The spectroscopy is done by analyzing the different displacements by a magnet. For equal charge, these displacements depend only on velocity, thus for the EI 50's constant kinetic energy conditions, these displacements are uniquely determined by a particle's mass.
Spectroscopy involves analyzing different magnetic shifts. When charges are equivalent, these shifts are solely based on velocity; therefore, for EI 50's fixed kinetic energy conditions, these shifts are precisely defined by a particle's mass.