Dynode multipliers

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

CE Photoionisation (PI) Ion-cyclotron resonance (ICR) Continuous dynode multiplier... 

At the electron multiplier detector, each arriving ion starts a cascade of electrons, just as a photon starts a cascade of electrons in a photomultiplier tube (Figure 20-12). A series of dynodes multiplies the number of electrons by 105 before they reach the anode where current is measured. The mass spectrum shows detector current as a function of mlz selected by the magnetic field. 

With the exception of an ICR-MS, nearly aU mass spectrometers use electron multipliers for ion detection. There are three main classes of electron multipliers discrete dynode multipliers, continuous dynode electron multipliers (CDEM), also known as channel electron multipfiers (GEM), and microchannel plate (MCP) electron multipliers, also known as multichannel plate electron multipliers. Though different in detail, aU three work on the same physical principle. An additional detector used in mass spectrometers is the Faraday cup. 

Continuous dynode multipliers with a channeltron . The ions are directed towards a collector whose entrance, in the form of a horn, is made of a lead doped glass with which acts as the conversion cathode. The ejected electrons are attracted towards a positive electrode (Figure 16.26) and their collisions against the internal walls give rise to multiplication, as with the separated dynodes. The assembly is usually mounted off-axis to avoid the impact of neutral species as well as photons emitted by the filament, equally susceptible to the removal of the electrons. 

The most common detector is the photomultiplier tube (PMT). A PMT is a sealed, evacuated transparent envelope (quartz or glass) containing a photoemissive cathode, an anode, and several additional electrodes called dynodes. The photoemissive cathode is a metal coated with an alkali metal or a mixture of elements (e.g., Na/K/Cs/Sb or Ga/As) that emits electrons when struck by photons. The PMT is a more sophisticated version of a vacuum phototube (Fig. 5.17), which contained only a photoemissive cathode and an anode the photocurrent was hmited to the electrons ejected from the cathode. In the PMT (Fig. 5.18), the additional dynodes multiply the available electrons. The ejected electrons are attracted to a dynode that is maintained at a positive... 

The discrete dynode multiplier is composed of a series of coppertberylium plates between which the electrons cascade. 

Truong QS, Keeffe R, Ellacott T, Desson K, Herber N (2001) In IAEA symposium on international safeguards, IAEA-SM-367/8/08/P Tuttas D, Schwieters JB, Quaas N, Bouman C (1998) Improvements in TIMS high precision isotope ratio measurements for small sample sizes. Application note 30136, Thermo Eisher Scientific, Bremen Tuttas D, Schwieters JB, Bouman C, Deerberg M (2005) New compact discrete dynode multipliers integrated into the thermo scientific TRITON variable multicollector array. Application note 30192, Thermo Fisher Scientific, Bremen United Nations Security Council (1991) Text of UNSC Resolution 687. Available at http //www.fris.org/ news/un/iraq/sres/sres0687.htm... 

A calculation method based on isotope ratios for the determination of dead time and its uncertainty in ICP-MS and application of the method to investigating some features of a continuous dynode multiplier. J. Anal. At. Spectrom., 14, 1075-1079. 

While the principles of operation are similar, the structures of discrete-dynode multipliers and CEMs are quite different. Figure 3.1 illustrates each type schematically. 

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. 

At present, TOF mass analyzers are not widely used in ICP-MS. However, there are commercial systems available (as described in Chapter 2) and the following brief description of detectors used in TOF-MS applications is included for completeness. There are two main types of detectors used in TOF-MS applications microchannel plates (MCPs) and fast discrete-dynode multipliers. 

The three main kinds of electron multipliers are (1) the discrete dynode electron multipliers, (2) the continuous dynode multipliers (Figure 2.4) that work in a similar way, and (3) microchannel plates that are discussed in Section 23.2 ... 

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