Electron multiplier detector

As with mass analyzers, many types of mass detectors equipped with an electron multiplier are available. Most common mass detectors are the chan-neltron, Daly detector, electron multiplier tubes, and the Faraday cup. All generate a current when charged analytes generated in the source and separated in the analyzer impinge on them. This current is recorded as a function of the masses selected by the electrical-field settings. 

DCEMS together with low-temperature and in-field measurements require much more sophisticated experimental equipment. Gas counters, scintillation detectors, electron multipliers (Channeltron, Ceratron), surface barrier silicon semiconductor detector, and electron energy analyzers belong to the most frequently used detectors applied in CEMS and CXMS, respectively. Differences among individual constructions can be found in Ref. 123. Commercially available version of CEMS/CXMS spectrometer is depicted in Fig. 18.36 [127]. Device is based on 27t proportional continuous gas flow counter for room-temperature zero-magnetic field measurements. 

After the APCI or ESI processes have been run, the analyte is focused by means of octapoles and lenses and fed into the mass filter, such as, for example, an ion trap. In this segment, the solvent has been completely separated, leaving the ions isolated in the ion trap. After separation on the basis of their mass to charge ratio, the ions are accelerated to the detector (electron multiplier). Figure 4.3 provides a schematic representation of a mass spectrometer with ESI sprayhead and ion trap and liquid chromatography apparatus. 

Qualitative analysis is the process of the determination of the presence (or absence) of a particular element or group of elements in a sample. The ability to perform a comprehensive qualitative analysis is directly related to the sensitivity of the analysis method and hence the detection capability. Ideally, it is desirable to determine major, minor, trace, and ultratrace concentration level elements simultaneously on the same sample aliquot, which requires an instrument and technique that exhibits a wide dynamic range of measurement of the ion currents for the various element isotopes. However, in practice, it is often difficult to determine the high intensity of major elements on the same sample dilution as that required for ultratrace concentration element determination (usually measured on undiluted sample). This problem is often accommodated by the use of very low abundance isotopes of the major concentration analyte, reducing the analysis sensitivity. Where no low abundance isotope is available, instrumentation with dual detectors (electron multiplier for low concentration analytes and Faraday analog detectors for high concentration elements) can be used effectively. 

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).

In the other types of mass spectrometer discussed in this chapter, ions are detected by having them hit a detector such as an electron multiplier. In early ICR instruments, the same approach was taken, but FT-ICR uses a very different teclmique. If an RF potential is applied to the excitation plates of the trapping cell (figure B 1.7.18(b)) equal to the cyclotron frequency of a particular ion m/z ratio, resonant excitation of the ion trajectories takes place (without changing the cyclotron frequency). The result is ion trajectories of higher... 

In TOF-SARS [9], a low-keV, monoenergetic, mass-selected, pulsed noble gas ion beam is focused onto a sample surface. The velocity distributions of scattered and recoiled particles are measured by standard TOF methods. A chaimel electron multiplier is used to detect fast (>800 eV) neutrals and ions. This type of detector has a small acceptance solid angle. A fixed angle is used between the pulsed ion beam and detector directions with respect to the sample as shown in figure Bl.23.4. The sample has to be rotated to measure ion scattering... 

Figure Bl.23.5. Schematic illustration of tlie TOE-SARS spectrometer system. A = ion gun, B = Wien filter, C = Einzel lens, D = pulsing plates, E = pulsing aperture, E = deflector plates, G = sample, PI = electron multiplier detector with energy prefilter grid and I = electrostatic deflector.

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

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). 

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... 

An incident ion beam causes secondary electrons to be emitted which are accelerated onto a scintillator (compare this with the operation of a TV screen). The photons that are emitted (like the light from a TV screen) are detected not by eye but with a highly sensitive photon detector (photon multiplier), which converts the photon energy into an electric current. 

The array detector (collector) consists of a number of ion-collection elements arranged in a line each element of the array is an electron multiplier. Another type of array detector, the time-to-digital converter, is discussed in Chapter 31. 

An array ion collector (detector) consists of a large number of miniature electron multiplier elements arranged side by side along a plane. Point ion collectors gather and detect ions sequentially (all ions are focused at one point one after another), but array collectors gather and detect all ions simultaneously (all ions are focused onto the array elements at the same time). Array detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of a substance are available. For these very short time scales, only the array collector can measure a whole spectrum or part of a spectrum satisfactorily in the time available. 

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. 

Another form of array is called a microchannel plate detector. A time-of-flight (TOP) mass spectrometer collects ions sequentially in time and can use a point detector, but increasingly, the TOP instrument uses a microchannel plate, most particularly in an orthogonal TOP mode. Because the arrays and microchannel plates are both essentially arrays or assemblies of small electron multipliers, there may be confusion over their roles. This chapter illustrates the differences between the two arrays. 

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. 

By placing a suitable detector at the focus (a point detector), the arrival of ions can be recorded. Point detectors are usually a Faraday cup (a relatively insensitive device) or, more likely, an electron multiplier (a very sensitive device) or, less likely, a scintillator (another sensitive device). 

Arrival of ions, which have a positive or negative charge, causes an electric current to flow either directly (Faraday cup) or indirectly (electron multiplier and scintillator detectors). 

Each element of an array detector is essentially a small electron multiplier, as with the point ion collector, but much smaller and often shaped either as a narrow linear tube or as somewhat like a snail shell. 

The ions in a beam that has been dispersed in space according to their various m/z values can be collected simultaneously by a planar assembly of small electron multipliers. All ions within a specified mass range are detected at the same time, giving the array detector an advantage for analysis of very small quantities of any one substance or where ions are produced intermittently during short time intervals. 

Mattauch-Herzog geometry, which simultaneously focuses all resolved masses onto one plane, allowing the integrating properties of an ion-sensitive emulsion to be used as the detector. Although electrical detection with an electron multiplier can be applied, the ion-sensitive emulsion-coated glass photographic plate is the most common method of detection and will be described in this article. 

Photocells and photomultipliers (secondary electron multipliers, SEM) are mainly employed in photometry. These are detectors with an external photo-effect . 

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. 

Soft X-ray absorption measurements are done at low-energy synchrotron X-ray facilities such as the UV ring at NSLS or the Advanced Photon Source (APS) at Lawrence Berkeley National Laboratory (LBNL). The beam size is typically 1 mm in diameter. The electron yield data are usually obtained in the total electron yield (EY) mode, measuring the current from a channel electron multiplier (Channeltron). Sometimes a voltage bias is applied to increase surface sensitivity. This is referred to as the partial electron yield (PEY) mode. Huorescence yield (EY) data are recorded using a windowless energy dispersive Si (Li) detector. The experiments are conducted in vacuum at a pressure of 2 X 10 torr. 

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