Detectors continuous dynode

The most connnon detectors used for TOF-SARS are continuous dynode channel electron multipliers which... 

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.

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. 

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. 

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 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. 

Variants of the EM include the discrete dynode, channeltron (or continuous dynode), the microchannel plate, and the Daly detector. The EM can detect ion currents below 1 ion/s. EM detectors are often operated in the pulse counting mode, in which the pulse produced by a single ion is amplified and detected as an individual events. Pulse counting offers excellent signal-to-noise ratio, high sensitivity, and insensitivity to change in EM gain. 

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. 

Whether an electron multiplier has a discrete-dynode or continuous-dynode design, it can be operated in both analog and pulse-counting modes. For analog detectors. 

Continuous dynode, discrete dynode, multichannel plate (MCP) photomultiplier detector (Daly)... 

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)). 

After separation of the ions produced, a detector, usually a continuous dynode version of an electron multiplier, is used to count the ions and generate a mass spectrum. Such a detector is shown schematically in Figure 10.8. Ions from the mass analyzer strike the semi-conductive surface and release a cascade of electrons. These are accelerated by a potential difference to another portion of the semi-conductive surface where a larger cascade of electrons results. This process is repeated several times until amplification of the original weak input is magged about 1-million-foId. 

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. 

The channel matrix is usually treated in such a way as to optimize the secondary emission characteristics of each channel, and thus each channel can be considered a continuous dynode stmcture. Parallel electrical contact to each channel is provided by the deposition of a metallic coating on the front and rear surfaces of the MCP these constitute the input and output electrodes respectively. The principles of construction of an MCP detector are shown in Rgure 13.14. 

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