Analysis of VX on Soil Particles Using Ion Trap Secondary Ion Mass Spectrometry

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• 作者：Gary S. Groenewold ; Anthony D. Appelhans ; Garold L. Gresham ; John E. Olson ; Mark Jeffery ; and J. B. Wright
• 刊名：Analytical Chemistry
• 出版年：1999
• 出版时间：July 1, 1999
• 年：1999
• 卷：71
• 期：13
• 页码：2318 - 2323
• 全文大小：127K
• 年卷期：v.71,no.13(July 1, 1999)
• ISSN：1520-6882

文摘

The direct detection of the nerve agent VX (methylphosphonothioic acid, S-[2-[bis(1-methylethyl)amino]ethyl] O-ethyl ester) on milligram quantities of soil particles hasbeen achieved using ion trap secondary ion mass spectrometry (IT-SIMS). VX is highly adsorptive toward a widevariety of surfaces; this attribute makes detection usinggas-phase approaches difficult but renders the compoundvery amenable to surface detection. An ion trap massspectrometer, modified to perform SIMS, was employedin the present study. A primary ion beam (ReO₄^-) wasfired on axis through the ion trap, where it impacted thesoil particle samples. [VX + H]⁺, [VX + H]⁺ fragment ions,and ions from the chemical background were sputteredinto the gas-phase environment of the ion trap, where theywere either scanned out or isolated and fragmented (MS²).At a surface concentration of 0.4 monolayer, intact [VX+ H]⁺, and its fragment ions, were readily observableabove background. However, at lower concentrations, thesecondary ion signal from VX became obscured by ionsderived from the chemical background on the surface ofthe soil particles. MS² analysis using the ion trap wasemployed to improve detection of lower concentrationsof VX: detection of the ³⁴S isotopic ion of [VX + H]⁺,present at a surface concentration of ~0.002 monolayer,was accomplished. The study afforded the opportunity toinvestigate the fragmentation chemistry of VX. Semiempirical calculations suggest strongly that the molecule isprotonated at the N atom. Deuterium labeling showed thatformation of the base peak ion (C₂H₄)N(i-C₃H₇)₂⁺ involvestransfer of the amino proton to the phosphonothioatemoiety prior to, or concurrent with, C-S bond cleavage.To manage the risk associated with working with thecompound, the vacuum unit of the IT-SIMS was locatedin a hood, connected by cables to the externally locatedelectronics and computer.