Primary Ion Depletion Kinetics (PIDK) Studies as a New Tool for Investigating Chemical Ionization Fragmentation Reactions with PTR-MS

We report on a new approach for studying fragmentation channels in Proton Transfer Reaction-Mass Spectrometry (PTR-MS), which we name primary ion depletion kinetics (PIDK). PTR-MS is a chemical ionization mass spectrometric (CIMS) technique deploying hydronium ions for the chemical ionization. Induced by extremely high concentrations of analyte M, depletion of the primary ions in the drift tube occurs. This is observed as quasi zero concentration of the primary ion H3O+, and constant MH+. Under these non-standard conditions, we find an overall changed fragmentation. We offer two explanations. Either the changed fragmentation pattern is the result of secondary proton transfer reactions. Or, alternatively, the fast depletion of H3O+ leads to reduced heating of H3O+ in the drift field, and consequently changed fragmentation following protonation of the analyte M. In any case, we use the observed changes in fragmentation as a successful new approach to fragmentation studies, and term it primary ion depletion kinetics, PIDK. PIDK easily yields an abundance of continuous data points with little deviation, because they are obtained in one experimental run, even for low abundant fragments. This is an advantage over traditional internal kinetic energy variation studies (electric field per number density (E/N) variation studies). Also, some interpretation on the underlying fragmentation reaction mechanisms can be gleamed. We measure low occurring fragmentation (<2% of MH+) of the compounds dimethyl sulfide, DMS, a compound that reportedly does not fragment, diethyl sulfide DES, and dipropyl sulfide DPS. And we confirm and complement the results with traditional E/N studies. Summing up, the new approach of primary ion depletion kinetics allows for the identification of dehydrogenation [MH+ -H2] and adduct formation (RMH+) as low abundant fragmentation channels in monosulfides.

本研究报道了一种用于研究质子转移反应-质谱（Proton Transfer Reaction-Mass Spectrometry, PTR-MS）中碎裂通道的新方法，我们将其命名为初级离子耗竭动力学（Primary Ion Depletion Kinetics, PIDK）。质子转移反应-质谱是一种采用水合氢离子进行化学电离的化学电离质谱（Chemical Ionization Mass Spectrometric, CIMS）技术。当分析物M的浓度极高时，漂移管内的初级离子会发生耗竭，表现为初级离子H₃O⁺的浓度近乎为零，而质子化分析物离子MH⁺的浓度保持恒定。在这类非标准实验条件下，我们观察到整体碎裂行为发生改变。对此我们提出两种解释：其一，碎裂模式的改变源于次级质子转移反应；其二，H₃O⁺的快速耗竭会降低其在漂移电场中的受热程度，进而使分析物M质子化后的碎裂行为发生变化。无论机制如何，我们将观测到的碎裂行为变化作为一种全新且有效的碎裂研究方法，并将其命名为初级离子耗竭动力学（PIDK）。相较于传统的内部动能变化研究（比场（Electric Field per Number Density, E/N）变化研究），初级离子耗竭动力学的优势在于：仅需单次实验即可获取大量连续且离散度极低的数据点，即使是丰度较低的碎裂产物也能被有效观测。此外，该方法还可用于解析碎裂反应的潜在机制。我们观测到了二甲基硫（Dimethyl Sulfide, DMS，据报道不会发生碎裂）、二乙基硫（Diethyl Sulfide, DES）以及二丙基硫（Dipropyl Sulfide, DPS）的低丰度碎裂产物（其信号强度低于MH⁺的2%），并通过传统的比场变化研究对实验结果进行了验证与补充。综上，初级离子耗竭动力学这一新方法可用于识别单硫醚类化合物中的两类低丰度碎裂通道：脱氢反应[MH⁺ - H₂]以及加合物形成（RMH⁺）。