SIMS Instrumentation: Mass Analyzers

Dynamic SIMS instruments use two kinds of mass analyzers, magnetic sector and quadrupole. Magnetic sector instruments are most common. As the ion beam passes through the magnetic field, the particles are acted on by a force at right angles, both to the direction of motion and to the direction of the magnetic field. The following equation shows the relationship between the magnetic field (B), the ion accerating voltage (V), the mass-to-charge ratio (m/q), and the radius of ion curvature (r) in the magnetic field. In atomic units, m/q becomes m/z where z is the number of charges on the ion.

Magnetic sector mass analyzer is shown in blue. The ion beam trajectories (indicated in red) are greatly exaggerated in the lateral directions.

Modern mass spectrometers use non-normal pole faces for entrance and exit of the ion beam to the magnetic sector. The fringings fields in this configuration compress the ion beam in the vertical direction (in and out of the screen) as it passes through the sector. Fewer ions strike metal surfaces and the ion beam focuses better at the exit slit with non-normal pole faces. The entrance and exit slits can be arranged at ion beam crossovers for the cleanest separation (highest mass resolution) between ions with similar m/z values. The green part of the beam represents ions with higher m/z values that do not pass through the spectrometer.

The combination of a magnetic and an electrostatic sector produces a double focusing instrument. A magnetic analyzer, by itself, introduces chromatic aberrations into an ion beam with dispersed ion energies. These aberrations reduce mass resolution. In a series arrangement of one electrostatic and one magnetic sector, the energy dispersion of the electrostatic sector can just compensate the energy dispersion of the magnet. The system will have the mass dispersive properties of the magnet, except that it will produce higher mass resolution. The spectrometer lens adjusts the cross-over from the electrostatic sector to the location required for the magnetic sector.

Quadrupole mass analyzers were invented in 1953. Many kinds of analysis, including SIMS, employ quadrupoles. Ideally, the rods have hyperbolic shapes, but this geometry can be approximated with closely spaced circular rods. In a typical quadrupole spectrometer, the rods are 1 cm in diameter and 20 cm long. In the diagram, ions enter from the left at a relatively low energy (~25 eV). Since SIMS ions can have a wider energy range than 25 eV, electrostatic sectors usually precede the quadrupole.

Alternating and direct voltages on the rods cause the ions to oscillate after entering the quadrupole. For a given set of voltages, Ions with a single mass-to-charge ratio undergo stable oscillation and traverse through the rods. All other ions have unstable oscillations and strike the rods. The alternating frequency and the ratio between the alternating and direct voltages remain constant. Scanning the voltages scans the mass spectrum.

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