Microplate biochemical determination of Russian VX: influence of admixtures and avoidance of false negative results.

Two microplate spectroscopic methods for determination of organophosphates, based on inhibition of acetylcholinesterase (AChE) activity, were further improved and evaluated for determination of the chemical weapon agent Russian VX (RVX) in aqueous solutions. The linear range of the Hestrin method (74.8 – 1120 pM) was 3.1-fold wider than that of the Ellman method (37.4 – 374 pM). Limits of detection and quantification of RVX for both methods were below the maximal allowable concentration of RVX in water-soluble washouts. One of the early product of RVX hydrolysis N,N-diethylaminoethanethiol, like GSH, caused false negative results in the Ellman method at concentrations exceeding 10 µM; individual blanks were necessary to eliminate the effect. The Hestrin method showed greater specificity (about three orders of magnitude) for analysis of samples containing mercaptans. A major product of RVX degradation, 2,2'-dithiobis(N,N-diethylethanamine), caused significant inhibition of AChE at concentrations of ≥0.1 mM (p<0,01) and had a false positive effect at higher concentrations (≥2mM). For environmental monitoring of RVX, the method based on Hestrin is preferred over that of Ellman, principally because the former method was less sensitive to interference from major admixtures and did not give rise to potentially dangerous false negative results.