Continuous-dynode electron multipliers

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

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]. 

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. 

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.)... 

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. 

After the ions have been separated by the analyser, they will be focused onto the detector, where they are converted into a measurable electrical current. This results in a signal in the form of a series of peaks showing the abundance of those particular ions. The most common type of detector is an electron multiplier, which exists in two forms the discrete dynode electron multiplier and the continuous dynode electron multiplier (also called a channel electron multiplier [CEM]). 

In comparison, the continuous dynode electron multiplier differs in that the amplification of the signal occurs by the electrons colliding with the internal surface of the detector. The detector is a continuous dynode that is horn shaped it is shown in Figure 5.22. 

Figure 3.29. Simplified sketch of a continuous dynode electron multiplier.

The continuous dynode electron multiplier (channeltron) is another device (Adams and Manley (1965)) which takes advantage of unique properties of amorphous semiconductors and considerable appUed research is devoted to the utilization of vitreous materials for optical mass memories (Ovshinsky (1969) Feinleib et al (1971)) high energy particle detectors (Srinivasan et al (1971)) ultrasonic delay lines (Krause et al (1970)) and magnetic bubble domain memories (Chaudhari etal (1973)). 

Most of the disadvantages of discrete dynode SEMs are overcome by the development of channel electron multipliers (CEMs) (sometimes also referred to as continuous dynode electron multipliers, CDEMs) that are by far the most widely used ion detectors in analytical mass spectrometers. 

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].

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