飞行时间质谱峰值的漂移

飞行时间质谱仪(time-of-flight mass spectrometer, TOF-MS)在分子团簇的激光电离/解离动力学研究中广泛使用。文章报道了在用脉冲355 nm的YAG激光进行水/甲醇二元团簇的多光子电离研究中，发现在电离激光相对于脉冲分子束的不同延时下，即激光作用于脉冲束的不同位置，飞行时间质谱仪测得的离子的质谱峰值发生漂移。在激光作用于脉冲束的中段时，离子的信号最强，同时离子的峰值漂移达到最大。分析认为：这种峰值漂移不是因为新质量数谱峰的出现，而是离子在穿越质谱仪的离子引出区和加速区极板时发生部分离子吸附，引起极板间电压的起伏造成的。离子在电场起伏下的数值模拟与实验中观测到的离子峰值漂移规律一致。

Deviation of Mass Peak Position in Time-of-Flight Mass Spectrum

Time-of-flight (TOF) mass spectrometer is widely used in many research areas including the study of dynamics of laser ionization/dissociation molecular clusters. A new phenomenon was observed when we used YAG 355 nm laser to ionize water/methanol mixture molecular beam and detect the ions with time-of-flight mass spectrometer: the peak positions of mass ions shift when changing the laser delay time to the molecular beam and keeping other conditions unchanged. The flight time shortens to a minimum value when the laser ionizes the middle part of the molecular beam, while the ions intensity reaches the maximum. The peak shift does not mean a new mass number ion appearing through mass spectra identification. The reason is deduced to be the voltage fluctuation on the extract electrode of TOF when ions pass through the grating electrode and part of them are being absorbed. The numerical simulation of charge-particle movement in electrical field also supports the deduction, and is consistent with the experimental results.