Electron multiplier array

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 array detector (collector) consists of a number of ion-collection elements arranged in a line each element of the array is an electron multiplier. Another type of array detector, the time-to-digital converter, is discussed in Chapter 31. 

Where space is not a problem, a linear electron multiplier having separate dynodes to collect and amplify the electron current created each time an ion enters its open end can be used. (See Chapter 28 for details on electron multipliers.) For array detection, the individual electron multipliers must be very small, so they can be packed side by side into as small a space as possible. For this reason, the design of an element of an array is significantly different from that of a standard electron multiplier used for point ion collection, even though its method of working is similar. Figure 29.2a shows an electron multiplier (also known as a Channeltron ) that works without using separate dynodes. It can be used to replace a dynode-type multiplier for point ion collection but, because... 

An array ion collector (detector) consists of a large number of miniature electron multiplier elements arranged side by side along a plane. Point ion collectors gather and detect ions sequentially (all ions are focused at one point one after another), but array collectors gather and detect all ions simultaneously (all ions are focused onto the array elements at the same time). Array 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 a substance are available. For these very short time scales, only the array collector can measure a whole spectrum or part of a spectrum satisfactorily in the time available. 

In modem mass spectrometry, ion collectors (detectors) are generally based on the electron multiplier and can be separated into two classes those that detect the arrival of all ions sequentially at a point (a single-point ion collector) and those that detect the arrival of all ions simultaneously (an array or multipoint collector). This chapter compares the uses of single- and multipoint ion collectors. For more detailed discussions of their construction and operation, see Chapter 28, Point Ion Collectors (Detectors), and Chapter 29, Array Collectors (Detectors). In some forms of mass spectrometry, other methods of ion detection can be used, as with ion cyclotron instmments, but these are not considered here. 

Other types of mass spectrometer can use point, array, or both types of ion detection. Ion trap mass spectrometers can detect ions sequentially or simultaneously and in some cases, as with ion cyclotron resonance (ICR), may not use a formal electron multiplier type of ion collector at all the ions can be detected by their different electric field frequencies in flight. 

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... 

Each element of an array detector is essentially a small electron multiplier, as with the point ion collector, but much smaller and often shaped either as a narrow linear tube or as somewhat like a snail shell. 

The ions in a beam that has been dispersed in space according to their various m/z values can be collected simultaneously by a planar assembly of small electron multipliers. All ions within a specified mass range are detected at the same time, giving the array detector an advantage for analysis of very small quantities of any one substance or where ions are produced intermittently during short time intervals. 

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. 

The mass range requirement invariably means that FAB is used in conjunction with a magnetic sector instrument. Conventional detectors, such as the electron multiplier, are not efficient for the detection of large ions and the necessary sensitivity is often only obtained when devices such as the post-acceleration detector or array detector are used. Instruments capable of carrying out high-mass investigations on a routine basis are therefore costly and beyond the reach of many laboratories. 

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 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. 

Another problem to be solved, involved the development of an electron multiplier array of sufficient length to cover the entire focal plane. Commercially available MCA s were nowhere near the required dimensions of approximately 1 mm high x 361... 

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. 

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