Discrete dynode

Figure 2.21. Schematic of (a) a photoplate detector (b) a Faraday cup (c) a discrete-dynode electron multiplier (EM) of Venetian blind type and (d) a continuous dynode EM. Parts (c) and (d) reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

In many applications, discrete dynode electron multipliers have been replaced by a less costly continuous dynode design. These conicalshaped devices (Fig. 11.14) are fabricated from resistive glass (doped... 

Figure 12.20 shows the structure of the side-window circular cage type and linear focused head-on type of photomultiplier which are both preeminent in fluorescence studies. The lower cost of side-window tubes tends to favor their use for steady-state studies, whereas the ultimate performance for lifetime studies is probably at present provided by linear focused devices. In both types internal current amplification is achieved by virtue of secondary electron emission from discrete dynode stages, usually constructed of copper-beryllium (CuBe) alloy, though gallium-phosphide (GaP) first dynodes have been used to obtain higher gains. 

Fig. 4.57. Discrete dynode electron multipliers, (a) Schematic of a 14-stage SEM. (b) Photograph of an old-fashioned 16-stage Venetian blind-type SEM clearly showing the resistors and ceramics insulators between the stacking dynodes at its side, (a) Adapted from Ref. [238] by permission. Springer-Verlag Heidelberg, 1991.

Fig. 1.31 Discrete-dynode electron multiplier. When the ions hit the surface of the detector electrons are emitted to form an avalanche of electrons which generates the signal.

Figure 14 Detectors (a) Discrete dynode electron multiplier, (b) Dual-mode discrete dynode electron multiplier detector, (c) Channeltron electron multiplier, (d) Faraday collector. (f) Daly detector.

There is another design of electron multiplier for which the discrete dynodes are replaced by one continuous dynode. A type of continuous-dynode electron multipliers (CDEM), which is called a channeltron, is made from a lead-doped glass with a curved tube shape that has good secondary emission properties (Figure 3.3). As the walls of the tube have... 

With the exception of an ICR-MS, nearly aU mass spectrometers use electron multipliers for ion detection. There are three main classes of electron multipliers discrete dynode multipliers, continuous dynode electron multipliers (CDEM), also known as channel electron multipfiers (GEM), and microchannel plate (MCP) electron multipliers, also known as multichannel plate electron multipliers. Though different in detail, aU three work on the same physical principle. An additional detector used in mass spectrometers is the Faraday cup. 

Figure 7-12 presents a conceptual diagram of the operation of a discrete dynode electron multiplier. When an ion strikes the first dynode, it causes the ejection of one or more electrons ( secondary electrons ) from the dynode surface. The electron is accelerated toward the second dynode by a voltage difference of -100 V. Upon strildng the second dynode, this electron causes the ejection of additional electrons, typically 2 or 3 in number. The second group of electrons is then accelerated toward the third d)mode, and upon strildng the third dynode, causes the ejection of several more electrons, The process is repeated through a chain of dynodes, num-... 

Figure 7-12 Discrete dynode electron multiplier showing dynode structure and generation of electron cascade.

The most common transducers for ICP-MS are electron multipliers. The discrete dynode electron multiplier operates much like the photomultiplier transducer for ultraviolet/visible radiation, discussed in Section 25A-4. Electrons strike a cathode, where secondary electrons are emitted. These are attracted to dynodes that are each held at a successively higher positive voltage. Electron multipliers with up to 20 dynodes are available. These devices can multiply the signal strength by a factor of up to 10. ... 

In an ion counting device (Fig. 33) the ions entering the analyzer are directed to a suitable surface held at high potential, so that the incident ions release several electrons. The surface may consist of a continuous dynode, the curvature of which guides the electrons, which increase in number at each interaction with the dynode surface and finally onto an anode. In the case of discrete dynodes the construction of the detector is similar to that of a photomultiplier. These detectors have a dead time, as the interaction of an ion takes a finite time (usually less than 10 ns), during which the detector is blind to the next ion. One can make a correction of the measured count rate (nmeas) for this dead time (r) as ... 

Figure 16.26 MS detectors, (a) Discrete dynode model with active film (reproduced courtesy of ETP Scientific Inc.) (b) Continuous dynode model. Diagram of a channeltron the funnel shaped cathode permits the recovery of ions issuing from different trajectories. The curvature has the effect of preventing the positive ions which appear by the impact of electrons on the residual molecules and restricting therefore the production of further electrons (c) MicroChannel plate. Each plate consists of an array of tiny glass tubes. Each channel becomes a continuous dynode electron multiplier (d) Details of the conversion cathode. Multiplication of the electrons in a microtube (from illustration by Galileo USA).

The most commonly used detectors are based on the electron multiplier, especially the discrete dynode type. Here, the signal is received and amplified. 

Figure 11-2a is a schematic of a discrete-dynode elec-Iron multiplier designed for collecting and converting positive ions iiilo an electrical signal, [his device is... 

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