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Computer Modeling of Ion Optics in Time-of-Flight mass Spectrometry using SIMION 3D

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Steven M. Colby and John J. Manura, Scientific Instrument Services, Inc., 1027 Old York Road, Ringoes, NJ 08551

It has been know for some time that non-ideal grids can be an important factor limiting the resolution of time-of-flight mass spectrometers. It has, however, been difficult to quantify the impact of grids in specific instrument designs. We report the development of a new tool for the simulation of ion scattering at grids. This tool is a accessory for use with the ion optics program SIMION 3D v.6.0. It allows the examination of the effects of grids on instrument resolution and sensitivity. Specific examples are presented illustrating the advantages and disadvantages of specific instrument designs in TOFMS. Opportunities for improvements in resolution and sensitivity are proposed.

The simulation of grid scattering is accomplished through the use of SIMION's user programming interface. Monte-Carlo methods are used to randomly alter the trajectories of ions as they pass through a grid. The probabilities of different angular deflections are based on either analytical calculations of the electric field near the grid surface or detailed simulations of the grid environment. For these calculations the program considers the electric field on each side of the grid as well as the kinetic energy of the ion.