Collector electrode

The most common selective detectors in use generally respond to the presence of a characteristic element or group in the eluted compound. This is well illustrated by the thermionic ionisation detector (TID) which is essentially a flame ionisation detector giving a selective response to phosphorus- and/or nitrogen-containing compounds. Typically the TID contains an electrically heated rubidium silicate bead situated a few millimetres above the detector jet tip and below the collector electrode. The temperature of the bead is maintained... 

Lead-acid Bipolar current collector, electrode additive... 

FIGURE 4-30 Cyclic voltammogram for ferrocene at a 3 pm width, 2 pm gap interdigitated microband (solid line). The dotted line represents the current of the collector electrode held at a potential of —0.1 V (Reproduced with permission from reference 95.)... 

An appreciable increase in working area of the electrodes can be attained with porous electrodes (Section 18.4). Such electrodes are widely used in batteries, and in recent years they are also found in electrolyzers. Attempts are made to use particulate electrodes which consist of a rather thick bed of particulate electrode material into which the auxiliary electrode is immersed together with a separator. Other efforts concern fiuidized-bed reactors, where a finely divided electrode material is distributed over the full electrolyte volume by an ascending liquid or gas flow and collides continuously with special current collector electrodes (Section 18.5). 

By locating the anode entirely upstream from the ionized gas volume, collection of long range beta particles is minimized in the displaced coaxial cylinder design, and the direction of gas flow minimizes diffusion and convection of electrons to the collector electrode. However, the free electrons are sufficiently mobile that modest pulse voltages (e.g., 50 V) are adequate to cause the electrons to move against the gas flow and be collected during. this time. 

The TID design proposed Patterson consists of an alkali metal doped cerwlc cylinder, containing an embedded heater surrounded by a cylindrical collector electrode [100]. The ceramic thermionic emitter is biased at a negative potential with respect to the collector electrode, and it is heated to a surface temperature of 400-800 C, depending on the mode of detection. The response of the detector to different elements depends on the electronic work function of the thermionic surface (i.e., the... 

Figure 5. Generation/collection experiment with poly(I)-coated microelectrodes in CH3CN/O.I M [n-Bu4N]PFg at 10 mV/s. The lower cyclic voltammograms are for the generator electrode as its potential is swept between -0.2 V and -0.9 V vs. Ag+/Ag while the potential of the collector electrodes is held at 0.0 V vs. Ag+/Ag.

Both half-cell reactions are fully electro-reversible. In practice, there are two types of lead the collector electrode is made of lead alloy mesh in order to give it greater structural strength, and is made with about 5 per cent antimony. Spongy lead (Pb + Pb02) is introduced into the holes of the mesh. 

CHO+, which reacts with water formed in the flame to form H30+, allowing a measurable electrical current to flow across an electrode gap. A schematic representation of an FID is shown in Fig. 14.7, showing the fuel and oxidant flows, flame tip, location of the flame and collector electrode. Like the carrier gas, the fuel (hydrogen) and oxidant (air) gases must be highly pure and carefully flow controlled. For each GC, the manufacturer provides recommendations. 

Figure 29.4, illustrates a schematic diagram of a flame-ionization detector. It comprises of a positively charged ring (also referred to as cylindrical collector electrode), whereas the flame jet serves as the negative electrode. The flame jet has two inlets from the bottom of the column effluent is introduced and from the side H2 to form the fuel, whereas air is let in uniformly around the base of the jet. 

Figure 29.5, depicts the diagram of an electron capture detector. The metal block of the detector housing itself serves as a cathode, whereas an electrode polarizing lead suitably positioned in the centre of the detector housing caters for a collector electrode (anode). The radioactive source from a beta-emitter is introduced from either sides of the detector housing below the electrode polarizing lead. 

To prepare the standardized dispersion electrode, a mixture of 100 mg soot, 100 g 97% sulfuric acid and 100 mg pigment was stirred direct into the dilute sulfuric acid of the measuring vessel, so that the concentration of sulfuric acid in the electrolyte after stirring in was about 4.5 N. The current collector electrode was a gold-platinum mesh (90/10) of about 4.5 cm2 area. The current/voltage characteristics shown in Fig. 12 make it possible to compare CoTAA with various phthalocyanines. 

Fig. 1. Photoionization cell for less volatile compounds 1—tube of stainless steel 2—LiF windows on Teflon rings 3—semicylindrical collector electrode for positive ions 4—capillary for vapor admission 5—to the vacuum line.

Closely spaced band electrodes (pairs or triples), with each electrode within the diffusion layer of the other, can be used for studying reactions, in a manner analogous to ring-disk generation-collection and redox recycling experiments (131,132). Unlike with rotating ring-disk electrodes, the product of the reaction at the collector electrode can diffuse back across the narrow gap to the... 

Continuous dynode electron multipliers (such as the Channeltron) are horn-shaped detectors (Fig. 3.14b). A high voltage is applied between the input and output ends of the detector. When an ion strikes the detector, secondary electrons are produced. These electrons in turn strike the wall of the detector, generating more electrons. Up to 108 electrons are produced and collected at a collector electrode at the output end of the detector for each incident ion, depending on the applied voltage. 

---