The Electron Multiplier

Although photoemissive detectors with no internal electron gain stages are available in the form of photodiodes, most state of the art devices incorporate an electron multiplier to boost photocathode output before it is coupled to the external electronics [5.2]. There are many advantages of electron-multiplication 

Certain dynode materials cannot be used in combination with certain photocathodes thus (CuBeO) is used with the S-1 surface rather than (CsSb). Low dynode photoemission may also be required in some devices, which then prohibits use of high gain but photoemissive (CsSb). 

Two additional special purpose multiplier designs are also quite common. The first is the crossed-field device [5.138] where a magnetic field is used in conjunction with a specified electrostatic field to force high-energy electrons into 

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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). 

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. 

The main advantages of the ms/ms systems are related to the sensitivity and selectivity they provide. Two mass analyzers in tandem significantly enhance selectivity. Thus samples in very complex matrices can be characterized quickly with Htde or no sample clean-up. Direct introduction of samples such as coca leaves or urine into an ms or even a gc/lc/ms system requires a clean-up step that is not needed in tandem mass spectrometry (28,29). Adding the sensitivity of the electron multiplier to this type of selectivity makes ms/ms a powerhil analytical tool, indeed. It should be noted that introduction of very complex materials increases the frequency of ion source cleaning compared to single-stage instmments where sample clean-up is done first. 

Ion intensities up to a count rate of 2 x 10 are measured using a secondary electron multiplier (SEM). When it becomes saturated above that value, it is necessary to switch to a Faraday cup. Its ion-current amplification must be adjusted to fit to the electron multiplier response. 

The mass range requirement invariably means that FAB is used in conjunction with a magnetic sector instrument. Conventional detectors, such as the electron multiplier, are not efficient for the detection of large ions and the necessary sensitivity is often only obtained when devices such as the post-acceleration detector or array detector are used. Instruments capable of carrying out high-mass investigations on a routine basis are therefore costly and beyond the reach of many laboratories. 

Error in measuring the gain or efficiency of the electron multiplier or ion counter... 

The advantage of the photomultiplier compared to the electron multiplier is the longer lifetime (several years). Channel electron multiplier and photomultiplier are mostly used in quadrupole instruments or ion traps. 

Figure 6. EE from the fracture of fiber/epoxy strands made with (a) E-glass, (b) S-glass, and (c d) graphite filaments, in Epon 828. (Detection in (d) is at a lower gain of the electron multiplier compared to (c) ).

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. 

Mass spectrometers work equally well for negative and positive ions by reversing voltages where the ions are formed and detected. To detect negative ions, a conversion dynode with a positive potential is placed before the conventional detector. When bombarded by negative ions, this dynode liberates positive ions that are accelerated into the electron multiplier, which amplifies the signal. 

The experiments of Kistiakowsky and Kydd [1] were done by single-pulse photolysis with a 500-J flashlamp, the reaction vessel contents being sampled via a pinhole leak into the electron ionization source of a Bendix time-of-flight (TOF) mass spectrometer. Mass spectra were obtained by pulsed extraction of ions from the ion source at 50-fis intervals after the flash. The signal from the electron multiplier detector was displayed on a cathode ray tube, which was photographed with a rotating drum camera. 

Fig. 3.4 Apparatus for the study of Rydberg states of alkali metal atoms a, the atomic beam source b, the electric field plates c, the pulsed laser beams and d, the electron multiplier...

Fig. 10.2 Major components of a thermal atomic beam apparatus for microwave ionization experiments,the atomic source, the microwave cavity, and the electron multiplier. The microwave cavity is shown sliced in half. The Cu septum bisects the height of the cavity. Two holes of diameter 1.3 mm are drilled in the side walls to admit the collinear laser and Na atomic beams, and a 1 mm hole in the top of the cavity allows Na+ resulting from a field ionization of Na to be extracted. Note the slots for pumping (from ref. 4).

Fig. 11.4 Two chambered cell allowing the use of field ionization to study collision processes. The lower chamber contains a relatively high pressure and the upper chamber, containing the electron multiplier, is at a lower pressure (from ref. 27).

Electrons from the heated tungsten filament are accelerated to the annular anode. Depending on the anticathode material a characteristic fluorescence radiation is emitted, passes through a thin Aluminum window and induces photoelectrons on the surface of the analytical sample. These photoelectrons are deflected in the spherical electrostatic analyzer, double focussed to eliminate stray electrons and finally counted by the electron multiplier. The whole system works under a vacuum of 10-s to 10 7 torr or even 10 10 torr, if surface properties have to be studied. This vacuum is generated by a Titanium... 

Detector The detector is the last major portion of the mass spectrometer, and it detects the presence, and preferably abundance, of ions after they have exited the mass analyzer. Examples include the electron multiplier, common on quadrupole instruments, and the microchannel plate (an array of electron multipliers), which have been common on TOF instruments. For most users, the actual detector is a relatively invisible portion of the instrument that needs little or no regular attention. 

For MS work, the electron impact (El) mode with automatic gain control (AGC) was used. The electron multiplier voltage for MS/MS was 1450 V, AGC target was 10,000 counts, and filament emission current was 60 pA with the axial modulation amplitude at 4.0 V. The ion trap was held at 200°C and the transfer line at 250°C. The manifold temperature was set at 60°C and the mass spectral scan time across 50-450 m/z was 1.0 s (using 3 microscans). Nonresonant, collision-induced dissociation (CID) was used for MS/MS. The associated parameters for this method were optimized for each individual compound (Table 7.3). The method was divided into ten acquisition time segments so that different ion preparation files could be used to optimize the conditions for the TMS derivatives of the chemically distinct internal standard, phenolic acids, and DIMBOA. Standard samples of both p-coumaric and ferulic acids consisted of trans and cis isomers so that four segments were required to characterize these two acids. The first time segment was a 9 min solvent delay used to protect the electron multiplier from the solvent peak. 

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