Multichannel electron multiplier

The detector may be a simple electrometer when using a cylindrical or spherical grid analyser. With the other types, fewer electrons are being collected and an electron multiplier, having much greater sensitivity, is necessary. This consists of a number of dynodes, each of which produces more electrons than it receives. For a measurable current, about 10 to 20 dynodes are required. Alternatively, a multichannel electron multiplier in the focal plane of the analyser can be used to collect simultaneously electrons with a range of energies. 

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. 

The basic components include a Nd YAG pulsed laser system which is coaxial with a He Ne pilot laser and visible light optical system. The latter system enables the analytical area of interest to be located. The TOF-MS has a flight path of 2m in length, with an ion detection system that includes an electron multiplier detector, a multichannel transient recorder, together with a computer acquisition and data processing system. 

Figure 5.6. Diagram of a low-energy, high-angle electron-impact spectrometer. (A) Electron gun (B) monochromator (180° spherical electrostatic energy selector) (C) electron optics (D) scattering chamber (E) analyzer (180° spherical electrostatic energy selector) (F) electron multiplier (G) amplifier and pulse discriminator (H) count-rate meter (I) multichannel scaler (J) X Y recorder (K) digital recorder. (After Kupperman et a/.<42))...

The detectors used in mass spectrometers for atmospheric applications are essentially the same as for other MS applications and are commonly electron multipliers, either channeltrons or multichannel plate... 

Figure 16.6—Linear time of flight (TOF) and principle of the reflectron. 1) Sample and sample holder 2) MALDI ionisation device 3 and 3 ) extraction and acceleration grid (5 000 V potential drop) 4) control grid 5) multichannel collector plate 6) electron multiplier 7) signal output. The bottom figure shows a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass, but which have different initial energies. This device increases resolution, which can attain several thousand.

Figure 16.22—MS detectors, a) Multiple stage electron multipliers (reproduced by permission of ETP Scientific Inc.) b) channeltron the conical shape of the cathode allows the detection of ions with slightly different trajectories c) electron multiplication within a channeltron d) entrance of a multichannel plate detector (microchanneltron).

The atomic beam was formed by a multichannel capillary array, placed perpendicular to the positron beam, with a 2.5 mm2 effusing area and a length-to-diameter ratio of 25 1. The head pressure behind the array was kept at 9 torr (ss 103 Pa) in the initial measurements. An annealed tungsten moderator was used to provide a beam of more than 105 positrons per second at 200 eV. A much more intense beam of electrons could also be obtained by reversing the electrostatic potentials on the various elements which made up the transport system. Channel electron multipliers (CEM1 and CEM2 respectively) were used to monitor the incident and scattered beams. In later versions of the apparatus, a third... 

The dispersion analyser yields the energy distribution directly. Electrons at the pass energy are detected using an electron multiplier in the counting mode and the counts stored in either a computer or in a multichannel analyser, whose channel... 

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. 

Two of the three laser ionization methods have already been discussed, namely one-photon PI and multiphoton MPI. The third type is resonance enhanced MPI, or REMPI. In the latter method the laser is tuned so that an intermediate state of the molecule is excited with one, two, or perhaps three photons. The excitation of the intermediate state determines the overall cross section for the process because the absorption of additional photons to reach the ionization continuum is generally rapid. In contrast to PI and MPI, REMPI is state selective if the absorption process is resonant between two bound and reasonably long-lived states of the molecule. It is an extremely sensitive method for product detection because the result of the REMPI process is an ion which can be detected with near 100% efficiency. Not only is the ion collection efficiency of the detector (e.g., by channeltron electron multiplier or a multichannel plate detector) extremely high (ca. 50%), but all ions regardless of their initial velocity vector can be collected by the application of appropriate electric fields. This is a major advantage... 

Multichannel Plate. A plate consisting of parallel, microscopically small channels oriented perpendicular to the surface. The inner walls of the channels have a conductive coating. When a voltage is applied between the opposite surfaces of the plate the channels work as electron multipliers. MCPs are used in photomultiplier tubes and image intensifiers. 

Multichannel electron detectors of various types can be placed to cover the exit plane of the analyzer. CCDs (discussed in Chapter 7), phosphor-coated screens and position-sensitive detectors are some of the multichannel devices in use. One type of position-sensitive detector consists of a microchannel plate (MCP) electron multiplier. The MCP [Fig. 14.7(b)] consists of a large number of very thin conductive glass capillaries, each... 

A Faraday-cup, a secondary electron multiplier, a scintillation counter or a multichannel plate are used for ion detection. 

All types of multipliers (electron multiplier, continuous dynode, multichannel plate) work on the same principle. 

The multichannel plate detector (MCP) is a porous glass plate in which each pore acts as a mini electron multiplier. The MCP is a set of parallel multipliers. 

Ion detection is provided by Faraday cups, electron multipliers, channel electron multipliers, cryogenic detectors, multichannel plate detectors, and electroop-tical detectors. 

---