2.1 Materials
CB (99%) was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Sulfuric acid (18.4 kmol/m3) was procured from Beijing Lanyi Chemical Products Co., Ltd. (Beijing, China). Lower-concentration sulfuric acid solutions were prepared by diluting the concentrated acid in water. Fuming nitric acid (98 wt%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). High-performance liquid chromatography (HPLC)-grade methanol (99.9%) was procured from Shanghai Titan Scientific Co., Ltd. (Shanghai, China). Nitrobenzene (>99.8%) as an internal standard was obtained from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Standard samples of o -NCB (>99%), p -NCB (>99.5%), m -NCB (>99.5%) and DNCB (99%) were procured from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China).