基于密度泛函理论的三聚氰胺质谱裂解过程研究

本研究运用密度泛函理论的第一性原理计算方法，在考虑准分子离子自旋的情况下，采用B3LYP/6-311++G（2d,2p）高精度基组全自由度优化了各质谱碎片准分子离子的稳定几何构型，计算了质谱分析中三聚氰胺形成的各碎片离子的键断裂能，分析了3种中间离子碎片Mulliken原子电荷分布，进而推导出三聚氰胺的质谱裂解途径。计算结果表明，三聚氰胺准分子离子m/z 127通过环断裂方式逐级裂解生成m/z 85和m/z 68碎片离子，其中部分m/z 68碎片离子进一步裂解形成m/z 43碎片离子。采用三重四极杆质谱仪，在正离子多反应监测扫描模式（MRM）下，观测到三聚氰胺质谱裂解的主要碎片离子有C₂H₅N₄⁺（m/z 85）、C₂H₂N₃⁺（m/z 68）和CH₃N₂⁺（m/z 43），且其质谱峰信号强度依次减小，该现象进一步验证了裂解能理论计算的可靠性。此外，实验还发现，准分子离子的质谱裂解途径主要受碎片离子Mulliken电荷分布及其化学键相互作用强弱的影响。该方法有助于三聚氰胺结构的准确鉴定，也可为三聚氰胺的检测、研究和应用提供理论依据。

Research of Melamine Mass Fragmentation Process Based on Density Functional Theory

Melamine analysis detection method has become one of the leading subjects of interest to experts in the field of chemical, food and the environment. The use of quantum chemistry and molecular mechanics, such as a computational chemistry research fragmentation mechanism, play an irreplaceable role in the experiment. According to the related reported experimental phenomenon, the first-principles calculation based on density functional theory was used considering spin polarization effect of atoms. The stable geometry of each mass fragment ion’s excimer was optimized for each fragment ions, and the fracture energy of bond formation was calculated at the level of B3LYP/6-311 G (2d,2p). Mulliken atomic charge distribution of three kinds of intermediate ion fragments was also analyzed, thus fragmentation pathway of melamine was obtained. The calculated results show that pseudo-molecular ions m/z 127 of melamine are cleaved by ring fracture mode, and a large number of fragment ions m/z 85 and m/z 68 are generated mainly by way of stepwise cracking. Subsequently, some fragment ions m/z 68 are further cleaved to form fragment ions m/z 43. Using positive ion multi-reactions monitoring (MRM) technology of triple-quadrupole mass spectrometer, major fragment ions of the melamine C₂H₅N₄⁺ (m/z 85), C₂H₂N₃⁺ (m/z 68) and CH₃N₂⁺ (m/z 43) were detected, and their signal intensity of mass spectrometry were in turn decreased in our experimental observations, which further verify the correctness of the theoretical calculation of the dissociation energy. In addition, the theoretical calculation also found that the molecular mass spectrometry of excimer ion was mainly influenced by the Mulliken charge distribution of the fragment ions and the chemical bonds between atoms. This method can be used to identify the melamine structure and provide a theoretical basis for the detection, research and application of melamine.