Dynode

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

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... 

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. 

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... 

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... 

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. 

Secondary Emission - Electrons striking the surface of a cathode could cause the release of some electrons and, hence, a net amplification in the number of electrons. This principle is used in the construction of photomultipliers where light photons strike a photoemitting cathode releasing photoelectrons. These electrons are subsequently amplified striking a number of electrodes (called dynodes) before they are finally collected by the anode. 

Photomultipliers Secondary electron multipliers, usually known as photomultipliers, are evacuated photocells incorporating an amplifier. The electrons emitted from the cathode are multiplied by 8 to 14 secondary electrodes dynodes). A diagramatic representation for 9 dynodes is shown in Figure 18 [5]. Each electron impact results in the production of 2 to 4 and maximally 7 secondary electrons at each dynode. This results in an amplification of the photocurrent by a factor of 10 to 10. It is, however, still necessary to amplify the output of the photomultipher. 

Fig. 18 Section through an RCA photomultiplier, schematic, dynodes, 10 anode.

Depending on their positioning the dynodes are referred to as being head-on or side-on . Commercial scanners mostly employ side-on secondary electron multipliers where, as the name implies, the radiation impinges from the side — as in Figure 19. Their reaction time is shorter than for head-on photomultipliers because the field strength between the dynodes is greater. 

Fig. 20 Cross section through a head on photomultiplier [51, 52]. — 1—10 dynodes, 11 anode.

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... 

Fig. 2-5a. Schematic diagram of No. 931-A multiplier phototube. 0 = photocathode 1-9 = dynodes 10 = anode. (Radio Corporation of America.)...

The multiplication process is repeated in each succeeding stage, the electrons from the (specially shaped) dynode 9 being collected by the anode 10. A multiplier phototube of this type is normally operated at 75 to 100 volts per stage, and an over-all gain of a million can be realized. 

Dry weight of living tissues, determined by long-wavelength x-ray absorptiometry, 297-300 Duane and Hunt, law of, 7 Dynodes, 56... 

Based on the photoelectric effect, electrons in evacuated tubes (photoelectrons) are released from a metal surface if it is irradiated with photons of sufficient quantum energy. These are simple photocells. Photomultipliers are more sophisticated and used in modem spectrophotometers where, via high voltage, the photoelectrons are accelerated to another electrode (dynode) where one electron releases several electrons more, and by repetition up to more than ten times a signal amplification on the order of 10 can be obtained. This means that one photon finally achieves the release of 10 electrons from the anode, which easily can be measured as an electric current. The sensitivity of such a photomultiplier resembles the sensitivity of the human eye adapted to darkness. The devices described are mainly used in laboratory-bound spectrophotometers. 

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

Mass spectrometry Ionization Accelerated ions Ionsensitive multiplier tubes, dynodes Spectrum Digitalized data... 

A schematic cross-section of one type of photomultiplier tube is shown in Figure 26. The photomultiplier is a vacuum tube with a glass envelope containing a photocathode and a series of electrodes called dynodes. Light from a scintillation phosphor liberates electrons from the photocathode by the photoelectric effect. These electrons are not of sufficient number or energy to be detected reliably by conventional electronics. However, in the photomultiplier tube, they are attracted by a voltage drop of about 50 volts to the nearest dynode. 

Using a voltage potential, the electrons are attracted and strike the nearest dynode with enough energy to release additional electrons. 

The second-generation electrons are attracted and strike a second dynode, releasing more electrons. 

At the final dynode, sufficient electrons are available to produce a pulse of sufficient magnitude for further amplification. 

PMTs contain a photosensitive cathode and a collection anode that are separated by electrical electrodes called dynodes, which provide electron multiplication or gain. The cathode is biased negatively by 400-2500 V with respect to the anode. An incident photon ejected by the photocathode strikes the first dynode... 

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