Dispersive mass spectrometers

Figure 9.18 Early mass spectrometer designs, (a) Aston, 1919 (b) Dempster, 1918 (c) Mattauch-Herzog, 1935 (d) Bainbridge, 1933. In each case, B signifies the magnetic fieid. Spectrometers (c) and (d) are dispersive mass spectrometers (c) the Mattauch-Herzog design is aiso a doubie-sector instrument, using an electric sector before the magnetic field. Modern versions of these spectrometers are still in use, with the photoplate replaced by one of the modern detectors discussed subsequently.

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint 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 any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

A mass spectrometer analyzer disperses ions according to their various m/z values. 

In a mass spectrometer, ions can arrive at a multipoint collector as a spatially dispersed beam. This means that all ions of different m/z values arrive simultaneously but separated in space according to each m/z value. Each element of the array, depending on its position in space, detects one particular m/z value (see Chapter 29, Array Collectors ). 

Alternatively, the ions in a mass spectrometer can also arrive at a multipoint collector as a temporally dispersed beam. Therefore, at any point in time, all ions of the same m/z value arrive simultaneously, and different m/z values arrive at other times. Ail elements of this collector detect the arrival of ions of one m/z value at any one instant of time. This type of detector, which is also an array, is called a microchannel plate collector of ions. 

After the analyzer of a mass spectrometer has dispersed a beam of ions in space or in time according to their various m/z values, they can be collected by a planar assembly of small electron multipliers. There are two types of multipoint planar collectors an array is used in the case of spatial separation, and a microchannel plate is used in the case of temporal separation. With both multipoint assemblies, all ions over a specified mass range are detected at the same time, or apparently at the same time, giving these assemblies distinct advantages over the single-point collector in the analysis of very small quantities of a substance or where ions are produced intermittently during short time intervals. 

The heart of the mass spectrometer is the mass analyzer, the function of which is to measure the mass-to-charge ratios of ions and provide a means of their identification. This is achieved by a combination of a dispersive action to separate the ions according to their m/e ratios and a focusing action to maximize the resolved ion intensities... 

The mass spectrometer sampling capillary or the dispersive infra-red analyzers used for continuous analysis and monitoring of the gas phase composition are situated between the reactor and the sampling valve, as close to the reactor as possible, in order to avoid any delay in the recording of changes in the composition of reactants or products. This delay should be taken into account when plotting simultaneously the time dependence of catalyst potential or current and gas phase concentration of the reactants or products. 

Belshaw NS, Freedman PA, O Nions RK, Frank M, Guo Y (1998) A new variable dispersion double focusing plasma mass spectrometer with performance illustrated for Pb isotopes. Inti J Mass Spectrom 181 51-58... 

Which directs them toweurds the analyzer slits. Alternatively, they may be extracted by the field penetration of the high voltage on the focusing electrodes. In both instances the ion beam is usually focused, collimated and accelerated to provide a beam of narrow energy dispersion that is capable of traversing the analyzer section of the mass spectrometer. In modern mass spectrometers the ionization source and analyzer sections are usually differentially pumped, allowing the source to operate at a distinctly higher... 

Stephens, W. E. A pulsed mass spectrometer with time dispersion. (Proc. Am. Phys. Soc.) Phys. Rev. 1946, 69, 691. 

GC-IR is becoming more widely used because FT spectrometers (p. 281) have virtually replaced the older dispersive types and even with computerized enhancements are much cheaper than mass spectrometers. 

Using the Tomtec Quadra 96 workstation, 0.1 mL of the ethyl acetate layer was transferred to a 96-well collection plate containing 0.4 mL of acetonitrile in each sample well. The solution was mixed 10 times by aspiration and dispersion on the Tomtec. The plate was then covered with a sealing mat and stored at 2 to 8°C until LC/MS/MS analysis. The HILIC-MS/MS system consisted of a Shimadzu 10ADVP HPLC system and Perkin Elmer Sciex API 3000 and 4000 tandem mass spectrometers operating in the positive ESI mode. The analytical column was Betasil silica (5 fim, 50 x 3 mm) and a mobile phase of acetonitrile water formic acid with a linear gradient elution from 95 5 0.1 to 73.5 26.5 0.1 was used for 2 min. The flow rate was 1.0 mL/min for the API 3000 and 1.5 mL/min for the API 4000 without any eluent split. The injection volume was 10 jjL and a run time of 2.75 min was employed. 

Field Free Region Any region of a mass spectrometer where the ions are not dispersed by a magnetic or electric field. 

W. E. Stephens. A Pulsed Mass Spectrometer with Time Dispersion. Phys. Rev., 69(1946) 691. 

The ability to separate ions spatially is called the dispersion of a mass spectrometer. Dispersion is simply the distance between the centers of two ion beams that differ in mass by Am at the collection plate. A simple sector instrument, where the ion beam enters and exist the magnetic field normal to the pole faces and the object and image distances are the same, is known as a symmetrical geometry analyser. Examples are shown in Fig. 8. In this case, the dispersion, D is given by ... 

Many older isotope ratio mass spectrometers employed symmetrical geometry. Most new instruments employ asymmetric design known as extended geometry, providing greater dispersion for the same radius ( 2x that of a symmetrical geometry). 

Fletcher IR, Maggi AL, Rosman KJR, McNaughton NJ (1997b) Isotopic abundance measurements of K and Ca using a wide-dispersion multi-collector mass spectrometer and low-fractionation ionisation techniques. Int J Mass Spectrom Ion Proc 163(1-2) 1-17... 

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