Ion trap mass analyzers operate similarly to quadrupole instruments. However, instead of filtering a flowing ion stream like quadrupoles, ion traps capture and store ions in three-dimensional space. Before saturation, ion traps can eject specified ions by mass for detection according to their m/z.

Within the ion trap, a series of experiments can fragment target ions to better define the parent ion based on the fragmentation pattern. The ability to garner more structural information on a single analyte by continuously fragmenting (i.e. fragmenting ions, selecting specific fragment ions, and repeating fragmentation) is known as MSn.

The electric field produced by applying RF voltages to the stacked or “sandwich” geometry of two end-cap electrodes traps ions in the space between the electrodes. Ramping or scanning the RF voltage causes ions in a resonant frequency or trapping state to be ejected. However, dynamic range is often limited. The finite volume and containment space for ions restricts the measurement range, especially for samples in complex matrices.

Given similar functionality, ion traps are combined with quadrupoles, marrying the ion current advantages of quadrupoles with the ion trapping behavior to improve sensitivity in ways not possible with either instrument alone, enabling on-the-fly experiments.

Reference:

https://www.waters.com/nextgen/cn/zh/education/primers/the-mass-spectrometry-primer/common-ionization.html