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.
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.
Reflectrons have two properties that enhance the resolving power of an instrument. First, the length of the flight path is increased, increasing flight time and allowing a larger temporal distribution between ions of similar m/z. Second, the reflectron provides temporal focusing that can be exploited to reduce the arrival time distribution at the detector. The combination of greater flight time and greater time between the arrival of similar m/z (increase t) and a reduction in the arrival time distribution (decrease dt) produce the resolution enhancement (resolution = t / 2dt).
How does path length increase? The key thing to remember is that the measurent in the experiment is time. Ions travel in and out of the reflectron or twice the length of the reflectron. Furthermore, the ions are decelerated, stop, and re-accelerated. For an ion mirror (single stage reflectron), the average velocity is one half the entrance/exit velocity. Therefore, two passes at one-half the velocity results in a four time increase in flight time. Because flight path increases the flight time of ions increases. Also, the spacing in time between similar m/z ions increases.
How does the arrival time distribution decrease? Ions of the same m/z but with different kinetic energies (different initial velocities from the desorption) will travel different path lengths in the reflectron. Ions with greater kinetic energy arrive at the reflectron first but penetrate deeper into the field thus traveling a longer flight path in the reflectron than ions with less kinetic energy. If the geometry of the instrument is optimized such that dt in the drift region is equal to dt in the reflectron, then the net dt = 0. Therefore, ions of the same m/z but different kinetic energies can arrive at the detector at or nearly the same time. Figure 3 contains a graphical illustration of the focusing properties of an ion mirror.