Discrete dynode secondary electron

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... 

Figure 12.20 shows the structure of the side-window circular cage type and linear focused head-on type of photomultiplier which are both preeminent in fluorescence studies. The lower cost of side-window tubes tends to favor their use for steady-state studies, whereas the ultimate performance for lifetime studies is probably at present provided by linear focused devices. In both types internal current amplification is achieved by virtue of secondary electron emission from discrete dynode stages, usually constructed of copper-beryllium (CuBe) alloy, though gallium-phosphide (GaP) first dynodes have been used to obtain higher gains. 

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. ... 

The pulse height distribution at high detector gain often shows a structure, probably due to the discrete numbers of secondary electrons emitted at the first dynode. An example for an H7422P-40 module is shown in Fig. 6.14. 

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 7.5 Schematic diagram of an off-axis detector for a mass spectrometer. The conversion dynode is maintained at a high potential (up to 10-20 kV) thus accelerating the ions to high velocities to improve the secondary emission efficiency (the diagram is drawn to illustrate the airangement for positive ions). The secondary elections are then accelerated to the first dynode (maintained at 2 kV) and the SEM then amphfies the secondary electron current as usual. For negative ions the conversion electrode is maintained at a high positive potential and secondary positive ions are accelerated on to the first dynode. This example portrays a quadrupole analyzer with a discrete dynode SEM. Reproduced from Photomultiplier Tubes Basics and Applications (3rd Edn), Hamamatsu Corporation, with permission.

Schematics illustrating the principles underlying CEMs are shown in Figure 7.7. Unlike the discrete dynode case, the potential gradient that accelerates the electrons along the device is not created in discrete steps using a resistor chain (Figure 7.3) but rather by adjusting the intrinsic resistance of the interior wall of the device such that a continuous potential gradient is provided this also results in a low background current that replenishes the surface charge lost to the secondary electron avalanche.

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.

---