Noyes Laboratory of Chemical Physics and the Beckman Institute, California Institute of Technology, Pasadena, California 91125, USA.
J Am Soc Mass Spectrom. 2009 Nov;20(11):2093-9. doi: 10.1016/j.jasms.2009.07.009. Epub 2009 Jul 18.
We present studies of the ionization mechanism operative in the ambient pressure pyroelectric ionization source (APPIS), along with applications that include detection of simulants for chemical nerve agents. It is found that ionization by APPIS occurs in the gas-phase. As the crystal is thermally cycled over a narrow temperature range, electrical discharges near the surface of the crystal produce energetic species which, through reactions with atmospheric molecules, result in reactant ions such as protonated water clusters or clusters of hydroxide and water. Reactant ions can be observed directly in the mass spectrometer. These go on to react with trace neutrals via proton transfer reactions to produce the ions observed in mass spectra, which are usually singly protonated or deprotonated species. Further implicating gas-phase ionization, observed product distributions are highly dependent on the composition of ambient gases, especially the concentration of water vapor and oxygen surrounding the source. For example, basic species such as triethylamine are observed as singly protonated cations at a water partial pressure of 10 torr. At a water pressure of 4 torr, reactive oxygen species are formed and lead to observation of protonated amine oxides. The ability of the APPIS source to detect basic molecules with high proton affinities makes it highly suited for the detection of chemical nerve agents. We demonstrate this application using simulants corresponding to VX and GA (Tabun). With the present source configuration pyridine is detected readily at a concentration of 4 ppm, indicating ultimate sensitivity in the high ppb range.
我们介绍了在环境压力热释光电离源 (APPIS) 中起作用的电离机制的研究，以及包括检测化学神经毒剂模拟物的应用。结果发现，APPIS 的电离发生在气相中。当晶体在很窄的温度范围内热循环时，晶体表面附近的放电会产生高能物质，这些物质通过与大气分子的反应，产生质子化水团簇或氢氧化物和水的团簇等反应物离子。反应物离子可以直接在质谱仪中观察到。这些离子通过质子转移反应与痕量中性分子进一步反应，生成在质谱中观察到的离子，这些离子通常是单质子化或去质子化的物质。进一步表明气相电离，观察到的产物分布高度依赖于环境气体的组成，特别是围绕源的水蒸气和氧气的浓度。例如，在水分压为 10 毫托的情况下，三乙胺等碱性物质以单质子化阳离子的形式被观察到。在水压力为 4 毫托的情况下，形成反应性氧物种，导致观察到质子化的胺氧化物。APPIS 源能够检测具有高质子亲和力的碱性分子，使其非常适合检测化学神经毒剂。我们使用对应 VX 和 GA（沙林）的模拟物证明了这一应用。在目前的源配置下，吡啶在 4 ppm 的浓度下很容易被检测到，表明在高 ppb 范围内具有最终灵敏度。
