Magnetic electron multiplier

SFC Electron inpact (El) Reversed geometry (BE) Magnetic electron multiplier... 

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

An AutoSpec-TOF mass spectrometer has a magnetic sector and an electron multiplier ion detector for carrying out one type of mass spectrometry plus a TOF analyzer with a microchannel plate multipoint ion collector for another type of mass spectrometry. Either analyzer can be used separately, or the two can be run in tandem (Figure 20.4). 

Thus, ions are produced, deflected in a magnetic field, then focused in an electric field, and finally detected by an electron multiplier or other ion detector. 

Reduction of the measurement time for element distributions is possible by simultaneous detection of several masses. This can be achieved only by use of a magnetic sector field spectrometer with Mattauch-Herzog geometry [3.49] (Fig. 3.20) and parallel detection of up to five masses by mechanically adjusted electron multipliers. 

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. 

The analyser will always be preceded by some form of collection optics, and followed by an ion detector (usually a channel electron multiplier which converts ions into electron showers). There are three types of analyser for use in SIMS spectrometers, the magnetic sector instrument, the quadrupole analyser and time-of -flight (TOF) systems. 

Rate constants are usually determined in the pulsed mode by switching on the electron gun for several microseconds followed by reaction of the ions in a field-free region. Magnetic mass analysis coupled with an electron multiplier in which the pulses are collected in a multiscaler provide a time development of the particular ions under analysis. There have been few reactions relevant to this review which have been studied by HPMS. 

Thermal-ionization mass spectrometers (TIMS) combine a hot-filament source with a magnetic-sector mass spectrometer. The mass spectrometers are operated at low to moderate mass-resolving power. A large number of elements can be measured with thermal ionization mass spectrometry. Special care is taken to purify the samples using ion exchange columns. Samples are loaded onto the filaments along with an emitter, and a typical run may take several hours. Modem systems have multiple collectors so that several isotopes can be measured simultaneously. High-precision measurements are done with Faraday cup detectors, but low-abundance isotopes can be measured on electron multipliers. Modem machines are capable of precisions of 0.1 to 0.01 permit. 

Fig. 2. Mass spectrometer with photoionization 1—built-in hydrogen lamp 2—vacuum monochromator filled with hydrogen 3—LiF window 4—ionic source container 5—photoionization space with the accelerating grids 6—fluorescent layer for intensity calibration of the incident u.v. light 7—photomultiplier 8—magnetic mass analyzer 9—electron 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. 

In concluding this section, it is pertinent to take note of a special kind of isotopic fractionation ubiquitous, often quite severe, and arguably the most important source of fractionation that must be taken into consideration in noble gas geochemistry. This fractionation arises in mass spectrometric analysis contributory effects can and do arise in gas extraction and transport through the vacuum system, in the ion source (especially when a source magnet is used), in beam transmission, and in ion collection and detection (especially when an electron multiplier is used). As noted in Section 1.3, sample data are corrected for instrumental (and procedural) discrimination, which is calibrated by analysis of some standard gas (usually air). This is a roundabout and imperfect near-equivalent to the 8 value convention, which is the norm in stable isotope geochemistry (O, C, H, S, N, etc.). The reproducibility of instrumental discrimination inferred from repeated calibration analysis is usually quite satisfactory, but seldom is any care taken to try to match operating conditions in samples and calibration analyses. It is thus a matter of faith - undoubtedly quite... 

---