Reflectron ion mirror

Fig. 11.13. Diagram of a TOF mass analyzer (with reflectron). Ions enter from an external source and are accelerated (orthogonally) by the pusher electrode toward the reflectron. The reflectron (ion mirror) retards, reverses and reaccelerates the ions back toward the micro-channel plate detector.

Fig. 24. Cartoon demonstrating the effect of the reflectron (ion mirror) on the ions flight path in...

Reflectrons (ion mirrors) are used to increase the resolution, as well as the ability to obtain accurate mass measurements, by counteracting the effect of the energy spread not compensated for by collisional cooling at the delayed extraction grid (in MALDI sources). A reflectron is a cylinder composed of concentric rings and grids... 

Fig. 17. Schematic representation of a reflectron (ion mirror) in a TOF analyzer Ions with the same m/z formed with higher initial velocity (--) penetrate deeper in the electrostatic field of the reflec-...

In (a), a pulse of ions is formed but, for illustration purposes, all with the same m/z value. In (b), the ions have been accelerated but, because they were not all formed in the same space, they are separated in time and velocity, with some ions having more kinetic energy than others. In (c), the ions approach the ion mirror or reflectron, which they then penetrate to different depths, depending on their kinetic energies (d). The ones with greater kinetic energy penetrate furthest. In (e), the ions leave the reflectron and travel on to the detector (f), which they all reach at the same time. The path taken by the ions is indicated by the dotted line in (f). 

By use of an electrostatic ion mirror called a reflectron, arrival times of ions of the same m/z value at the detector can be made more nearly equal. The reflectron improves resolution of m/z values. 

The mass accuracy and resolution of TOF analyzers was improved by the reflectron design in which traveling ions are reflected by an ion mirror and are turned around in their flight paths. 

The reflector or reflectron has been developed by Mamyrin. [35] In the reflector TOF analyzer - often abbreviated ReTOF - the reflector acts as an ion mirror that focuses ions of different kinetic energies in time. Its performance is improved by using two-stage or even multistage reflector designs. 

The reflectron is situated behind the field-free region opposed to the ion source. The detector is positioned on the source side of the ion mirror to capture the arrival of ions after they are reflected. There are two common methods of positioning the detector. It may be coaxial with the initial direction of the ion beam. This detector has a central hole to transmit the ions leaving the source. However, the most common instrument geometry has the detector off-axis with respect to the initial direction of the ion beam. Indeed, adjusting the reflectron at a small angle in respect to the ions leaving the source allows the detector to be positioned adjacent to the ion source. 

The resolution is limited by the length of the flight tube. A longer tube will result in improved separation between ions with different m/z. An ion mirror or reflectron (see below) can be used to double the length of the flight tube without compromising the dimensions of the instrument. 

ToF measurements can be performed using a linear or reflectron mass analyzer. The former comprises a basic ToF layout, whereas the latter uses an ion mirror to time-focus the ions onto the detector. The linear detector is used for intact protein analysis owing to its ability to detect a very wide mass range, up to 100,000 Da (26). The reflectron geometry offers higher mass resolution capabilities, but has a more limited mass range, typically below 5000 Da. 

The technology of MALDI-MS has been considerably improved in recent years. One factor that has led to much better resolution is the addition of a reflectron. This is an ion mirror with the electrode voltage arranged in such a way that it largely corrects for the initial velocity distribution of a packet of particular ion species. The correction increases the mass resolution, which is especially useful in case a mixture of analytes is assayed. 

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