Magnetic sector field

Apart from the quadrupole and TOP analyzers described in Sect. 3.2.2, the most important types of mass analyzer used in common dynamic SIMS instruments employ a magnetic-sector field. 

Magnetic Sector Field. In a magnetic field B an ion with the velocity v and the charge q experiences a centripetal force, the Lorentz force P ... 

Two types of commercial magnetic sector field instrument will be described in the following ... 

Fig. 3.19. Basic set-up of a direct imaging magnetic sector instrument. The stigmatic secondary ion optics consists of an electrostatic analyzer (ESA) and a magnet sector field.

The direct imaging magnetic sector mass analyzer (Fig. 3.19) has the unique property that all parts (lenses, electrostatic analyzer and magnetic sector field) of the secondary ion optics are stigmatic (comparable with light microscopes). This means that all points of the surface are simultaneously projected into the analyzer. 

Magnetic sector field instruments have mass resolutions up to m/Am = 20000 quad-rupole instruments are limited to a mass resolution of approximately m/Am = 500. For both types of instrument the a mass range extends to 500. 

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. 

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

The most commonly used mass separators are quadrupoles, ions traps and time-of-flight analyzers, for which the principle of mass separation is discussed below. Additionally, other types such as magnetic sector field or Fourier-trans-form cyclotron-resonance instruments are available. 

Quadrupole Magnetic sector field Electric sector field Time of flight (ToF) Electrical Ion-trap Magnetic Ion-trap... 

Magnetic sector field (B) Combination of magnetic (B) and electric sector fields (E) Quadrupole mass analyzer (Q) Time-of-flight mass analyzer (ToF)... 

Figure 1.2 Principle of the operation of a mass spectrometer including sample introduction system, ion source, mass separator (e.g., a magnetic sector field) and ion detector system (e.g., double ion collectors for simultaneous measurements of two separated ion beams).

A quite different type of mass spectrometer - the first 180° magnetic sector field mass spectrometer (see Figure 1.7), with directional focusing of ions for isotope analysis, was constructed by Dempster, independently of other instrumental developments in mass spectrometry, in 1918. 

Double-focusing mass spectrometers with high mass resolution are rather bulky and expensive instruments so that the development of single magnetic sector field mass spectrometers was of significance in the following years. 

Figure 1.14 Schematic of a 60° magnetic sector field mass spectrometer (Nier s mass spectrometer, 1940) 5, - entrance slit S2 - exit slit (H.Kienitz (ed.), Massenspektrometrie (1968), Verlag Chemie, Weinheim. Reproduced by permission of Wiley-VCH.)...

In general, ions generated by thermal ionization possess low initial energies (0.1-0.2eV), therefore mostly single magnetic sector field mass spectrometers are used for ion separation. 

The ions formed are extracted by the extraction lens, EL, perpendicular to the electron beam. Due to the low energy spread of ions formed in an electron ionization source (< 1 eV) one single-focusing magnetic sector field mass spectrometer or quadrupole analyzer is sufficient for... 

The radius of trajectory rm in cm of an ion on its circular path through a homogeneous magnetic sector field with magnetic force (gauss) and accelerating voltage V (volts) is determined by ... 

For a symmetric magnetic sector field, the entrance slit and the exit slit at A and A", respectively, are at the same distance from the magnetic field boundary. From the lens equation of a symmetric magnetic sector field with Iff = Iff Equation (3.6) is vahd... 

But in practice no ion emitting point (A ) exists, because mass spectrometers use slit systems (entrance and exit slits) of finite width (b and b", respectively), and so parameters b and b" are introduced to describe the defined object and image widths of the entrance and exit slits (A and A"), respectively. The ratio b"/b is defined as the so-called transverse magnification Gm of a homogeneous magnetic sector field ... 

By using non-normal beam incidence and curved pole pieces, the effective radius of curvature of the magnetic sector field is extended, which results in an increase in dispersion and an improvement in the abundance sensitivity of the mass spectrometric system.4... 

The prism effect of a homogeneous magnetic sector field is crucial for the separation of different types of ions according to their mass-to-charge ratio. In Figure 3.3 the principle of mass separation... 

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