Tang Caiming, Tan Jianhua, Fan Yujuan, Zheng Ke, Yu Zhiqiang, Peng Xianzhi
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences Guangzhou 510640 China
Guangzhou Quality Supervision and Testing Institute Guangzhou 510110 China.
RSC Adv. 2020 Apr 4;10(23):13749-13758. doi: 10.1039/c9ra09155f. eCollection 2020 Apr 1.
Revelation of chlorine and bromine isotope effects in fragmentation is crucial for compound-specific isotope analysis of chlorine/bromine (CSIA-Cl/Br) using gas chromatography-electron ionization mass spectrometry (GC-EI-MS), but theoretical fundamentals of the isotope effects remain unclear. Herein, this study provides a theoretical basis for elucidating the details and implications of chlorine and bromine isotope effects occurring in dehalogenation reactions in EI-MS. Inter-ion and intra-ion isotope effects can occur in dehalogenation reactions in EI-MS, and affect chlorine/bromine isotope ratios. In a dehalogenation reaction, inter-ion isotope effects increase the isotope ratio of a precursor ion but decrease that of its product ion. On the other hand, intra-ion isotope effects can only affect (increase) the isotope ratio of a product ion, and have no effect on its precursor ion. The chlorine/bromine isotopologue distributions of ions measured by EI-MS are deduced to be non-binomial (nonrandom), regardless of the initial isotopologue distributions prior to fragmentation. The bulk chlorine/bromine isotope ratio of an ion cannot be exactly achieved with a calculation scheme using an arbitrary pair or pairs of neighboring chlorine/bromine isotopologues, but can be calculated with complete isotopologues of the ion. The isotope ratio of a compound calculated with a pair/pairs of neighboring isotopologues could not accurately reflect the trueness, even though it has been calibrated with external isotopic standard(s), due to different isotopologue distributions of the analyte and external isotopic standard(s). The conclusions derived from theoretical derivation have been experimentally proven with the isotopically distinct standards of tetrachloroethylene and trichloroethylene from different manufacturers. The findings of this study are conducive to CSIA-Cl/Br using GC-EI-MS to obtain high-quality data, and provide new insights into the actual chlorine/bromine isotopologue distributions of chlorinated/brominated organic compounds.
揭示碎片化过程中的氯和溴同位素效应对于使用气相色谱 - 电子电离质谱法（GC - EI - MS）进行氯/溴化合物特异性同位素分析（CSIA - Cl/Br）至关重要，但同位素效应的理论基础仍不清楚。在此，本研究为阐明EI - MS中脱卤反应中发生的氯和溴同位素效应的细节及影响提供了理论依据。EI - MS中的脱卤反应会发生离子间和离子内同位素效应，并影响氯/溴同位素比值。在脱卤反应中，离子间同位素效应会增加前体离子的同位素比值，但会降低其产物离子的同位素比值。另一方面，离子内同位素效应只会影响（增加）产物离子的同位素比值，而对其前体离子没有影响。据推断，无论碎片化之前的初始同位素异构体分布如何，通过EI - MS测量的离子的氯/溴同位素异构体分布都不是二项式的（非随机的）。使用任意一对或多对相邻的氯/溴同位素异构体的计算方案无法准确实现离子的总体氯/溴同位素比值，但可以使用离子的完整同位素异构体进行计算。即使使用外部同位素标准进行了校准，由于分析物和外部同位素标准的同位素异构体分布不同，用一对或多对相邻同位素异构体计算的化合物同位素比值也无法准确反映真实性。从理论推导得出的结论已通过不同制造商生产的四氯乙烯和三氯乙烯的同位素不同标准进行了实验验证。本研究的结果有助于使用GC - EI - MS的CSIA - Cl/Br获得高质量数据，并为氯化/溴化有机化合物的实际氯/溴同位素异构体分布提供新的见解。
