uniform momentum heavier ions may move at a slower velocity while lighter ions may move at a faster velocity as ions 307 pass through aperture 313 in ion lens 310 the electrostatic force generated by ion lens 310 may focus or defocus ions 307 in some embodiments as ions 307 travel from ion lens 310 to end cap 316 the lighter ions will be accelerated by ion lens 310 in the y direction perpendicular to the axis connecting aperture 313 and aperture 319 more than the heavier ions as discussed above in situations where ions 307 have uniform velocity the larger acceleration causes larger deflection of the lighter ions however when ions 307 enter ion lens 310 with uniform momentum the lighter ions may be traveling at a faster velocity than the heavier ions therefore even if the lighter ions experience greater acceleration in the y direction the lighter ions also traverse the distance between ion lens 310 and end cap 316 more quickly accordingly the lighter ions traverse this distance in less time which results in smaller deflections in the y direction before the lighter ions arrive at end cap 316 the heavier ions on the other hand travel the distance between ion lens 310 and end cap 316 more slowly allowing for more time during which the heavier ions are deflected in the y direction in some embodiments the fact that lighter ions are accelerated in the y direction more than the heavier ions but the heavier ions take longer to arrive at end cap 316 than the lighter ions may result in lighter ions and heavier ions being deflected by relatively the same amount therefore.