The stability of soman and its stereoisomers in aqueous solution: toxicological considerations

The paucity of information regarding the characteristics of soman (pinacolyl methylphosphonofluoridate) in aqueous solution has limited its use as a toxicological or pharmacological reagent. We report here on the stability of soman under conditions in which it may normally be found during use and storage in the laboratory. Solutions of 1 mM soman in normal saline were not hydrolyzed after 5 months of storage at -90 degrees C. Samples that were repeatedly thawed but not allowed to warm to room temperature and then immediately refrozen showed no apparent hydrolysis. Portions of the same solution, stored in the refrigerator just above freezing, exhibited 50% hydrolysis after 150 days. When portions of this solution were stored at 21 degrees C, the time for 50% hydrolysis was in excess of 5 days. This rate of hydrolysis was the same for all four of the soman stereoisomers. In buffered solutions at pH 7.4, 8.0 and 8.6 the half-times were 6.6, 3.2 and 2.2 h at 27 degrees C and 4.8, 1.6 and 1.2 h at 37 degrees C, respectively. Hydrolysis rates were not significantly influenced by the presence of a carbodimide stabilizer in the agent. There is no reason to expect any deviation from a direct correlation between total soman concentration and toxicity.