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Note 69: Application of SIMION 6.0 To a Study of the Finkelstein Ion Source: Part 1

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By Steven Colby

Presented at EAS, Summerset, NJ., November 1997

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INTRODUCTION

In 1940, A. Theodore Finkelstein described a unique ion source for the generation of intense ion beams [1]. This source was unique in that an ionizing beam of electrons was introduced co-linear to the final beam of ions. The co-propagation of charged particles was assisted by a magnetic field and dramatically increased the flux of ions from the source. We investigate the possibility of using the Finkelstein source for the introduction of ions into a mass spectrometer. Simulations are performed using the SIMION 3D software program. Both a generic conventional ion source and a Finkelstein source are modeled and comparisons are made of their efficiencies. In order for these results to be of general use, the simulation process is described in sufficient detail so that the reader can use these methods on more specific examples.

Software

The results reported in this poster were generated with the ion optics program SIMION 3D v.6.0. This software was developed by David Dahl at the Idaho National Engineering Laboratory [2]. The latest version allows for greatly expanded simulation capabilities. These include larger array sizes (10 million points) and three dimensional modeling. The new capabilities of dynamic parameter variation, time varying potentials, and user programming are also employed in the work below.


Figure 1A


Figure 1B

Fig. 1 - Simulated quadrupole mass spectrometer: (A) Conventional generic source, mass filter, and detector. (B). Cutaway of Finkelstein source. (Not shown are a filament shield and second magnetic pole located behind the filament.)

Simulation

SIMION was used to model the quadrupole mass spectrometers shown in Figure 1. Details of the simulation of the conventional ion source have been reported previously [3]. The simulation was divided into five sections. Each of these was designed and "refined" individually. (Refining is SIMION's method of calculating the potentials on non-electrode points.) The sections are then placed on an Ion Optics Bench (IOB) in their proper positions. The IOB feature allows items to be reused in different simulations. For example, parts of the quadrupole mass filter in this work were taken from an example that is included with the software. The IOB also permits the use of pieces whose symmetries differ. It is often helpful to incorporate elements of symmetry in the simulation because they dramatically reduce the number of array points required. Likewise, sections of the simulation can have a different number of grid points per unit area. This allows for more accurate simulations where needed, such as around the filament, without requiring such high precision everywhere else in the instrument.

The five sections (or instances) used in our experiments were a magnet, the source, the region between the source and the start of the quadrupoles, the quadrupoles, and the volume including the end of the quadrupoles and detector. The source instance consisted of a cylindrical piece (1.5 cm long X 1.0 cm dia.) with a flat plate at one end. In the conventional source a "repeller" was positioned on the side facing away from the quadrupoles. The plate included a 2 mm aperture to pass ions into the mass analyzer. Two planes of symmetry along the main axis of the instrument were used to divide the number of points in the calculation by a factor of four. A 2 x 3 mm slit was placed in the source to allow electrons from the filament to enter. (Because of the symmetry used, our simulation included two identical filaments and slits.) A magnetic instance was placed over the source so that an appropriate magnetic field was generated. The instance between the source and the quadrupoles included an electrode with another 2 mm aperture. In many actual instruments, an einzel lens is placed in this area. The instance also incorporated a section of the quadrupoles in order to properly model the transition between these two regions of the instrument. The quadrupoles were modeled in two dimensions and then "extruded" along the axis of the instrument in the IOB. The final instance modeled the transition between the end of the quadrupoles and the detector.

In the Finkelstein source, the -70V filament was placed behind the source (See Fig. 1B). An electron entrance hole was placed at the bottom of the source volume. The entire source was held at ground with the exception of the place that included the ion exit aperture. This surface was held at a potential of -1V. A ring magnet was placed around the ion exit and an opposite pole (not shown) was placed beneath the filament. Also not shown in Fig. 1B is a shield, held at -70V, directly behind the filament. The magnetic field within the center of the ion source had a strength of approximately 350 Gauss.

Electrons and ions were simulated using SIMION's trajectory calculations. The potentials of the quadrupoles were varied at a radio frequency of 1.1 MHz in order to pass ions of 100 m/z. Control of the time dependent potentials was accomplished using SIMION's user programming interface. Each simulation was performed using groups of ions. Within each group, electrons were assumed to leave a filament at 10 different points covering a linear range of 2 mm. At each of these points, five electrons were generated with different angular trajectories. All electrons were given 0.25 eV of kinetic energy. Before the simulation of each group, a random number was generated for use in determining where electrons should turn into positive ions of 100 m/z. This was intended to simulate the electron impact ionization of neutral species. The simulation was repeated a large number of times in order to model the random generation of ions in the region between the two filaments. The complete source code for all user programs used in this simulation are available on the Internet [4].

Fig 3aA
Figure 2A

Fig 3bB
Figure 2B

C
Figure 2C

Fig. 2 - Sample simulations in conventional source. (In A and B only the source region is shown. Views are along the y-axis).

Figure 3

Fig. 3 - Sample Simulation In the Finkelstein Source

Results

Figures 2 and 3 show the simulation of three ion groups from each of the two ion sources. These are intended to show examples of the possible fates of the simulated ions. In the first case (A), ions are formed too close to the filament. They are therefore drawn back toward the filament. This was the most common path for ions to take in both sources. In the second case (B), ions are generated closer to the center of the source. In the conventional source these are eliminated by striking the front and side walls. In the final case (C), ions are shown passing into the mass filter. Some of these ions reach the detector. However, the random initial thermal kinetic energy of the ions is sufficient to prevent some of these from reaching the detector.

Figuare 4

Fig. 4 - Location of Ion Impacts For the Two Sources

The probabilities of ion elimination occurring in each region of the instrument are compared in Figure 4. In the conventional source, over 64% of the ions are lost through the filament opening. This is due to both the magnetic and electric fields of the source. The fraction of ions lost through this route is a function of the size and depth of the rectangular opening used to admit electrons into the source. A plot of potential energy contours of the conventional source is shown in Figure 5A. Ions are accelerated in directions perpendicular to the red lines shown. It is clear that only in the center region are ions directed towards the exit aperture. Those ions that do move towards the exit are accelerated into the plane of Figure 5 (xz plane) by the magnetic field. As a result, the ions that strike the front plate of the source do so, on average, below the level of the principal instrument axis. As was shown in Figure 3, those ions that do not travel towards the filament still have only a small chance of reaching the detector. The Finkelstein source has an attractor in the source rather than a repeller. This generates the fields shown in Figure 5B and dramatically improves the acceleration of ions toward the ion exit.

Figure 5

Fig. 5 - Potential Energy Contours of the Source Regions

Using SIMION's data recording menu, it is possible to determine the starting positions of the ions that reach the detector. This enables us to determine the zone in which detectable ions may be generated. The volumes found were approximately 0.15 x 1.7 x 1.4 mm for the conventional source and 10 x 0.8 x 2.8 mm for the Finkelstein source.

Limitations of the Simulation (Warnings!)

Our goal in this presentation has been to demonstrate methods for instrument analysis and design and examine a possible source design. We are not attempting to characterize a specific instrument. However, as with any computer modeling, there are limitations to these simulations that must be kept in mind when analyzing the results. For example, the user program allows electrons to generate ions before they reach a significant kinetic energy. The simulated system was not optimized so the potentials chosen are not likely to be the best for ion collection in this system. Likewise, the specific geometry used was not based on an actual instrument or an optimal design. We have also not included the einzel lenses found in many commercial instruments. This will certainly have a dramatic influence on the collection efficiency of ions.

There are also some clear disadvantages to the Finkelstein source. For example, photons from the filament are likely to generate significant noise at the detector. The ions produced in the Finkelstein source also have a kinetic energy distribution of 0.6 eV while those in the conventional source have a distribution of 0.02 eV. This is will have a significant effect on the performance of the mass filter.

Current Work

Our next project will be a further characterization of the Finkelstein source. This will include an examination of the effects of the initial ion kinetic energy on the instrument resolution and the possibility of using a magnetic bottle to contain the electrons.

Conclusions

We have demonstrated the use of SIMION 3D in modeling two ion source regions for a quadrupole mass spectrometer. The software is an excellent tool for the investigation of ion optics within the instrument. We believe that the work of David Dahl will contribute to a large variety of applications.

SIMION 7 and 8 users can download source files here: download simion-finksim.zip. (Note: these files are fairly old. They are based on the "quad" example in SIMION 6.0, which has since been updated in SIMION 8.0/8.1.

Continue to Part II...

References

1. A. Theodore Finkelstein, Review of Scientific Instruments, v.11 (1940) p94.


2. David A. Dahl 43ed ASMS 1995, pg. 717.
3. S. M. Colby, C. W. Baker, and J. J. Manura 44th ASMS 1996 pgxx.
4. http://www.sisweb.com