Assay of the Chiral Organophosphate, Soman, in Biological Samples

The anticholinesterase, soman, (CH 3 ) 3 CC(H)CH 3 O(CH 3 )P(0)F, consists of four stereoisomers assigned as C( +)P( +)-soman in which C stands for chirality in the pinacolyl moiety and P for chirality at phosphorus. The four stereoisomers are separated by gas chromatography on an optically active Chirasil—Val column, synthesized and coated in house, or on a Chirasil—Val column identical with the commercially available column when combined with a Carbowax 20M column. This method in combination with an assay based on acetylcholinesterase inhibition shows that the two isomers which do not have anticholinesterase activity, i.e. C( +)P( +)- soman, are rapidly degraded in rat blood due to hydrolysis by phosphorylphosphatases. Epimeric soman isomers, e.g. C( +)P( — )-soman, can be separately assayed on a Carbowax or a CPSiI 8 column, using 2 H-labeled soman isomers as internal standards. 2 H-labeled soman stereoisomers serve as internal standards in GC- assay of all four stereoisomers on Chirasil—Val. For work-up of the four stereoisomers from rat blood the sample is first stabilized by (i) acidification to pH 4.2 at 0°C to suppress hydrolysis by phosphorylphosphatases, (ii) addition of aluminum ions for complexation of fluoride ions to prevent regeneration of C( +)P(—)-soman by free fluoride ions from soman-inhibited carboxylesterase, and (iii) addition of (CH 3 ) 3 CCH 2 O(CH 3 )P(0)F to occupy covalent binding sites for C( +)P(—)-soman, before extraction with a Sep-Pak C 18 cartridge and elution with ethyl acetate. Using a splitless or on-column injection technique and alkali flame ionization detection, the minimum detectable concentration is 30 pg/3-ml blood sample.