Assay of the Chiral Organophosphate, Soman, in Biological Samples

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Abstract
The anticholinesterase, soman, (CH3)3CC(H)CH3O(CH3)P(O)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 phosphoryl-phosphatases. Epimeric soman isomers, e.g. C(±)P(-)-soman, can be separately assayed on a Carbowax or a CPSil 8 column, using 2H-labeled soman isomers as internal standards. 2H-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[ddot]C to suppress hydrolysis by phosphoryl-phosphatases, (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 (CH3)3CCH2O(CH3)P(O)F to occupy covalent binding sites for C(±)P(-)-soman, before extraction with a Sep-Pak C18 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.

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