The sample digestion was conducted based on the recommended method in literature[13]. 100 mg of samples (GBW07401 soil and NIES No. 8 vehicle exhaust particulates) was weighed and placed into high-pressure PTFE vessels. 4 mL HNO3, 1 mL HClO4 and 1 mL HF were added into the vessels and the resulting mixture was digested in a microwave oven (MDS-2003F, Sineo Microwave Chemical Technology, Co. Ltd, Shanghai China). The digestion program is detailed in Table 3.