Electron multipliers with discrete dynodes

Figure 2.26 Basic operating principle of an electron multiplier, (a) Continuous dynode electron multiplier (b) electron multiplier with discrete dynodes. Reproduced with permission of Dr. Paul Gates, School of Chemistry, University of Bristol, from [114].

FIGURE 2.4 Electron multipliers with discrete dynodes (a) and continuous dynode (b). 

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. ... 

Detecting ions in GC/MS is performed almost exclusively using an electron multiplier. There are two types of electron multipliers the continuous dynode type and the discrete type. Both operate on the principle that ions with sufficient kinetic energy will emit secondary electrons when they strike a metal surface. The discrete type of electron multiplier has a series of... 

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.

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 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).

In the discrete dynode electron multiplier, the ions from the analyser are converted into electrons by a dynode (an electron used to provide secondary emission). The dynode surface is typically composed of CsSb, BeO, or GaP, which are secondary emitting materials. This means that the electrons are emitted or released from atoms in the surface layer with the number of electrons released depending upon... 

FIGURE 7.7 Discrete-dynode electron multiplier. (From Babis et al.. Performance evaluation of a miniature ion mobility spectrometer drift cell for application in handheld explosives detection ion mobility spectrometers, Anal. Bioanal. Chem. 2009, 395, 411-419. With permission.)... 

FIQURE 11 >2 (a) Discrete-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 to Mass Spectrometry, 3rd ed., pp. 334-35, New York Raven Press, 1997. With permission.)... 

Figure 4.17 Schematic illustrations of (a) a Discrete Dynode Electron Multiplier, (b) a Channeltron, and (c) a Micro-Channel Plate, along with the electron cascades generated when these detectors are biased. Reprinted with permission from van der Heide (2012) Copyright 2012 John Wiley and Sons.

Figure 4.18 Schematic example of Pulse Height Distribution analysis carried out for 4500 eV secondary ions impinging on an ETP Discrete Dynode Electron Multiplier operated at the listed voltages. Pulse counting was carried out using custom built ECL logic pre-amplifier/discriminator units. Discriminator voltage in this case should be set at 5 mV. Reproduced with permission from van der Heide and Fichter (1998) Copyright 1998 John Wiley and Sons.

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