Analysis of the Enantiomers of VX Using Normal-Phase Chiral Liquid Chromatography with Atmospheric Pressure Chemical Ionization-Mass Spectrometry

Abstract

The chemical warfare nerve agent O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothioate (VX) is a mixture of two enantiomers resulting from the chiral center at the phosphorus atom. Significant differences exist in the reported toxicity and acetylcholinesterase inhibition rates of the two enantiomers. This makes the ability to distinguish between them desirable for either toxicological studies or the development of antidotal therapies. Using a Chiralcel OD-H column with normal-phase liquid chromatography, the enantiomers were baseline resolved in less than 7 min. Atmospheric pressure chemical ionization was utilized as the interface between a liquid chromatograph and mass spectrometer. The mass spectra of the two enantiomers were virtually identical. The protonated molecule was readily observed at m/z 268. VX was incubated with human plasma for 13 min, followed by hexane extraction. The areas of the first and second eluting VX enantiomers decreased by approximately 40% and 6%, respectively, when compared with VX-spiked plasma samples that were not allowed an incubation phase. Currently, research by others has been directed towards the identification, isolation, and possible modification of enzymes capable of hydrolyzing VX. The method presented here provides an analytical tool capable of monitoring the stereospecificity of enzymes that react with VX.