The stability of mutagenic chemicals stored in solution

Using the Salmonella/microsome assay, we have evaluated the stability of mutagenic responses of chemicals stored frozen over a period of 18 months. Each of the standard mutagens was prepared in January 1982, and aliquots were stored at -20 degrees C and at -80 degrees C. Sodium azide (NaN3) was dissolved in water; 4-nitro-o-phenylenediamine (4NOP), 4-nitroquinoline-N-oxide (4NQO), benzo(a)pyrene (B[a]P), and 2-aminoanthracene (2AA) were dissolved in DMSO, all at 100 micrograms/ml. At various times, aliquots were removed, thawed, and tested in parallel with freshly prepared mutagen samples using strain TA100 in a standard plate test and freshly prepared Aroclor 1254-induced rat liver S-9 mix where needed. 4NOP (2-10 micrograms/plate), 4NQO (0.01-0.10 micrograms/plate), B(a)P (0.5-2.5 micrograms/plate), and 2AA (0.25-2.0 micrograms/plate) showed no significant differences between the freshly prepared solutions and the solutions stored at -20 degrees C and -80 degrees C. NaN3 (0.1-0.8 micrograms/plate) did show a statistically significant difference, with the fresh samples giving the lowest mean responses (over all doses) and the -80 degrees C treatment giving the highest. The freezing of mutagen solutions is adaptable to routine use and provides the advantage of reducing the time required to prepare positive control chemicals and reducing the exposure of laboratory personnel to known mutagens.