Magnetic field focusing

No distortion, neither geometrical neither due to magnetic fields, thanks to proximity focusing. The non sensitivity of the tube to magnetic fields enables software enhancement of the images such as flat field corrections. 

Particles can be manipulated in suspension using strongly focused laser beams ( optical tweezers ) [25] or magnetic fields [26] and by collecting statistics on tire particle movements using video microscopy, infonnation on the particle interactions can be obtained. 

Through the use of sequential electric (electrostatic) and magnetic fields (sectors) and various correcting lenses, the ion beam leaving the ion source can be adjusted so that it arrives at the collector in focus and with a rectangular cross-section aligned with the collector slits. For the use of crossed electromagnetic fields. Chapter 25 ( Quadrupole Ion Optics ) should be consulted. 

An ion beam containing just two types of ion of m/z values 100 and 101 dispersed in space on passing through a magnetic field. After dispersal, ions of individual m/z value 100 or lOI are focused at points close to the entries of two elements of an array collector. Each element of the array is a point ion collector. 

By adjusting the magnetic field, the dispersed ion beam in Figure 30.2 can be moved up or down so that ions of specific m/z values can be focused at a point ion collector. 

Certain regions of a mass spectrometer have no electric or magnetic fields to affect an ion trajectory (field-free regions). Figure 32.3 illustrates three such regions in a conventional double-focusing instrument. 

The electric fields in such instruments are used to focus the fast-moving ion beam according to the kinetic energies of the ions contained in it. This property allows ions of individual m/z values to be focused sharply before or after deflection in the magnetic field. 

Thus, ions are produced, deflected in a magnetic field, then focused in an electric field, and finally detected by an electron multiplier or other ion detector. 

As with a light beam and glass lenses, an ion beam can be directed and focused using electric and magnetic fields, often called lenses by analogy with their optical counterparts. 

Electric and magnetic fields can be used sequentially to focus the beam of ions. The use of crossed electromagnetic fields is described in the discussion of quadrupoles (Chapter 25). 

By changing the strengths of the electric and magnetic fields, ions of different m/z values can be focused at just one spot (the collector). 

Magnetic analyzer. A direction-focusing device that produces a magnetic field perpendicular to the direction of ion travel. The effect is to bring to a common focus all ions of a given momentum with the same mass-to-charge (m/z) ratio. 

Once the primary electron beam is created, it must be demagnified with condenser lenses and then focused onto the sample with objective lenses. These electron lenses are electromagnetic in nature and use electric and magnetic fields to steer the electrons. Such lenses are subject to severe spherical and chromatic aberrations. Therefore, a point primary beam source is blurred into a primary beam disk to an extent dependent on the energy and energy spread of the primary electrons. In addition, these lenses are also subject to astigmatism. AH three of these effects ultimately limit the primary beam spot size and hence, the lateral resolution achievable with sem. 

Fig. 7. Thermomagnetic recording, (a) A focused laser beam generates a thermal profile in the magnetic layer, (b) The coercive force in the layer is reduced and its magnetisation can be reversed by a small magnetic field, here 30 kA/m. At room temperature, the coercive force is high and the written domains are...

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