A VERSATILE METHOD OF SIMULATION OF THE OPERATION OF ION TRAP MASS SPECTROMETERS

A program is described that simulates ion motion in a quadrupole ion trap. Based on the Taylor series method, a numerical solution to the Mathieu differential equation is obtained. Trapping conditions can be varied during the trajectory computation, and this allows simulation of the complex operating modes used when the ion trap is utilized as a mass spectrometer. Such features as the mass-selected instability scan mode, mass range extension via resonant ion ejection, and the use of r.f.-d.c. for ion isolation, are all included in the simulation. Other aspects of operation, such as helium collisions and space charge effects, are also implemented in the model. Results are presented to demonstrate some of the capabilities of the program. Besides ion trajectories, simulated mass spectral peaks can be obtained, and kinetic energy distributions and phase-of-ejection distributions can be calculated. Such a simulation program can assist in understanding fundamental aspects of ion trap mass spectrometry as well as having didactic value in allowing visualization of ion behavior in different experiments. Attention is also given to the limitations of such simulation programs.