Back-ionization

Back corona (back ionization) The discharge phenomenon that takes place from a particulate layer in an electrostatic precipitator. 

Back ionization Excessive buildup of charged particles limiting further deposition onto substrate. 

Fig. 2 is a schematic representation of the electric field lines from the gun to a part that has a protrusion at its center. In this case, the electric field lines will concentrate at the protrusion. The charged powder and free ions will build up faster here and contribute to back-ionization. Because the powder particles are strong dielectrics, their charge will be retained for many hours, depending on a number of factors such as powder resistivity, as well as the ambient relative humidity. 

Another technique to reduce back-ionization is to control the gun current. Controlling gun current helps reduce excessive free ions by automatically reducing the electrode voltage when the gun-to-part distance decreases. This method reduces back-ionization and the Faraday Cage effect (see below). 

In dynamic FAB, this solution is the eluant flowing from an LC column i.e., the target area is covered by a flowing liquid (dynamic) rather than a static one, as is usually the case where FAB is used to examine single substances. The fast atoms or ions from the gun carry considerable momentum, and when they crash into the surface of the liquid some of this momentum is transferred to molecules in the liquid, which splash back out, rather like the result of throwing a stone into a pond (Figure 13.2). This is a very simplistic view of a complex process that also turns the ejected particles into ions (see Chapter 4 for more information on FAB/LSIMS ionization). 

Referring back to equation 47, the other quantity necessary in calculating the gas conductivity is the coUision cross section, Gases contain at least four types of particles electrons, ionized seed atoms, neutral seed atoms, and neutral atoms of the carrier gas. Combustion gases, of course, have many more species. Each species has a different momentum transfer cross section for coUisions with electrons. To account for this, the product nQ in equation 47 is replaced by the summation where k denotes the different species present. This generalization also aUows the conductivity calculation to... 

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleo diilic substitution reactions. They are primary and do not form caibocation intermediates, but the /-butyl substituent efiTectively hinders back-side attack. The rate of reaction of neopent>i bromide with iodide ion is 470 times slower than that of n-butyl bromide. Usually, tiie ner rentyl system reacts with rearrangement to the /-pentyl system, aldiough use of good nucleophiles in polar aprotic solvents permits direct displacement to occur. Entry 2 shows that such a reaction with azide ion as the nucleophile proceeds with complete inversion of configuration. The primary beiuyl system in entry 3 exhibits high, but not complete, inversiotL This is attributed to racemization of the reactant by ionization and internal return. 

The enzyme carbonic anhydrase promotes the hydration of COg. Many of the protons formed upon ionization of carbonic acid are picked up by Hb as Og dissociates. The bicarbonate ions are transported with the blood back to the lungs. When Hb becomes oxygenated again in the lungs, H is released and reacts with HCO3 to re-form HgCOj, from which COg is liberated. The COg is then exhaled as a gas. 

Figure 14.10 Schematic diagram of the aromatics analyser system BP, back-pressure regulator CF, flow controller CP, pressure controller Inj, splitless injector with septum purge V, tliree-way valve column I, polar capillary column column 2, non-polar capillary column R, restrictor FID I, and FID2, flame-ionization detectors.

Figure 14.17 Schematic diagram of the on-line coupled LC-GC system VI, valve foi switcliing the LC column outlet to the GC injector V2, valve for switching the LC column to back-flush mode V3, LC injection valve RI, refractive index monitor detector UV, ulti avio-let monitor detector FID, flame-ionization detector.

The ease of removal of an electron from a gaseous atom, the ionization energy, is one of the factors that is important in fixing E°. Refer back to Table 21-111 and predict the trend in E° that this factor would tend to cause. 

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