Electron multipliers

An electron multiplier can amplify an ion current up to a factor of 10 . Each ion which impinges on the metal surface of the first dynode of the multiplier causes the emission of a number of secondary electrons. These are accelerated to another electrode inducing consecutive, additional electron emission thus producing a cascade of electrons. 

This process is repeated several times, using ten stages or more. The multiplication factor depends on the material and number of the dynodes and of the voltage between them. After amplification the signals can be registrated with a recorder, oszillograph or on-line with a computer and stored on magnetic tapes or disks. 

Ions are directed onto the first plate (dynode) of an electron multiplier. The ejected electrons are accelerated through an electric potential so they strike a second dynode. Suppose each ion collision causes ten electrons to be ejected, and, at the second dynode each of these electrons causes ten more to be ejected toward a third dynode. In such a situation, arrival of just one ion causes 10 X 10 = 10 = 100 electrons to be ejected from the second dynode, an amplification of 100. Commercial electron multipliers routinely have 10, 11, or 12 dynodes, so amplifications of 10 are 

Ions traveling at high speed strike the inside of the metal (Faraday) cup and cause secondary electrons to be ejected. This production of electrons constitutes a temporary flow of electric current as the electrons are recaptured. 

Each ion from an incident ion beam causes two electrons to be emitted from the first dynode. These electrons are accelerated to the second dynode, where each causes two more electrons (now four in all) to be ejected. These in turn are accelerated to a third dynode and so on, eventually reaching, say, a tenth dynode, by which time the initial two electrons have become a shower of 2 electrons. 

Eventually, multipliers become less sensitive and even fail because of surface contamination caused by the imperfect vacuum in the mass spectrometer and the impact of ions on the surfaces of the dynodes. 

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. 

Dual-pressure linear ion trap mass filter 

Before I go on to describe disaete dynode detectors in greater detail, it is worth looking at two of the earlier designs—the channel electron multiplier (Channeltron ) and the Faraday cup—to get a basic understanding of how the ICP-MS ion detection process works. 

FIGURE 11.1 The location of the detector in relation to the mass analyzer. 

FIGURE 11.2 Basic principles of a channel electron mnltiplier. (From Channeltron , Electron Multiplier Handbook for Mass Spectrometry Applications, Galileo Electro-Optic Corp., 1991.) 

Practical Guide to ICP-MS A Tutorial for Beginners, Second Edition 

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Another example of a teclmique for detecting absorption of laser radiation in gaseous samples is to use multiphoton ionization with mtense pulses of light. Once a molecule has been electronically excited, the excited state may absorb one or more additional photons until it is ionized. The electrons can be measured as a current generated across the cell, or can be counted individually by an electron multiplier this can be a very sensitive technique for detecting a small number of molecules excited. 

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

In the other types of mass spectrometer discussed in this chapter, ions are detected by having them hit a detector such as an electron multiplier. In early ICR instruments, the same approach was taken, but FT-ICR uses a very different teclmique. If an RF potential is applied to the excitation plates of the trapping cell (figure B 1.7.18(b)) equal to the cyclotron frequency of a particular ion m/z ratio, resonant excitation of the ion trajectories takes place (without changing the cyclotron frequency). The result is ion trajectories of higher... 

In TOF-SARS [9], a low-keV, monoenergetic, mass-selected, pulsed noble gas ion beam is focused onto a sample surface. The velocity distributions of scattered and recoiled particles are measured by standard TOF methods. A chaimel electron multiplier is used to detect fast (>800 eV) neutrals and ions. This type of detector has a small acceptance solid angle. A fixed angle is used between the pulsed ion beam and detector directions with respect to the sample as shown in figure Bl.23.4. The sample has to be rotated to measure ion scattering... 

Figure Bl.23.5. Schematic illustration of tlie TOE-SARS spectrometer system. A = ion gun, B = Wien filter, C = Einzel lens, D = pulsing plates, E = pulsing aperture, E = deflector plates, G = sample, PI = electron multiplier detector with energy prefilter grid and I = electrostatic deflector.

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

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. 

As mentioned previously, a particle such as an ion traveling at high speed causes a number of secondary electrons to be ejected when it strikes a metal surface. This principle is utilized in the electron multiplier (Figure 28.3). 

A scintillator, sometimes known as the Daly detector, is an ion collector that is especially useful for studies on metastable ions. The principle of operation is illustrated in Figure 28.4. As with the first dynode of an electron multiplier, the arrival of a fast ion causes electrons to be emitted, and they are accelerated toward a second dynode. In this case, the dynode consists of a substance (a scintillator) that emits photons (light). The emitted light is detected by a commercial photon... 

An ion beam causes secondary electrons to be ejected from a metal surface. These secondaries can be measured as an electric current directly through a Faraday cup or indirectly after amplification, as with an electron multiplier or a scintillation device. These ion collectors are located at a fixed point in a mass spectrometer, and all ions are focused on that point — hence the name, point ion collector. In all cases, the resultant flow of an electric current is used to drive some form of recorder or is passed to an information storage device (data system). 

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... 

Idealized face view of a set of small electron multipliers arranged over a plane. Some typical individual multipliers are shown in later figures. 

A typical single microchannel electron multiplier. Note how the primary ion beam causes a shower of electrons to form, The shower is accelerated toward the other end of the microchannel, causing the formation of more and more secondary electrons. 

A different form of miniature electron multiplier. The curved shape is used to reduce backscattering of the electrons. The final output of electrons flows along a wire to an amplifier. 

The ions are detected by an electron multiplier placed in line with the beam. 

Thus, ions are produced, deflected in a magnetic field, then focused in an electric field, and finally detected by an electron multiplier or other ion detector. 

By placing a suitable detector at the focus (a point detector), the arrival of ions can be recorded. Point detectors are usually a Faraday cup (a relatively insensitive device) or, more likely, an electron multiplier (a very sensitive device) or, less likely, a scintillator (another sensitive device). 

Arrival of ions, which have a positive or negative charge, causes an electric current to flow either directly (Faraday cup) or indirectly (electron multiplier and scintillator detectors). 

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. 

Electron multiplier. A device to multiply current in an electron beam (or in a photon or particle beam after conversion to electrons) by incidence of accelerated electrons upon the surface of an electrode. This collision yields a number of secondary electrons greater than the number of incident electrons. These electrons are then accelerated to another electrode (or another part of the same electrode), which in turn emits secondary electrons, continuing the process. 

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