Magnetic analysers

Figure Bl.6.3 Electron energy analysers that use magnetic fields (a) the trochoidal analyser employing an electromagnet, (b) the Wien filter and (c) the sector magnet analyser. Trajectories for electrons of different energies are shown.

Reverse-geometry double-focusing mass spectrometer A double-focusing mass spectrometer in which the magnetic analyser precedes the electrostatic analyser. 

Double-focusing mass spectrometer with EB configuration in which a deflection of 90° electrostatic analyser (E) is followed by a 90° magnetic analyser (B) (or reverse geometry - BE). 

Greater resolution can be achieved when a magnetic analyser is coupled with an electrostatic analyser (in a double focusing" mass spectrometer).. Using this combination of analysers, mass accuracies of around 1 part per million (ppm) can be obtained. However, magnetic sector instruments have relatively low sensitivity and are very expensive. 

Currently used magnetic analyser mass spectrometers constitute a logical evolution of the previously described instrument. They provide very accurate m/z values, but are limited for high mass analyses (problems produced in the magnetic sector). They also include an electrostatic sector E placed after ion acceleration and before the magnetic field, B (Fig. 16.3). 

Figure 16.3—Magnetic analyser-based instrument. A model JMS AX505 from Jeol is shown. The characteristic shape of the electromagnet can be seen on this photograph (magnetic sector). The detector lies at the extreme right of the photograph and the ion source appears at the centre of the picture. It should be noted in the figure that the instrument is also coupled to a GC. (Reproduced by permission of Jeol, Japan.)...

Figure 16.5—Magnetic analyser mass spectrometer, a) Nier Johnson system, b) directional focusing properties of the magnetic field, c) principle of a double focusing mass spectrometer d) Mattauch-Herzog system.

Magic angle, 130 Magnetic analyser, 293, 296 Magnetic prism, 297 Magnetisation vector, 132 MALDI, 310... 

For every value of 5, the ions with the same charge and the same momentum (mv) have a circular trajectory with a characteristic r value. Thus, the magnetic analyser selects the ions according to their momentum. However, taking into account the kinetic energy of the... 

A peak position is given by a time value (or a Hall voltage value in a magnetic analyser or a voltage value in a quadrupole), which must be converted into a mass value. This supposes a preliminary calibration with known products (PFK, (Csl) , etc.). The masses furnished by the calibrating product are stored in the computer. A relationship of the type m = ax + b is calibrated with two known peaks and is checked or corrected with known peaks located in other mass areas. 

Magnetic analyser an instrument that causes a direction focusing produced by a magnetic field that is perpendicular to the ions displacement direction. Its effect is to bring to a common focus all the ions of a given momentum. This can be converted to m /z ratios if all the ions have the same kinetic energy. 

Equipment fitted with magnetic analysers has the advantage of an exceptional mass resolution (up to A//AA/ = 100000) to the detriment, however, of sensitivity. Time of flight spectrometers, coupled to pulsed primary beams, offer excellent resolution without any intrinsic limitation in terms of mass. 

Figure 16.5 Mass spectrometer with magnetic analyser, (a) Nier-Johnson assembly (b) View of the directional focusing by the magnetic sector (the entrance and exit planes are oblique with respect to the angle of incidence of the beam, ensuring focusing) (c) Mattauch-Herzog assembly (d) Arrangement of a double focusing spectrometer (e.g. R = 40 cm and R = 60 cm).

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