Computer Simulation of the Operation of a Three-Dimensional Quadrupole Ion Trap

A computer simulation of the motion of ions in a three-dimensional (3-D) quadrupole ion trap has been performed with a Gateway 386 PC/AT computer. The SIMION program was used as the main program to calculate the potential array of the ion trap space. Several user-written programs were interfaced to the SIMION program to simulate the effects of changing various operating conditions, such as the radio-frequency (rf) potential, the collisional buffer gas, external ion injection, dc ejection from the trap, and resonance ejection. With the use of this simulation, the total storage mass range could be obtained as a function of rf voltage and frequency. The simulations show, as expected, that the collisional buffer gas plays an important role in both stabilizing the trajectory of high-kinetic-energy ions (hot ions) inside the ion trap and trapping ions injected from an external source. Several different buffer gases were studied for their effects upon the trapping motion. In addition, both the total mass ejection that results from applying a dc pulse on the output end-cap electrode and the ion ejection that results from applying an rf frequency to the end cap to produce resonance ejection were also studied with this simulation program. It is demonstrated that a simple PC computer using a modified SIMION program provides results very similar to those expected from theory or from previous work.