evacuated via four stages respectively of a four stage turbo pump not shown via vacuum pumping ports 21 23 25 and 27 respectively ions are created in electrospray ion source 1 from liquid sample solution 2 flowing into sample probe 3 sample ions move from the ion source 1 through dielectric capillary 10 into the vacuum stage 18 the ions continue to pass through skimmer 14 rf multipole ion guide 16 and are focused into a collimated ion beam 33 by electrostatic lens assembly 29 the collimated ion beam 33 passes into orthogonal acceleration oa pulsing region 34 between pusher electrode 36 and grid intermediate electrode 37 during the u2018 ion filling u2019 period that is while the oa pulsing region 34 is field free the primary ion beam enters a region in the pulsing region 34 along an axis 17 that is parallel to the pusher electrode 36 and the grid intermediate electrode 37 surfaces during this u2018 ion filing u2019 period a segment of the ion beam 33 is pulse accelerated as an ion packet 35 from the pulsing region 34 toward tof flight tube 43 when pulse voltages are abruptly applied to and maintained on the solid pusher electrode 36 and grid puller electrode 38 during the u2018 pulse active u2019 period the ion.