Dynode Electron Multiplier

FIGURE 11.3 Schematic of a discrete dynode electron mnltiplier. (From K. Hnnter, Atomic Spectroscopy, 1S[1], 17-20,1994.) 

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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In many applications, discrete dynode electron multipliers have been replaced by a less costly continuous dynode design.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... 

Fig. 11.14. Diagram of a single-channel (continuous dynode) electron multiplier. Gains of 105 or 106 are achievable with modern electron multipliers.

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.

The names of both detectors reflect that these devices are channels which act as continuous dynode electron multipliers. If there is one channel, it is called a channeltron (channeltron electron multiplier, CEM), if many microchannels are used to form a plate it is called a microchannel electron multiplier plate (in short a microchannelplate, MCP, or channelplate), see Fig. 4.17. A comprehensive description of these devices is given in [Wiz79]. 

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.

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. 

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... 

Continuous dynode electron multiplier, also known as the channeltron. O, incident ions , secondary particles. Reproduced (modified) from Finnigan MAT documentation, with permission. 

The amplifying power is the product of the conversion factor (number of secondary particles emitted by the conversion dynode for one incoming ion) and the multiplying factor of the continuous dynode electron multiplier. It can reach 107 with a wide linear dynamic range (104-106). Their lifetime is limited to 1 or 2 years because of surface... 

Another type of continuous dynode electron multipliers is the microchannel plate (MCP) detector. It is a plate in which parallel cylindrical channels have been drilled. The channel diameter ranges from 4 to 25 pm with a centre-to-centre distance ranging from 6 to 32 pm and a few millimetres in length (Figure 3.4). The plate input side is kept at a negative potential of about 1 kV compared with the output side. 

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. ... 

Continuous dynode electron multipliers are also popular. These are trumpetshaped devices made of glass heavily doped with lead. A potential of 1.8 to 2 kV is imposed across the length of the device. Ions that strike the surface eject electrons that skip along the inner surface, ejecting more electrons with each impact. 

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).

FIGURE 11-2 (a) Oiscrete-dynode electron multiplier. Oynodes are kept at successively higher voltages via a multistage voltage divider, (b) Continuous-dynode electron multiplier. (Adapted from J, T. Watson. Introduction lo Mass Spectrometry, 3rd ed pp. 334-35, New Vork Raven Press. 1997. With permission.)... 

Figure 9.26 (a) A schematic discrete-dynode electron multiplier, showing the electron gain at... 

The microchannel plate is a spatially resolved array detector formed of 10 -10 continuous-dynode electron multipliers, each only 10-100 ptm in diameter. This detector is used in focal plane mass spectrometers as a replacement for photograph plate detectors and is used in some TOFMS instruments. 

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