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Note 70: Application of SIMION 6.0 To a Study of the Finkelstein Ion Source: Part II

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By Steven Colby

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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. This is the second part of an investigation of 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 1 A

Figure 2

Figure 1 B

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.

Figure 2

Figure 2. Side Views of the Finkelstein Source With the Filament In Two Different Positions

In the Finkelstein source, the -70V filament was placed in one of two positions behind the source (See Fig. 2). An electron entrance hole (2.0 mm Dia.) was placed at the back 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 either -1V or -2V. A ring magnet was placed around the ion exit and an opposite pole was placed behind the filament. Also shown in Figure 2 is a shield, held at -70V, directly behind the filament. Three different magnetic field strengths were examined. Their strengths were 265, 530, and 795 Gauss as measured in the center of the ion source.

Electrons and ions were simulated using SIMION's trajectory calculations. The potentials of the quadrupoles were varied at a frequency of 1.1 MHz and tuned to pass ions of 100 m/z. Control of the time dependent potentials was accomplished using SIMION's user programming interface. Each simulation was started with electrons leaving the filament over a 2mm length. The initial energy, position, and velocity vectors were randomly generated. The electrons were given 0.25 eV +/- 10% of kinetic energy. Once the electrons entered the source, they were randomly turned into ions of 100 AMU. The probability of ionization was a function of the distance traveled during each step in the trajectory calculations. The random conversion of electrons into ions 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].

Figure 3

Fig. 3 - Location of Ion Impacts For the Two Sources

Background

We have already reported results comparing the fate of ions in the standard and Finkelstein sources [5]. These results are shown in Figure 3. 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. In the standard source only ions produced in the center region are directed towards the exit aperture. 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 fields that dramatically improve the acceleration of ions toward the ion exit.

While the increase in ion collection shown in Figure 3 was very encouraging, 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. Because of the electric fields in the Finkelstein source, the ions generated may also have a larger energy distribution. This could have a significant effect on the performance of the mass filter.

We now report the results of further investigation of the Finkelstein source. This includes a comparison of the ion energy and angular distributions resulting from each source. We also examine the effects of various electrostatic and magnetic fields.

Results

The results obtained in this work are summarized in Tables 1 through 3. Table 1 shows the percent of ions detected for a variety of source conditions. We found that increasing the strength of the magnetic field and the negative potential of the attractor both improved the efficiency of ion detection. In comparison, the collection efficiency of the standard source was 1.0%. (This value differs from that shown in Figure 3 because we have now made the assumption that ions are only formed within the source). Note that moving the filament back by 6.9 mm resulted in a significant improvement. This occurred because the effect of the -70V filament on the fields within the source was reduced. The number of electrons entering the source did not change significantly because of the magnetic field.

Table 1 Percent Ions Detected

Finkelstein Source Far Filament Finkelstein Source Close Filament
Attractor voltage (V) Attractor voltage (V)
Magnetic Field (G) -1 -2 Magnetic Field (G) -1 -2
265 22% 29% 265 17% xx%
530 23 530 xx
795 27 30 795 xx
Standard Source: 1.0% detected

While the greater magnetic fields and extraction voltage improved the collection of ions, we were concerned that the broadened spatial and kinetic energy distribution of the ions would adversely effect the mass filtering properties of the quadrupoles. We therefore examined the kinetic energy of ions as they exited the source and their spatial distribution as the entered the quadrupoles. The results are shown in Table 2. We first noted that the increased fields did indeed increase the standard deviation of both the spatial and kinetic energy distributions although this was only a small effect. We also observed that the spatial and kinetic energy distributions were wider for the detected ions than for all of the ions that entered the quadrupoles. This was a surprise since we had expected that the ouliers would have a reduced chance of passing completely through the mass filter. Note the especially high standard deviation of the ion positions along the z-axis. The filament lies parallel to this axis.

Table 2. Spatial and Energy Distributions of ions {values for detected ions are in ()

KE (eV) Position y-axis (mm) Position z-axis (mm)
Average Std. Dev. Average Std. Dev. Average Std. Dev.
Standard Source
1.49(1.51) 0.11(0.10) 0.78(-0.03) 0.84(0.78) -0.01(-0.05) -0.36(0.29)
Finkelstein Source
265 G, -1V 0.50(0.50) 0.21(0.22) -0.01(-0.01) 0.78(0.25) 0.01(0.06) 0.19(1.23)
265 G, -2V 1.09(1.17) 0.35(0.34) 0.01(0.00) 0.91(0.26) 0.00(0.05) 0.24(1.17)
795 G, -1V 0.55(0.56) 0.18(0.19) 0.03(0.00) 0.72(0.23) 0.00(-0.01) 0.27(1.37)

In order to examine the effects of the wider distributions we tuned the quadrupoles to pass ions of 101 m/z and recorded the reduced transmission of 100 AMU ions. The results are shown in Table 3. Unexpectedly, he efficiency of the mass filter was not lower when using the Finkelstein source. In most cases the filtering was even better than the standard source.

Table 3. Percent Ions Detected With Quads Focused For 101 m/z and 100 AMU ions.

Finkelstein Source Far Filament
Attractor voltage (V)
Magnetic Field (G) -1 -2
265 12.2% 15.9%
795 15.2 16.5
Standard Source: 0.69% detected

Limitations of the Simulation (Warnings!)

Our goal is to demonstrate methods for instrument development and examine a possible source design. We were 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 [5]. None of the geometries were used with fully optimized potentials. It is likely that the standard source would perform better with an added set of lenses.

Conclusions

We have demonstrated the use of SIMION 3D in modeling two ion source regions for a quadrupole mass spectrometer. The Finkelstein source has continued to show promise.

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.

4. http://www.sisweb.com

5. S. M. Colby, Eastern Analytical Symposium, 1997.