxide (H₂O₂, 30%) were all supplied by Beijing Chemical Works, China. All materials were used without further purification.

Bisphenol A-based benzoxazine (BZ) monomer was synthesized from aniline, bisphenol A and paraformaldehyde by using a solvent method described in our previous paper [49 (link)]. Phenyltrimethoxysilane (PTMS) was supplied by Zhejiang Chemical Technology Company, Hangzhou, China. Dichloromethylvinylsilane (DMVS) and triethylamine were supplied by Alfa-Aesar, Tianjin, China. Sodium hydroxide (NaOH), isopropanol, toluene, acetone, chloroform, hydrogen peroxide(H₂O₂), acetic acid (HAc) and concentrated sulfuric acid (H₂SO₄) were purchased from Beijing Chemical Works, Beijing, China.

¹³C₁₂-labeled standard solutions of PCDD/Fs (purity > 99.9%) (calibration standard EPA 1613-CVS, surrogate standard EPA 1613-LCS and injection standard EPA 1613-IS) and PCBs (purity > 99.9%) (calibration standard EPA 68C-CVS, surrogate standard EPA 68C-LCS and injection standard EPA 68C-IS) were all provided by Wellington Laboratories (Guelph, Canada). Pesticide grade dichloromethane (DCM), n-hexane and toluene were purchased from Fisher (Fair Lawn, NJ, USA). Analytical grade anhydrous sodium sulfate (Na₂SO₄), sodium hydroxide (NaOH) and concentrated sulfuric acid (H₂SO₄) were supplied by Beijing Chemical Factory (Beijing, China). Silica gel 60 (0.063–0.100 mm), basic alumina, active carbon and diatomaceous earth were obtained from Merck (Darmstadt, Germany), Sigma-Aldrich (St. Louis, MI, USA), Supelco (Bellefonte, PA, USA) and DIONEX (Sunnyvale, CA, USA) respectively. The preparation methods of 30% (w/w) sulfuric acid-impregnated silica gel and 1.2% (w/w) sodium hydroxide impregnated silica gel have been reported previously [16 (link)].