Wien Filter

A Wien filter, also known as a velocity filter, is used to separate ions according to their velocity and thus indirectly according to their mass-to-charge ratio (m/z). This separation is based on the simultaneous application of an electric and a magnetic field, which are arranged orthogonally to each other. A Wien velocity filter consists of a homogeneous electric field and a homogeneous magnetic field that are perpendicular to each other. The ions enter perpendicular to both fields and only those ions that have the appropriate velocity pass through the velocity filter. The forces of the electric and magnetic fields are equal, slower or faster ions are deflected and moved out of the way. As the speed of the ions depends on their mass-to-charge ratio, the Wien filter can be used as a mass selection tool. Ions with different m/z values have different velocities and are therefore deflected to different degrees.

In TOF-SIMS, the separation of ion masses is mainly realized by their time of flight, which is why Wien filters are only used here in certain cases. In such special applications, a Wien filter could be used to preselect the ions or to improve mass selection, especially when using pulsed ion sources or when combining TOF-SIMS with other analytical methods.

This is because with a complex sample shape, the emission angle of the secondary ions is strongly influenced by the electric field. These uncertainties occur due to differences in the initial energy and emission angle, even if the ions in question have the same mass. Therefore, in some ToF measurement systems, the time-of-flight differences of the emitted secondary ions are compensated by Wien filters. The goal of such measures is to improve the mass resolution and, if necessary, to achieve better delineated imaging.

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