The Magnetic Sector Mass Analyzer

As can be seen from Equation 8.16, the greater the value of m/z, the larger the radius of the curved path. The analyzer tube of the insttument is constracted to have a fixed radius of curvature. A particle 

FIGURE 8.10 Schematic of a magnetic sector mass analyzer. (From Smith, R. M., Understanding Mass Spectra, A Basic Approach, 2nd ed., John Wiley and Sons, New York, 2004. Reprinted with permission.) 

An important considexation in mass spectrometry is resolution, defined according to the relationship 

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 

Either the Magnetic Field strength (B) or the Accelerating Potential (V) can be varied to bring different m/z ions into focus (detection) 

Ions with different m/z values have different radii of curvature through the magnetic field as described by Equation 3.15 

FIGURE 3.11 Schematic of a magnetic sector mass 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... 

Fig. 2.9. General schematic of a sector mass analyzer. Ions extracted from the ion source are accelerated by an electrostatic field (accelerating potential, 10 and enter the sector analyzer with velocity, v. Electric (electric flux density, E) or magnetic (magnetic flux density, 6) fields bend the trajectory of the ions into curved paths with radius, r. Trajectories of ions with larger m/z are affected more than smaller ones. An illustration of the direction-focusing ion beam approach in a magnetic sector mass analyzer is shown in the insert. Due to the dependence of the radius of an ion s trajectory on its kinetic energy ( ) in the electrostatic sector mass analyzer and on its momentum (mv) in the magnetic sector mass analyzer, the systems are also referred to as ion energy and ion momentum filters.

Since r is fixed, there are two parameters that can be varied to change the mass of the ions that passes through the magnetic sector mass analyzer to the ion detector. These are B, the magnetic field strength, and V, the accelerating potential of the source. To maximize the transmission of ions out of the ion source into the mass analyzer, and then out of the mass analyzer into the ion detector, the... 

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. 

The mean free path is the average distance a molecule travels before colliding with another molecule. The mean free path, X, is given by X = kT/ jr2 itP). where k is Boltzmann s constant, Tis the temperature (K), P is the pressure (Pa), and cr is the collision cross section. For a molecule with a diameter d, the collision cross section is ltd2. The collision cross section is the area swept out by the molecule within which it will strike any other molecule it encounters. The magnetic sector mass spectrometer is maintained at a pressure of 10-5 Pa so that ions do not collide with land deflect) each other as they travel through the mass analyzer. What is the mean free path of a molecule with a diameter of 1 nm at 300 K in the mass analyzer ... 

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.)...

Figure 3.8 / magnet sector mass analyzer bends the ion trajectory by 60°. The entrance and exit slits allow selected ions to pass through to the detector based on their momentum and charge... 

---