Discrete dynode secondary electron multipliers

The (current) amplification gain (Gsg, ) of a discrete dynode SEM, for ions of a specified m/z value and chemical type accelerated on to the first (conversion) dynode 

The so-called dark noise introduced by a discrete dynode SEM when operated under appropriate conditions is largely Johnson noise, the result of random thermionic emission of electrons from the dynode surfaces, and is minimized by appropriate choice of materials. When SEMs are used appropriately the dark noise is generally low ( lO A, negligible in applications of interest here) even when operated to provide Gsem = 10 . (The maximum output current from such devices can be as high as 10 A before saturation occurs.) In addition, some dark current can arise from leakage through the ceramic supports for the dynodes, particularly when they become 

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With no stable isotope pair within the U system or a suitable AME, a standard-sample bracketing protocol is usually employed to correct for mass bias. Human urine generally contains very low concentrations of U (generally 1-5 ng/L), so an isotope dilution strategy is required, together with ion-counting detection (ideally a Daly photomultiplier or discrete dynode secondary electron multiplier) and a multi-static (rather than multi-dynamic) peak-jumping routine, for precise measurement of the total U concentration and the minor isotopes of and even... 

Detecting ions in GC/MS is performed almost exclusively using an electron multiplier. There are two types of electron multipliers the continuous dynode type and the discrete type. Both operate on the principle that ions with sufficient kinetic energy will emit secondary electrons when they strike a metal surface. The discrete type of electron multiplier has a series of... 

Secondary electron multiplier for high amplification factors, usually built from discrete dynodes which are electrically connected by resistors to provide a voltage ramp across the number of dynodes (contrary to a CDEM). 

There is another design of electron multiplier for which the discrete dynodes are replaced by one continuous dynode. A type of continuous-dynode electron multipliers (CDEM), which is called a channeltron, is made from a lead-doped glass with a curved tube shape that has good secondary emission properties (Figure 3.3). As the walls of the tube have... 

Figure 7-12 presents a conceptual diagram of the operation of a discrete dynode electron multiplier. When an ion strikes the first dynode, it causes the ejection of one or more electrons ( secondary electrons ) from the dynode surface. The electron is accelerated toward the second dynode by a voltage difference of -100 V. Upon strildng the second dynode, this electron causes the ejection of additional electrons, typically 2 or 3 in number. The second group of electrons is then accelerated toward the third d)mode, and upon strildng the third dynode, causes the ejection of several more electrons, The process is repeated through a chain of dynodes, num-... 

The most common transducers for ICP-MS are electron multipliers. The discrete dynode electron multiplier operates much like the photomultiplier transducer for ultraviolet/visible radiation, discussed in Section 25A-4. Electrons strike a cathode, where secondary electrons are emitted. These are attracted to dynodes that are each held at a successively higher positive voltage. Electron multipliers with up to 20 dynodes are available. These devices can multiply the signal strength by a factor of up to 10. ... 

In the discrete dynode electron multiplier, the ions from the analyser are converted into electrons by a dynode (an electron used to provide secondary emission). The dynode surface is typically composed of CsSb, BeO, or GaP, which are secondary emitting materials. This means that the electrons are emitted or released from atoms in the surface layer with the number of electrons released depending upon... 

The electron multiplier consists either of a series of discrete dynodes or of a continuous channel of dynodes. Figure 1.26 illustrates the continuous-dynode type, also called "channeltron," which is the most widely used electron multiplier in modem instruments. A high negative potential is applied to the channeltron entrance, while the opposite end (anode) is usually grounded. Secondary electrons produced at the entrance by impinging ions or electrons... 

Figure 4.18 Schematic example of Pulse Height Distribution analysis carried out for 4500 eV secondary ions impinging on an ETP Discrete Dynode Electron Multiplier operated at the listed voltages. Pulse counting was carried out using custom built ECL logic pre-amplifier/discriminator units. Discriminator voltage in this case should be set at 5 mV. Reproduced with permission from van der Heide and Fichter (1998) Copyright 1998 John Wiley and Sons.

Discrete-dynode multipliers consist of an array of separate dynodes with high secondary electron yield surfaces. CEMs consist of a lead-silicate glass tube processed to have a resistive inner surface with a suitably high secondary electron emission to multiply electrons. The following discussion is restricted to discrete-dynode detectors, which are the most common type used in ICP-MS. However most of the principles described can be readily applied to CEMs. 

The basic principles of discrete-dynode electron multiplier operation are shown schematically in Figure 3.1. When an ion strikes the first dynode of a discrete-dynode electron multiplier (or conversion dynode) it liberates secondary electrons. The electron-optics of the dynodes then accelerates these electrons to the next dynode in the multiplier, which in turn produces a greater number of secondary electrons. This process is repeated at each subsequent dynode, generating a cascade of millions of electrons, which are finally captured (as an output pulse , hence the term pulse counting) at the multiplier output electrode. The gain of an electron multiplier can be defined as the average number of electrons collected at the multiplier s output electrode for each input ion that initiates an electron cascade. Similarly, it can be described as the current measured from the output divided by the input ion current. It should be noted that this second definition includes the ion detection efficiency of the multiplier. 

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