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Computer Modeling of Ion Optics in Time-of-Flight Mass Spectrometry Using SIMION 3D

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Pitt Con, 1997

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

Overview: 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 macro for use with the ion optics program SIMION 3D v.6.0 [1]. It allows the examination of the effects of grids on instrument resolution and sensitivity. Specific examples are presented to illustrate the advantages and disadvantages of specific instrument designs in TOFMS. Opportunities for improvements in resolution and sensitivity are proposed. Simulation: The simulation of grid scattering is accomplished through the use of SIMION's user programming interface. Each time an ion experiences a sharp change in electric field, as is found at a grid interface, the ion is "jumped" to a simulation of a small piece of grid. Monte-Carlo methods are used to randomly position the ion near the grid so that trajectories through all parts of a grid opening are examined. The program considers the electric field experienced by the ion before jumping to adjust the fields around the grid. In this manner one piece of simulated grid can be used for all grid transitions found within the instrument. The curved fields found at the grid openings deflect ions at various angles as a function of the position of the ions within the grid opening, the relative electric fields, and the velocity of the ions. An important aspect of this simulation is that the cumulative effects of multiple grid transitions are observed. Results: Our initial results showed that grids could have a considerable effect on the trajectory of the ion [2]. The scattering depended greatly on the electric fields and grid density. Table 1 shows the deflection for ions passing between a field free and high field region. Effects were most pronounced when an ion was decelerated after passing through a grid. In these cases the perpendicular velocity introduced by the field non-homogeneity at the grid could become a significant fraction of the total velocity.

Table 1: Deflection angles

Starting

Region

Grid Size

(lines/inch)

Field

Strength

(V/cm)

Deflection

Angle.*,a

(deg)

Perpendicular

Velocity *,a

(%)

Deflection

Angle.*,b

(deg)

Perpendicular

Velocity *,b

(%)

Field Free 70 2000 2.33 4.0 0.91 1.6
  70 10,000 3.62 6.3 0.99 1.7
High Field 70 any 0.453 0.27 0.153 0.27

*Average Values; a At 1.5 times grid wire spacing; b After 1.0 cm.

We also examined the cumulative effects of grid scattering in a Reflectron TOFMS. We assumed 70 line per inch grids at 2 different drift energies; 2000 and 10,000 eV. (100 AMU ion). A two stage source and a two stage reflector were used. In the source the ion was given 10% of its drift energy in the first stage. The center grid in the reflector was held at a potential equivalent to 90 % of the drift energy. A 20 mm diameter detector was placed 500 mm from the reflector. Full details are available on the web site listed below. The simulation results are listed in Table 2. A surprisingly large fraction (94 %) never reached the detector. Results were independent of drift energy. Table 2: Ion Loss Locations (%). Detector Reflector Grid Other 6 36 29 29 References: 1. David A. Dahl 43ed ASMS Conference on Mass Spectrometry and Allied Topics, May 21-26 1995, Atlanta, Georgia, 717. 2. S. M. Colby; C. W. Baker; J. J. Manura Proc, 41st ASMS Conf. 1996. (Available at www.sisweb.com application note #47)