Discrete dynode multipliers

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. 

The discrete dynode multiplier is composed of a series of coppertberylium plates between which the electrons cascade. 

Truong QS, Keeffe R, Ellacott T, Desson K, Herber N (2001) In IAEA symposium on international safeguards, IAEA-SM-367/8/08/P Tuttas D, Schwieters JB, Quaas N, Bouman C (1998) Improvements in TIMS high precision isotope ratio measurements for small sample sizes. Application note 30136, Thermo Eisher Scientific, Bremen Tuttas D, Schwieters JB, Bouman C, Deerberg M (2005) New compact discrete dynode multipliers integrated into the thermo scientific TRITON variable multicollector array. Application note 30192, Thermo Fisher Scientific, Bremen United Nations Security Council (1991) Text of UNSC Resolution 687. Available at http //www.fris.org/ news/un/iraq/sres/sres0687.htm... 

While the principles of operation are similar, the structures of discrete-dynode multipliers and CEMs are quite different. Figure 3.1 illustrates each type schematically. 

Discrete-dynode multipliers consist of an array of separate dynodes with high secondary electron yield surfaces. CEMs consist of a lead-silicate glass tube processed to have a resistive inner surface with a suitably high secondary electron emission to multiply electrons. The following discussion is restricted to discrete-dynode detectors, which are the most common type used in ICP-MS. However most of the principles described can be readily applied to CEMs. 

At present, TOF mass analyzers are not widely used in ICP-MS. However, there are commercial systems available (as described in Chapter 2) and the following brief description of detectors used in TOF-MS applications is included for completeness. There are two main types of detectors used in TOF-MS applications microchannel plates (MCPs) and fast discrete-dynode multipliers. 

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. 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... 

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-... 

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