Sector mass analyzers

Sector-type mass analyzers are named for the magnet (B) and electric (E) sectors, which bend the trajectory of ions through curvature paths induced by sector-shaped magnets or curved metal electrodes, respectively. S-MS utilizes the full expression of the Lorenz force equation, which describes magnetic (F ) and electric (F ) forces acting on an ion with charge q  

In contrast to B sectors, E sectors utilize the component of Equation 3.27. Such instm-ments select ions according to their kinetic energy rather than momentum (as in B sectors). The radius of the circular ion path formed in the static electric sector can be derived by combining centrifugal force (Equation 3.28) with electric force, similar to Equation 3.29 but by replacing with Fg, to give  

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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. 

Because there is so little mass bias in the mass analyzer, a discussion of ion transfer optics and collectors is not presented. The ion transfer optics of the magnetic sector mass analyzer, and the collectors used for isotope ratio measurements are critical design elements in all isotope ratio mass spectrometers and recent reviews of these items can be found in Habfast... 

Magnetic sector mass spectrometers are the most common type used in cosmochemistry. In these devices, ions are separated based on their mass-to-charge ratio. The equation governing the magnetic sector mass analyzer is... 

FIGURE 2.2. Schematic diagram of a single-focusing, 180° sector mass analyzer. The magnetic field is perpendicular to the page. The radius of curvature varies from one instrument to another. 

Walder, A. J., and Freedman, P. A. (1992) Isotopic ratio measurement using a double focusing magnetic sector mass analyzer with an inductively coupled plasma as an ion source. J. Anal. Atomic Spect. 7, 571-575. 

A dispersive magnetic sector mass analyzer does not use a flight tube with a hxed radius. Since all ions with the same kinetic energy but different values of m/z will follow paths with different radii, advantage can be taken of this. The ions will emerge from the magnetic held at different positions and can be detected with a position-sensitive detector such as a photoplate or an array detector. Examples of dispersive magnetic sector systems are shown in Fig. 9.16(c) and (d). 

Fig. 19.3 Magnetic sector mass analyzer, with m/z 32 impinging on the detector note flat-topped peak profile. (Used with permission of ThermoOnix Inc.)...

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