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Application of SIMION 6.0 to Problems in Time-of-flight Mass Spectrometry


ASMS, 1996

Steven Colby, Scientific Instrument Services

The release of the software SIMION 6.0 has greatly expanded the range of electron and ion optics modeling that can be practically attempted. We use the new program to examine three design problems found in the ion optics of time-of-flight mass spectrometers. The first of these is the scattering of ions as they pass through the grid material commonly used to separate regions of differing electric fields. As a result of the fields found on the either side of the grid, each grid hole can form a small electrostatic lens. Ions passing through different positions within a grid hole can then experience dissimilar accelerations that result in a range of trajectories. This scattering enables ions to reach the detector through a variety of paths of differing lengths. Under some circumstances the various routes traveled by detected ions may be the primary factor limiting mass spectral resolution. The degree of ion scattering is a function of the electric fields, hole size, and the energy of the ions. SIMION has made it possible for us to model the amount of scattering and affect on resolution as a function of the experimental parameters.

As a consequence of ion scattering at grids a number of reflectron TOF instruments have been designed with gridless reflectors. We use the new SIMION to demonstrate the design of these reflectors and identify optimum shapes for electrostatic rings. Both one and two stage reflectors are examined.

Finally we examine the use of electrostatic lenses and wire guides to focus sample ions in time-of-flight instruments. Our results demonstrate that the technique used can have a significant affect on mass spectral resolution.