Electrostatic mirrors

Figure 5 shows a ray path in TEM equipped with a Castaing-Henry imaging filter lens (Zeiss CEM-902). The imaging filter lens consists of a double magnetic prism and an electrostatic mirror. There is a limitation to accelerating... 

Time Focusing Devices. The resolution of the TOF analyzer is limited by the initial velocity spread of the ions. However, there are powerful devices that can compensate for this velocity distribution, and the most widespread techniques at present are the electrostatic ion reflector (electrostatic mirror) and time-lag focusing (delayed extraction). 

J. Joret, H. LeBeyec, Y. TOF-MS With a Compact Two-Stage Electrostatic Mirror Metastable-Ion Studies With High Mass Resolution and Ion Emission From Thick Insulators. Rapid Commun. Mass Spectrom. 1991,5,40-43. 

The second way to improve the mass resolution significantly is to use an electrostatic mirror (mass reflectron) placed in the drift region of ions (Fig. 1.27). 

Figure 16.6—Linear time of flight (TOF) and principle of the reflectron. 1) Sample and sample holder 2) MALDI ionisation device 3 and 3 ) extraction and acceleration grid (5 000 V potential drop) 4) control grid 5) multichannel collector plate 6) electron multiplier 7) signal output. The bottom figure shows a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass, but which have different initial energies. This device increases resolution, which can attain several thousand.

An electrostatic mirror can be produced by an electrode at a potential energy that is greater than the kinetic energy divided by the charge of the particle. The bending and focusing power of electrostatic systems are limited by the maximum electric fields that can be applied across the electrodes. Extensive electrostatic systems have been constructed for the transport of low-energy beams, KE < 50 keV, for example, beams extracted from ion sources are usually transported with electrostatic elements. 

Another interface that needs to be mentioned in the context of polarized interfaces is the interface between the insulator and the electrolyte. It has been proposed as a means for realization of adsorption-based potentiometric sensors using Teflon, polyethylene, and other hydrophobic polymers of low dielectric constant Z>2, which can serve as the substrates for immobilized charged biomolecules. This type of interface happens also to be the largest area interface on this planet the interface between air (insulator) and sea water (electrolyte). This interface behaves differently from the one found in a typical metal-electrolyte electrode. When an ion approaches such an interface from an aqueous solution (dielectric constant Di) an image charge is formed in the insulator. In other words, the interface acts as an electrostatic mirror. The two charges repel each other, due to the low dielectric constant (Williams, 1975). This repulsion is called the Born repulsion H, and it is given by (5.10). 

For the evaluation of Aq, the same model as in the homogeneous case is used, yet the electrode is replaced by the electrical image of reactant for the initial system (or that of the product for the final system) through the electrode, which acts as a electrostatic mirror. However, since these images are imaginary, only the real contributions are considered for the final evaluation of Aq, given in Eq. (106). 

Figure 16.6 A simplified schematic of a time of flight spectrometer and the principle of the ion reflector (reflectron). (1) sample and sample holder (2) MALDI ionization device by pulsed laser bombardment (3 and (3 ) ions are formed between a repeUer plate and an extraction grid (PD 5000V) then accelerated by an other grid (4) control grid (5) microchannel collector plate (6) signal output. Below, a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass but which have initially different energies. The widths of the peaks are of the order of 10 and the resolution ranges between 15 to 20 000.

After that investigation, a series of questions naturally arose that led to the need to understand the real role of the surface. Is it an active region for sample ionization, or does it simply behave as an electrostatic mirror leading to better focusing into the entrance capillary orifice of ions previously produced If the latter hypothesis is true, how are the ions generated ... 

Figure 9.8 Schematic of a reflectron time of flight mass analyser. Reflectron lenses act as an electrostatic mirror to both increase the effective length of the flight path, but also to compensate for ion kinetic energy variations (Uq), resulting in higher mass accuracy relative to purely linear time of flight mass analyzers. Consequently, linear time of flight analyzers are nowadays largely obsolete.

The TOF is the most widely used analyzer for SIMS experiments. The TOF is based on the measurement of the time elapsed between the impact of the pulsed primary beam on the sample surface and the detection of the emitted secondary ions by the ion detector. The flight time of ions with different m/z (typically larger than 1 ps) is proportional to the ratio itself and is used to analyze the different ions. TOF analyzers are often used in conjunction with pulsed primary ion sources because the latter offer the possibility to synchronize the ion detection with the primary ion source pulse frequency. Reflectron TOF analyzers compensate for the secondary ion kinetic energy dispersion by using an electrostatic mirror that gradually reflects ions with the same m/z but with different kinetic energy. 

In this section, we will see that bipolar plates serve as electrostatic mirrors they replicate the disturbance induced by the resistive spot to a number of adjacent cells (Kulikovsky, 2007c). An analysis of the governing equations shows that the number of cells affected by this mirroring is inversely proportional to the square root of the BP electric conductivity. 

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