External considerations Reduction of exposure or accessibility D. Reduction/elimination of impurities 1. Generation of impurities of different chemical classes 2. Presence of toxic homologs(同系物) 3. Presence of geometric, conformational or stereoisomers 四大学化学学院 2002年7月26日四川大学化学学院 2002年7月26日 9 External considerations------ Reduction of exposure or accessibility ◼ D. Reduction/elimination of impurities ◼ 1. Generation of impurities of different chemical classes ◼ 2. Presence of toxic homologs（同系物） ◼ 3. Presence of geometric, conformational or stereoisomers