Potential drop accelerators

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.

In El, it is customary to use an ionisation energy of 70 eV. This is achieved by accelerating the electron produced by the filament through a potential drop of 70 V, applied between the filament and the chamber. Ionisation efficiency in El is in the order of one ion produced for every 10000 molecules. In some cases, reducing... 

Positive ions drift to the sheath edge where they encounter the strong field. The ions are then accelerated across the potential drop and strike the electrode or substrate surface. Because of the series capacitor or the dielectric coating of the electrodes, the negative potentials established on the two electrodes in a plasma system may not be the same. For instance, the ratio of the voltages on the electrodes depends upon the relative electrode areas... 

Thus, accelerating a population of ions across a chosen potential drop (/, allows them all to achieve the same kinetic energy eJJ, where e is the electronic charge. 

This wave-particle duality applies also to matter. Electrons, protons, neutrons, and other material panicles have been found to have properties which we usually correlate with wave motion. For example, a beam of electrons can be diffracted in the same way as a beam of X-rays. The wavelength associated with an electron depends upon the speed with which it is traveling. For electrons which have been accelerated by a potential drop of 40,000 volts, the wavelength is 0.06 A. 

A new accelerator, the synchrotron, proposed by Professor E. M. McMillan of the University of California and independently by V. Veksler in Russia, and now under construetion, is expected to yield particles wdth speeds corresponding to a potential drop of several billion volts. 

Ion collisions at either of the cathodes cause secondary electron emission. These electrons are accelerated by a sizeable potential drop toward the center of the anode but are constrained by the magnetic field to pass through the anode cylinder rather than travelling directly to the anode. The paths of electrons not initially normal to the anode axis are necessarily helical. As the electrons pass through the anode they are slowed down and reflected before the other plane cathode. Thus, the electrons are caused to execute oscillatory helical trajectories back and forth through the cylindrical anode. The trajectories of the electrons have been considered by... 

That is, a fast ion, formed by electron impact and accelerated toward the plasma surface, excites the H ion to form H , and then the H ion reacts via a third body electron capture process to form the ion inferred from the experiments. Only the ions formed at the surface of the plasma could be accelerated through the same potential drop as the primary H ions. Further, Schnitzer and Anbar argue that since there is no significant change in the Ha ion current as the ion source pressure is varied, the formation of does not occur within the... 

Rather than use either microwave (MW, 2.45 GHz) or radio frequency (RF, 13.56 MHz) power to sustain our plasma, we often combine the two power sources to generate a so-called "mixed" (or dual-) frequency plasma, as shown in a schematic view of the plasma tqrparatus. Fig. 3 [15]. While MW excitation generates a high concoitration of active i redes in the gas phase, as pointed out above, the role of the RF power is to create a negative DC self-bias voltage Vg on the powered, electrically isolated substrate holder electrode. This causes ions to be accelerated 1 the potential drop (Vp-Vg) across the RF-induced plasma sheath, to their maximum kinetic energy... 

Thus far very little has been said about how intact ionized molecules, e.g. those formed by soft ionization techniques like the API methods (Sections 5.3.3-5.3.6), can be induced to dissociate for subsequent MS/MS analysis. (In fact any ion produced in an ion source can be miz selected and subjected to MS/MS analysis). Soft ionization does not produce metastable ions (see above) in any abundance if at aU. Historically the most common method of ion activation has heen coUisional activation (CA), wherehy ions are accelerated through a defined potential drop to transform their electrical potential energy into kinetic (translational) energy, and are then caused to collide with gas molecules that are dehherately introduced into the ion trajectory the history of this approach has heen described in an excellent overview (Cooks 1995). This involves conversion of part of the ions kinetic energy into internal energy that in turn leads to fragmentation. It is still by far the most commonly used method. 

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