[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.