Reflectrons are ion optical devices that reverse the direction of travel of an ion (reflect) in a mass spectrometer. There are many types of reflectrons but the most common is the ion mirror. The ion mirror is a series evenly spaced electrodes onto which a single, linear, electric field is applied. Figure 1 contains a simple diagram of the potential energy of a TOF mass spectrometer that incorporates an ion mirror. Us represents the potential energy of the ion source and Ur represents the potential energy of the ion mirror. Notice how the potential of the ion mirror is greater than the potential of the ion source. Ions formed in the source obtain kinetic energy less than or equal to the potential applied to the source. If the potential energy of the ion mirror is greater than the source potential, ions that enter the ion mirror travel up the potential hill to the point that matches the energy obtained from the source, they stop, and then return back down the hill.
Figure 1. Illustration of the potential energy relationship in a TOF mass spectrometer with an ion mirror.
A detector is positioned on the entrance side of the ion mirror to capture the arrival of ions after they are reflected. There are two common methods of positioning the detector: (1) co-axial with the initial direction of the ion beam and (2) off-axis with respect to the initial direction of the ion beam. Figures 2 illustrates the positioning of the ion mirror and detector with respect to the initial direction of the ion beam. The off-axis method is the most common instrument geometry.
Figure 2. Instrument geometry for TOF mass spectrometers with co-axial (top) and off-axis (bottom) ion mirrors.