Answer 4.9

a) In an ICR cell the magnetic field is only able to radially confine the ions by forcing them on a circular path. Why do ions not exit the cell in axial direction?

The front and back ends of the (cylindrical) ICR cell are equipped with an endcap electrode, typically a flat disk to which low trapping voltages are applied. Provided the ions are entering the ICR cell at very low kinetic energy (about 1 eV), this trapping voltage creates a portentail well just high enough to prevent the ions from escaping.

b) Which side effect is caused by axial trapping in ICR cells?

Axial ion trapping forces the ions to oscillate axially inside the ICR cell. The electric trapping field suffers from curvature in regions closer to the trapping electrodes, and thus, introduces a component in radial direction. The radial component now also affects the circular ICR motion. In other words, axial motion and ICR motion become coupled to some extent. This causes splitting of the ideal cyclotron motion and also the corresponding frequency into three components: reduced cyclotron frequency, magnetron frequency, and trapping oscillation.

c) Can you name some techniques employed to reduce the influence of the magnetron motion?

ICR cells can be designed in numerous geometries. The most common approaches to reduce curved electric trapping fields towards the ends of the cell walls are to use either axially segmented ICR cells or segmented trapping plates. More recently, a dynamically harmonized ICR cell has been introduced that enables a 5-10 times higher resolving power than the aforementioned techniqes.