MicroChannel plate electron multiplier

Complete MCP s can be stacked to provide even higher gains. For response in the vacuum ultra-violet spectral region (50-200 nm) a SSANACON, self-scanned anode array with microchannel plate electron multiplier, has been used (36). This involves photoelectron multiplication through two MOP S, collection of the electrons directly on aluminum anodes and readout with standard diode array circuitry. In cases where analyte concentrations are well above conventional detection limits, multi-element analysis with multi-channel detectors by atomic emission has been demonstrated to be quite feasible (37). Spectral source profiling has also been done with photodiode arrays (27.29.31). In molecular spectrometry, imaging type detectors have been used in spectrophotometry, spectrofluometry and chemiluminescence (23.24.26.33). These detectors are often employed to monitor the output from an HPLC or GC (13.38.39.40). 

Similar to mass calibration, quantity calibration can be done internally or externally by using appropriate calibration compounds. The calibration curve covers the intensity range corresponding to the intensities of analyte peaks. In order to obtain the highest accuracy, the calibrants used in quantity calibration should have similar atomic composition, molecular structure, and mass as analytes. This requirement is because ionization efficiency depends on molecular structure, and detection efficiency is mass-dependent. In fact, ion detectors rely on secondary electrons produced by the impact of primary ions with detector surfaces, including microchannel plates, electron multipliers, and many others. 

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. 

An AutoSpec-TOF mass spectrometer has a magnetic sector and an electron multiplier ion detector for carrying out one type of mass spectrometry plus a TOF analyzer with a microchannel plate multipoint ion collector for another type of mass spectrometry. Either analyzer can be used separately, or the two can be run in tandem (Figure 20.4). 

A fuller description of the microchannel plate is presented in Chapter 30. Briefly, ions traveling down the flight tube of a TOF instrument are separated in time. As each m/z collection of ions arrives at the collector, it may be spread over a small area of space (Figure 27.3). Therefore, so as not to lose ions, rather than have a single-point ion collector, the collector is composed of an array of miniature electron multipliers (microchannels), which are all connected to one electrified plate, so, no matter where an ion of any one m/z value hits the front of the array, its arrival is recorded. The microchannel plate collector could be crudely compared to a satellite TV dish receiver in that radio waves of the same frequency but spread over an area are all collected and recorded at the same time of course, the multichannel plate records the arrival of ions not radio waves. 

Another form of array is called a microchannel plate detector. A time-of-flight (TOP) mass spectrometer collects ions sequentially in time and can use a point detector, but increasingly, the TOP instrument uses a microchannel plate, most particularly in an orthogonal TOP mode. Because the arrays and microchannel plates are both essentially arrays or assemblies of small electron multipliers, there may be confusion over their roles. This chapter illustrates the differences between the two arrays. 

An assemblage (array) of single-point electron multipliers in a microchannel plate is designed to detect all ions of any single m/z value as they arrive separated in time. Thus, it is not necessary for each element of the array to be monitored individually for the arrival of ions. Instead, all of... 

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

Each element of an array or a microchannel plate ion collector is essentially an electron multiplier, similar in operation to the type used for a point ion collector but very much smaller. 

After the analyzer of a mass spectrometer has dispersed a beam of ions in space or in time according to their various m/z values, they can be collected by a planar assembly of small electron multipliers. There are two types of multipoint planar collectors an array is used in the case of spatial separation, and a microchannel plate is used in the case of temporal separation. With both multipoint assemblies, all ions over a specified mass range are detected at the same time, or apparently at the same time, giving these assemblies distinct advantages over the single-point collector in the analysis of very small quantities of a substance or where ions are produced intermittently during short time intervals. 

Detector The detector is the last major portion of the mass spectrometer, and it detects the presence, and preferably abundance, of ions after they have exited the mass analyzer. Examples include the electron multiplier, common on quadrupole instruments, and the microchannel plate (an array of electron multipliers), which have been common on TOF instruments. For most users, the actual detector is a relatively invisible portion of the instrument that needs little or no regular attention. 

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]. 

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. 

The snowball effect within a channel can multiply the number of electrons by 10s. A plate allows an amplification of 102-104, whereas by using several plates the amplification can reach 108. This detector is characterized by a very fast response time because the secondary electron path inside the channel is very short. In consequence, it is well suited to TOF analysers, which need precise arrival times and narrow pulse widths. Furthermore, the large detection area of the microchannel plate allows the detection of large ion beams from the analyser without additional focalization. However, the microchannel plate detectors have some disadvantages. They are fragile, sensitive to air and their large microchannel plates are expensive. 

A newer and less expensive alternative to the microchannel plate is the microsphere plate (MSP). As illustrated in Figure 3.6, this electron multiplier consists of glass beads with diameters from 20 to 100 pm that are sintered to form a thin plate with a thickness of 0.7 mm. This plate is porous with irregularly shaped channels between the planar faces. The surfaces of the beads are covered with an electron emissive material and the two sides of the plate are coated to make them conductive. The operating principle of this electron multiplier is similar to that of the microchannel plate. A potential difference of between 1.5 and 3.5 kV is applied across the plate, with the output side of the plate at the more positive potential. When particles hit the input side of the microsphere plate, they produce secondary electrons. These electrons are then accelerated by the electric field through the porous plate and collide with other beads. Secondary electron multiplication in the gaps occurs and finally a large number of secondary electrons are emitted from the output side of the plate. 

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