Predicted By Determining The Initial Ionization Mass Spectral Characteristics Of The Indole Alkaloids Fragmentation Regularity

Mass spectrometry has become an indispensable instrument in modern analytical technology due to its high sensitivity,rapid analysis,low sample and accurately identifying unknown compounds. Particluarly when various chromatography coupled to the high separation efficency of different kinds of gas chromatography (GC), it becomes a powerful tool for complex mixtures analysis. However, the relationship between mass spectrometry and molecular structures of compounds is very complex, when compared with the nuclear magnetic resonance, the information providing by mass spectrometry is very less. the interpretion of mass spectral fragmentation patterns, in terms of structural elucidation of the preionization molecules, is still a challenging puzzle for many compound classes. The aim of this work is to study on the fragmentation mechanisms of Electron-Impact organic mass spectrometry by quantum chemistry based on the electron transfer law and point of view of energy, through mass spectral interpretation of indole alkaloid derivatives under the electron-impact.To predict preferred decomposition pathways in electron-impact mass spectrometry, a new approach, which is applied to determine the most likely initial ionization site by the natural spin density and the change of the charges and molecular structures from neutral to cationic form based on the DFT theory at B3LYP/6-31+G (d, p) level, has been proposed for the first time in this study. The characteristic mass spectral fragmentation of simple indole alkaloids are elucidated by determining the localization of intial ionization site using this method and the predicted results are in good agreement with the experimental mass spectra from National Institute of Standards and Technology (NIST). Results show that this method is superior to the traditional method, which is based on the ionization energy of comparable functional groups.To further test the reliability and accuracy of this new method, the energy and structural properties of reactants, products, neutral fragments, and transition states related to the dissociation of indole alkaloid derivatives in El source are calculated and analysed based on the Density Functional Theory (DFT) at the B3LYP/6-31+(d, p) level. Results show that the method presented in this study is feasible and reliable for determination of favorable ionization site in El mass spectra. More importantly, it has been further demonstrated that for those compounds containing heteroatoms such as nitrogen, the main primary a-cleavage fragmentations could be directly determined as long as the initial ionization site is assured.