Electric potential applied

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. 

A large electric potential applied to a needle provides a very intense field at its tip, where the radius of curvature is small. 

Figure 4-8. Basic components of a simple mass spectrometer. A mixture of molecules is vaporized in an ionized state in the sample chambers.These molecules are then accelerated down the flight tube by an electrical potential applied to accelerator grid A. An adjustable electromagnet, E, applies a magnetic field that deflects the flight of the individual ions until they strike the detector, D.The greater the mass of the ion, the higher the magnetic field required to focus it onto the detector.

The mass spectrometer is a homemade radial cylindrical energy analyzer. A radial electric field is produced by an electrical potential applied to the inner and outer concentric cylindrical electrodes. The radii of the inner and outer electrodes are 18 cm and 26 cm, respectively. In order to reduce... 

To use their nanotuhe nanotweezer, Kim and Lieher applied opposite electrical charges to the two gold electrodes (and, hence, to the two MWNTs). Since they carried opposite electrical charges, the two MWNTs were attracted to each other with a force that was proportional to the voltage applied to the electrodes. This force of attraction caused the ends of the two MWNTs to approach each other. The stronger the electrical potential applied, the closer the approach of the ends of the nanotweezers. Using this device, Kim and Lieher were able to pick up very small objects, such as polystyrene spheres with diameters of about 500 nm. 

A battery (or galvanic or voltaic cell) is a device that uses oxidation and reduction reactions to produce an electric current. In an electrolytic cell, an external source of electric current is used to drive a chemical reaction. This process is called electrolysis. When the electric potential applied to an electrochemical cell is just sufficient to balance the potential produced by reactions in the cell, we have an electrochemical cell at equilibrium. This state also occurs if there is no connections between the terminals of the cell (open-circuit condition). Our discussion in this chapter will be limited to electrochemical cells at equilibrium. 

Fig. 24.6. Inside an acid cooler. Fig. 9.5 gives an external view. Tubes start through the tube sheet , shown here. They extend almost to the far end of the cooler where there is another tube sheet . Cool water enters at this end and flows through the tubes to the far end. Between the tube sheets , the tubes are surrounded by warm acid moving turbulently around them. Heat transfers from the warm acid to the cool water (through the tube walls). The tube entering from the right contains a thermocouple. The polymer tubes in the foreground surround metal rods. The rods are bare between the tube sheets. An electrical potential applied between them and the water tubes anodically protects the tubes against acid side corrosion.

The relative elution volumes of the main peaks were 0.94 of Vo and 1.27 of Vo for two different electric potentials applied (100 and 150 mV, respectively). The relative elution volumes were inversely proportional to the relative velocities with respect to the average velocity of the carrier liquid and corresponded to the positions of the focused zones inside the channel. 

Electroseparation is defined as the use of electricity or electromagnetic fields to produce and enhance chemical or physical separation [2]. Tsouris and DePaoli [3,4] have presented brief reviews of this topic. Essentially the electrical potential applied between two electrodes is used to promote physical or chemical processes that are not favorable or are too slow under nonelectric process conditions. In the past few decades, scientists have tried to combine the advantages of both electrical and membrane processes. Electrodes with the porosity of a membrane (i.e., electromembranes) offer an advantage in terms of contact pollutants that are forced through the pores of an electromembrane are more likely to be adsorbed, decomposed, oxidized, or reduced than when passing along the relatively low surface area of a dense electrode. 

FIG. 27 Adsorption isotherms for ethylenediamine on a commercial activated carbon (Nuchar WV-M, Westvaco) , no electric potential applied , -0.5 V , —1.0 V. (Adapted from Ref. 670.)... 

Since the definition of E° in Eq. (96) refers to standard conditions, that is, conditions in which no external electrical field applies on the reactant or the product, it is more convenient to consider the act of electron transfer as a succession of three individual steps as outlined in Scheme 5, which is reminiscent of that in Scheme 3 established for the homogeneous analogous situation (Sec. II.C.l). In Scheme 5, Rq or Pq relates to R or P in the closest plane to the electrode where no electrical potential applies, that is, at the end of the diffuse layer, denoted Xq in Fig. 14d. Rd, and Pelectron transfer Xd, usually considered close to or slightly within the OHP, where an electrical potential Os (i.e., the electrical potential at the electron transfer site) applies. 

In Ref. 7, the case was studied in which the zeta potential of the wall was the linear function of the longitudinal coordinate. This situation may happen when the value of the zeta potential is controlled by the external electrical potential applied to the wall. Electrical potential value inside the capillary is naturally a linear function of the longitudinal coordinate x therefore, if the electrical potential applied to the outer boundary of the capillary wall is constant, then the potential difference across the wall is a linear function of X. The theoretical approach used in Ref. 7 is similar to the one in Ref. 5. Secondary parabolic flow was shown to be generated, leading to the increase of HETP. It was predicted theoretically, and verified experimentally, that a pressure profile superimposed on the capillary can, in some cases, compensate for the disturbed profile and reduce the HETP value. 

Now, ESI can be considered as an electrolysis cell and the ion transport takes place in the liquid, not the gas phase. The oxidation reaction yield depends on the electrical potential applied to the capillary, as well as on the electrochemical oxidation potentials from the different possible reactions. Kinetic factors can exhibit only minor effects, considering the low current involved. 

In the case of an electric potential applied to the metal, as in electrocatalysis, U(r) = U(r) + eV. However, the modified energy is the total energy, so the kinetic operator has to be also modified in an unknown form. We define the Bloch electrons as those obeying the periodic Schrodinger equation and the free electrons as those obeying a zero periodic potential. 

An electrochemical cell is a type of electrical circuit. As such, it may be modeled with an electrical analog circuit. The potentiometric cell can be considered to be an electrical potential applied to a capacitor and a resistor in series. The capacitor represents the interface between the electrode and the solution, the applied potential is the solution Eh, and the resistor represents the heterogeneous kinetics of the aqueous redox species. The term "heterogeneous kinetics" denotes electron transfer between different phases, in this case aqueous species and the noble-metal electrode. The time required for the capacitor to equilibrate with the applied potential depends on the size of the capacitor and the electrical current. 

Overall, it appears that the effects of electric fields on detachment, adhesion, and biofilm formation on electrodes are a result from electric current exchanged rather than the electric potential applied (Fig. 18.1b). 

A selective electrochemical modification of metallic SWNTs that have been wired to electrodes on a surface, has been achieved by Balasubramanian et al. [122]. The authors used, similarly to Strano et al., a diazonium agent to functionalize metallic SWNTs. Here, the reaction is driven by an electrical potential applied between electrodes in contact with tubes and a counter-electrode. The authors found conditions in which primarily metallic SWNTs react, once the semiconducting SWNTs are driven into the nonconducting state by an appropriate gate voltage. An experimental verification of the selective modification is given by transport measurements in which the signature of metallic SWNTs disappears after treatment. 

The electrospray nebulization is the result of a 2-5 kV electrical potential applied to the solvent emerging from a capillary. The field pulls out the liquid from the inlet capillary into a conical tip. 

The charges, with the signs opposite to the sign of the electric potential applied to the MS inlet, accumulate on the liquid meniscus at the capillary outlet. This process is followed by the formation of a Taylor cone and Coulomb explosions which lead to the formation of fine droplets. The mechanism responsible for generation of charged droplets and ions in Pl-ESI appears to be similar to that found in the conventional ESI process (Section 2.4). 

FIGURE 12.15 Schematic representation of the switching of mixed TEGT-biotinylated peptide SAMs between a bioactive and bioinactive state. Depending on the electrical potential applied, the peptide can expose (-1-0.3 V) or conceal (—0.4 V) the biotin site and regulate its binding to neutravidin [33]. For a color version of this figure, see the color plate section. 

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