Continuous dynode

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

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... 

CE Photoionisation (PI) Ion-cyclotron resonance (ICR) Continuous dynode multiplier... 

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In many applications, discrete dynode electron multipliers have been replaced by a less costly continuous dynode design.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

The operating principle of an MCP-PM is based on electron multiplication using a continuous dynode structure of ca. 10 um diameter holes, giving a more compact and hence faster time response when compared with conventional photomultipliers. Rise-times of 150 psec and transit-time jitter (i.e., impulse response) of ca. 25 psec FWHM at 200 counts/sec noise at room temperature have been recorded with the 6 fun channel Hamamatsu R3809 MCP-PM.(87)... 

A thin plate that contains a closely spaced array of channels that each act as a continuous dynode particle multiplier. A charged particle, fast neutral particle, or photon striking the plate causes... 

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. 

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 ... 

Continuous dynode multipliers with a channeltron . The ions are directed towards a collector whose entrance, in the form of a horn, is made of a lead doped glass with which acts as the conversion cathode. The ejected electrons are attracted towards a positive electrode (Figure 16.26) and their collisions against the internal walls give rise to multiplication, as with the separated dynodes. The assembly is usually mounted off-axis to avoid the impact of neutral species as well as photons emitted by the filament, equally susceptible to the removal of the electrons. 

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. 

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