Reflectron = Ion Mirror

Even with pulsed extraction, there is is still a distribution of kinetic energies for the same molecules, which means that resolution and accuracy drop. The reflectron compensates for the range of velocities: Ions of the same mass, but higher start energy drift deeper into the counter field, have thus to take a longer distance and catch up with the slower moving ions at a certain point after the reflectron. This is exactly the location, where the detector should be placed ("focusing point").

The focusing point would be different for different m/z ranges. In practice, instead of moving the detector for optimized measurements in different m/z ranges, the voltages in the reflectron plates are varied.

This is not necessary, when a quadratic field reflectron is employed. In a quadratic field reflectron the resolution of the fragment peaks is constant over the complete mass range making it useful for PSD analysis. Only one spectrum has to be acquired to obtain a complete PSD spectrum.