Comparison of sensitivity for methods of positive and negative ion mass spectrometry, exemplified by nitrotoluenes

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.