Theoretical Study of High-Order Velocity Focusing Achieved with Single-Stage Reflectron Time-of-Flight Mass Spectrometry

This study explores unexplained fundamental principles of reflectron time-of-flight (R-TOF) mass spectrometry (MS). Conventional calculations focusing on the ion trajectory in reflectors concluded that multistage reflectors are necessary to achieve second-order velocity focusing at ion detectors. This study demonstrates that in an instrument equipped with a matrix-assisted laser desorption/ionization (MALDI) ion source a single-stage reflector can achieve second-order velocity focusing when the optimal experimental parameters predicted using the coupled space and velocity focusing (CSVF) principle are used. The optimization model indicates that the delayed extraction technique is more effective in compensating for the initial ion velocity spread than reflectors. The calculation shows that for ions with m/z of 10,000, the predicted maximum mass resolving power (Rm) can reach 750,000 using a single-stage R-TOF MS with an effective total length of about 2.4 m, or approximately 130,000 when accounting for the temporal response limit of ion detectors. The calculation model also reveals that in second-order focusing conditions, ions have two focusing points along the flight path, instead of just one at the detector as conventionally believed. The result indicates that the new model is critically important for the advancement of R-TOF MS.