For a model series of nitro aromatic compounds comprising mono-, di- and trinitrotoluenes, and dinitrobenzene, using the same commercial experimental set up consisting of gas chromato-graphy/mass spectrometry (GC/MS) instrument equipped with a quadrupole analyzer, the two types of mass spectra were recorded, of positive ions (in the conventional, standard mode of electron ionization at 70 eV) and of negative ions (NI, generated at low, pre-ionization electron energies by resonant electron capture, REC). The comparative analysis of characteristic fragmentation processes was carried out for molecular ions of both signs. The sensitivity of REC NI MS method was established to enhance upon increasing the number of electron-withdrawing nitro groups, reaching or even outperforming that of the classical method of electron ionization (EI). Thus, the detection limit for trinitrotoluene in a solution was estimated as 5 ppm, which is by an order of magnitude lower as compared to the conventional EI mode of analysis. Comparison of the ion composition of NI mass spectra and of the relative intensities of ion peaks with the corresponding data obtained previously for the studied group of compounds by means of different types of mass spectrometers, has revealed a satisfactory inter-instrumental reproducibility of the NI mass spectra. The results reported herein indicate that the NI generation/registration mode, which can technically be readily available even without modifications of design of commercial GC/MS instrument in their basic configuration, is suitable to be introduced in laboratory practice as the additional tool for increasing the selectivity and reliability in detection/identification of trace amounts of compounds possessing a high electron af nity and for solving extraordinary structural and analytical problems.

中文翻译：

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正负离子质谱方法的灵敏度比较，以硝基甲苯为例

对于包含单、二和三硝基甲苯和二硝基苯的硝基芳族化合物模型系列，使用由配备四极杆分析仪的气相色谱/质谱 (GC/MS) 仪器组成的相同商业实验装置，两个记录了正离子（在 70 eV 的常规标准电子电离模式下）和负离子（NI，通过共振电子捕获在低电离前电子能量下产生，REC）的质谱类型。对两种符号的分子离子进行了特征碎裂过程的比较分析。REC NI MS 方法的灵敏度随着吸电子硝基数量的增加而提高，达到甚至超过经典电子电离 (EI) 方法的灵敏度。因此，溶液中三硝基甲苯的检测限估计为 5 ppm，与传统的 EI 分析模式相比低一个数量级。NI 质谱的离子组成和离子峰的相对强度与之前通过不同类型的质谱仪为研究的化合物组获得的相应数据的比较表明，NI 质量的仪器间重现性令人满意光谱。此处报告的结果表明，即使不修改商业 GC/MS 仪器的基本配置设计，NI 生成/注册模式在技术上也很容易获得，