Channel electron multiplier arrays

For modem time of flight (TOF) analyzers that can provide resolving power (FWHM definition) 10 via incorporation of the ion detector as an intrinsic component of the m/z analysis, such CEM response time characteristics are inadequate. This is easily illustrated by an ullra-simplified calculation for a typical case in which the spatial and time focusing properties of the TOF analyzer (Section 6.4.7) are sufficient that the resolving power (RP) is limited by the time response of the detector. The time of flight of an ion is given by Equation [6.19]  

Consider an ion with m/z 1000 accelerated through Vjcc = 20 kV in an instrument with a flight path length 

Xj = 2 m. Then evaluation of Equation [6.19] gives tf = 32 xs. For this example with fixed and differentiation of Equation [6.19] and simple algebraic rearrangement gives  

A wire mesh is usually placed in front of the entrance of a CEMA plate, typically 20 x 20 or 30 x 30 wires 

CEMA plates (usually in a chevron configuration) are required and such an application inherently demands some kind of pulse-counting strategy. 

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A small fraction of the orthopositronium atoms produced pass through the cw-excitation beam, where they are promoted to the 23Si level and then through a multi-pass doubled-YAG beam at 532 nm, where they are photo-ionized. The photo-ionized positron is electro-statically accelerated and magnetically-guided into a channel-electron multiplier array (CEMA) where it is detected. The time-of-Hight between the incident positron pulse and the photo-ionization pulse determines the range of positronium velocities detected. 

One fundamental question which arises in these peeling experiments is from which surface are the particles being emitted Two experiments we have performed helped to answer this question. The first employed a charged particle imaging device called a micro-channel plate or Channel Electron Multiplier Array (CEMA), which produces on a phosphor screen an intensity pattern of the charged particles incident upon the plate. The patterns observed from peeling 3M Magic Tape from a solid surface depended on the orientation of the peel test and substrate relative to the CEMA. 

Figure 7.9 (a) Sketch of a cutaway view of a channel electron multiplier array plate. Nickel-chromium electroding is applied to both surfaces of a microchannel plate to provide electrical contact and also penetrates into the channel. The penetration depth is minimized on the input face (0.3-0.7 of the channel diameter) to maximize the first strike conversion efficiency of incoming ions/elecrons channel. Reproduced from Wiza, Nucl. Instrum. Methods 162,587 (1979), copyright (1979), with permission from Elsevier. 

The sample inlet system for a typical mass spectrometer is versatile enough to handle gas, liquids, and solids. The device is usually held at 200°C and 0.02 torr pressure. Accordingly, any solids must have a sufficient vapor pressure under these conditions to allow transport to the ion source as a gas before a spectrum can be recorded. Modern instruments usually incorporate an electron multiplier or channel electron multiplier array as a detecting system. Both these devices work on the principle of electrons released from a material on ion impact. The electrons are... 

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 ideal high-throughput analytical technique would be efficient in terms of required resources and would be scalable to accommodate an arbitrarily large number of samples. In addition, this scalability would be such that the dependence of the cost of the equipment to perform the experiments would scale in a less than linear manner as a function of the number of samples that could be studied. The only way to accomplish this is to have one or more aspects of the experimental setup utilize an array-based approach. Array detectors are massively multiplexed versions of single-element detectors composed of a rectangular grid of small detectors. The most commonly encountered examples are CCD cameras, which are used to acquire ultraviolet, visible and near-infrared (IR) photons in a parallel manner. Other examples include IR focal plane arrays (FPAs) for the collection of IR photons and channel electron multipliers for the collection of electrons. 

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). 

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).

Most mass spectrometers measure one m/z value at a time. A single-channel ion detector is used for these instruments, either an electron multiplier (EM) or a Faraday cup. TOF, ion trap, and FTICR mass spectrometers have the ability to extract ions of many m/z values simultaneously, so simultaneous detection of these ions is desirable. One approach to multiple ion detection has been to use multiple detectors. Multiple detectors are also used for high-resolution magnetic sector MS instruments designed for very precise isotope ratio determination and for quantitative analysis using isotope dilution. Instruments with multiple detectors are called multicollectors. New detector developments in array detectors now permit simultaneous m/z measuranent over a wide mass range, such as the SPFCTRO MS instrument. 

A microchannel plate (MCP) is an array of 10" -lO miniature electron multipliers, oriented parallel to one another. Standard devices have channel diameters in the range 10-100 pm, and their length-to-diameter ratio is of the order 40-100. Typically, the channel axes are oriented at a small angle ( 8°) to the MCP input surface. 

The first two of the above demands are met effectively with microchannel plate (MCP) electron multipliers. These are flat arrays (of various dimensions) of micrometre-sized channels, each one acting as a very fast electron multiplier when a voltage gradient exists along it. Most high-resolution TOF instruments now use a microchannel plate for the first element of the detector. 

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