用测得的氯同位素效应观察全氯乙烯的协同和分步多重脱氯反应
Observation of Concerted and Stepwise Multiple Dechlorination Reactions of Perchlorethylene in Electron Ionization Mass Spectrometry According to Measured Chlorine Isotope Effects
摘要：
有机氯的脱氯碎片离子通常可在电子电离质谱（EI-MS）上发现。然而，目前还不清楚EI-MS中发生的多个脱氯反应是协同的还是逐步的。根据不同脱氯反应中氯同位素效应和质谱信号强度，研究了四氯乙烯（PCE）在EI-MS中多个脱氯反应的协同性。不同离子之间同位素比值的波动表明，EI-DFS-HRMS中观察到的多个脱氯反应是逐步的。EI-qMS检测到的离子同位素比值的变化趋势可能表明，在该MS中观察到的多个脱氯反应是一致的。这些协同/分步多脱氯反应的机制进行了初步解释。除了同位素效应的证据外，根据准平衡理论，MS信号强度还表明EI-HRMS中的多个脱氯反应可能是分步进行的，而EI-qMS中的多个脱氯反应可能是协同进行的。从电离区到质谱仪入口的距离可能是影响离子飞越此距离时脱氯反应的关键因素。EI-qMS中的该距离明显短于EI-HRMS中的该距离。因此，EI-qMS中检测到的主要离子可能是在电离和源内碎裂过程中产生的，而EI-HRMS中检测到的主要离子可能是在飞越距离的前体离子碎裂过程中产生的。我们推断，在电离区的主要脱氯反应是一致的，而发生在飞行过程中的前体离子的距离从电离区的质量分析器入口的逐步。
英文摘要：
Dechlorinated fragmental ions of organochlorines can be commonly found on electron ionization-mass spectrometry (EI-MS). Yet it is still unclear whether multiple dechlorination reactions taking place in EI-MS are concerted or stepwise. This study investigated the concertedness of the multiple dechlorination reactions of perchlorethylene (PCE) in EI-MS in light of the observed chlorine isotope effects during different dechlorination reactions along with the detected MS signal intensities. The fluctuant isotope ratios among different ions revealed that the observed multiple dechlorinated reactions in the EI-DFS-HRMS were stepwise. And the changing trends of isotope ratios of the ions detected by EI-qMS might suggest that the observed multiple dechlorination reactions in this MS were concerted. Mechanisms for these concerted/stepwise multiple dechlorination reactions are tentatively interpreted. Besides the evidence of isotope effects, according to the quasi-equilibrium theory, the MS signal intensities also suggested that the multiple dechlorination reactions in the EI-HRMS were likely stepwise, while those in the EI-qMS might be concerted. The distance from the ionization region to the mass analyzer entrance in an MS may be a critical factor affecting the dechlorination reactions of ions flying over this distance. This distance in the EI-qMS is significantly shorter than that in the EI-HRMS. Thus, the main detected ions in the EI-qMS might be generated during the ionization and in-source fragmentation processes, while the main ions detected in the EI-HRMS might be produced during the fragmentation of precursor ions flying over the distance. We deduce that the main dechlorination reactions in the ionization region were concerted, while those taking place during the flight of precursor ions over the distance from the ionization region to the mass analyzer entrance were stepwise.
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