Electrical desorption

There are many other experiments in which surface atoms have been purposely moved, removed or chemically modified with a scanning probe tip. For example, atoms on a surface have been induced to move via interaction with the large electric field associated with an STM tip [78]. A scaiming force microscope has been used to create three-dimensional nanostructures by pushing adsorbed particles with the tip [79]. In addition, the electrons that are tunnelling from an STM tip to the sample can be used as sources of electrons for stimulated desorption [80]. The tuimelling electrons have also been used to promote dissociation of adsorbed O2 molecules on metal or semiconductor surfaces [81, 82]. 

Bombardment of a liquid surface by a beam of fast atoms (or fast ions) causes continuous desorption of ions that are characteristic of the liquid. Where the liquid is a solution of a sample substance dissolved in a solvent of low volatility (often referred to as a matrix), both positive and negative ions characteristic of the solvent and the sample itself leave the surface. The choice of whether to examine the positive or the negative ions is effected simply by the sign of an electrical potential applied to an extraction plate held above the surface being bombarded. Usually, few fragment ions are observed, and a sample of mass M in a solvent of mass S will give mostly [M + H] (or [M - H] ) and [S -I- H]+ (or [S - H] ) ions. Therefore, the technique is particularly good for measurement of relative molecular mass. 

The process of field ionization presupposes that the substance under investigation has been volatilized by heat, so some molecules of vapor settle onto the tips held at high potential. In such circumstances, thermally labile substances still cannot be examined, even though the ionization process itself is mild. To get around this difficulty, a solution of the substance under investigation can be placed on the wire and the solvent allowed to evaporate. When an electric potential is applied, positive or negative ions are produced, but no heating is necessary to volatilize the substance. This technique is called field desorption (FD) ionization. 

In field ionization (or field desorption), application of a large electric potential to a surface of high curvature allows a very intense electric field to be generated. Such positive or negative fields lead to electrons being stripped from or added to molecules lying on the surface. The positive or negative molecular ions so produced are mass measured by the mass spectrometer. 

Field desorption. The formation of ions in the gas phase from a material deposited on a solid surface (known as an emitter) that is placed in a high electrical field. Field desorption is an ambiguous term because it implies that the electric field desorbs a material as an ion from some kind of emitter on which the material is deposited. There is growing evidence that some of the ions formed are due to thermal ionization and some to field ionization of material... 

The minimum number of postulates of the model of a desorption process with no explicit analytical expression of the heating schedule are required if the primary output data are treated according to Eqs. (10) and (12), viz. by numerical or graphical derivations and integrations of the recorded pressure data. After an adaptation of the analyzer, these operations can be performed by means of electrical circuits. The known temperature-time relationship (either in the form of an analytical function or established... 

Figure 1.4. Possible pathways of 0(a) and Na(a) adsorbed species created at the three-phase boundaries via application of electric current (a) Desorption (b) Reaction (c) Backspillover.

Fig. 4 shows the TPD experimental results for lwt%-PA/AC with toluene and MEK. The desorption characteristics of lwt%-PA/AC with toluene and MEK were determined by raising the room temperature by 5°C /min to 300 0. Maximal desorption concentrations and temperatures of both toluene and MEK were 1.13,1.25 and 15010, loot, respectively. The toluene and MEK adsorbed on lwt%-PA/AC was completely desorbed when the adsorbent was heated in an electric furnace. The temperature of the adsorbent was programmed, and concentration of VOCs desorbed could be controlled at some extents. The desorbed efficiencies of toluene and MEK were 98.1% and 99.1%, respectively. Similar TPD... 

Figure 6. The sequence of events in a laser desorption FTMS experiment, (a) The laser beam enters the cell and strikes the crystal, (b) Some of the desorbed molecules are ionized by an electron beam, (c) Ions are trapped in the analyzer cell by the magnetic and electric fields, (d) Ions are accelerated by an RF pulse and the resulting coherent image current signal is detected. Reproduced with permission from Ref. 18. Copyright 1935, North-Holland Physics Publishing.

Electrode polarization is associated with a change in EDL charge density at the electrode surface. Other changes in surface state of the electrode are possible, too (e.g., the adsorption or desorption of different components, which also involve a consumption of electric charge). By convention, we describe this set of nonfaradaic processes as charging of the electrode surface. 

As it was mentioned in paper [48], expressions (2.44) and (2.45) perfectly describe experimental situation with kinetics of tiie change of electric conductivity of sintered and partially reduced ZnO film during adsorption and desorption of molecular oxygen. Expression (2.44) describes the kinetics of the change of a during adsorption of O2 on ZnO with the surface rich in donors due to photolytic decomposition of ZnO in vacuum fairly well [74]. 

Taking into account the studies of electric conductivity as a function of Phj as well as the studies of kinetics of electric conductivity accompanying desorption of hydrogen the authors of paper [89] established that apart from processes (2.59) - (2.61) there is a recombination... 

Thermal desorption of solid traps by microwave energy is unsuitable for thermally labile compounds. In microwave thermal analysis [431] the (solid) sample is heated directly via interactions of the microwaves with the sample, providing more even heating and reduction of temperature gradients in comparison to heating with electrical furnaces. By passing air over a microwave-heated volatile sample evolved gases may be collected [432]. 

Electroneutral substances that are less polar than the solvent and also those that exhibit a tendency to interact chemically with the electrode surface, e.g. substances containing sulphur (thiourea, etc.), are adsorbed on the electrode. During adsorption, solvent molecules in the compact layer are replaced by molecules of the adsorbed substance, called surface-active substance (surfactant).t The effect of adsorption on the individual electrocapillary terms can best be expressed in terms of the difference of these quantities for the original (base) electrolyte and for the same electrolyte in the presence of surfactants. Figure 4.7 schematically depicts this dependence for the interfacial tension, surface electrode charge and differential capacity and also the dependence of the surface excess on the potential. It can be seen that, at sufficiently positive or negative potentials, the surfactant is completely desorbed from the electrode. The strong electric field leads to replacement of the less polar particles of the surface-active substance by polar solvent molecules. The desorption potentials are characterized by sharp peaks on the differential capacity curves. 

Thermal conduction (also referred to as electrical conductive heating or in situ thermal desorption) supplies heat to the soil through steel wells or with a blanket that covers the ground surface. As the polluted area is heated, the contaminants are destroyed or evaporated. Steel wells are used when the polluted soil is deep. The blanket is used where the polluted soil is shallow. Typically, a carrier gas or vacuum system transports the volatilized water and organics to a treatment system. 

It is suggested that the movement of the front by migration (electrical potential), diffusion (chemical potentials), and advection (hydraulic potentials) will cause desorption of cations and other species from clay surfaces and facilitate their release into the fluid.34... 

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