Optimization of Gas Flow Through Multiple FAIMS Electrode Geometries
Michael Belford, Satendra Prasad, Jean-Jacques Dunyach
Thermo Fisher Scientific, San Jose, CA
Overview
Results
Purpose: The gas flow profile for a number of electrode geometries was studied.
Nitrogen is introduced into the electrode from a port located above the ion inlet. Once
in this region between the ground electrode and the entrance plate, the gas flows into
the ion inlet of the FAIMS cell with any remaining gas exiting the entrance plate to aid
ion desolvation in the source housing. Figure 2 shows this flow through a benchmark
cylindrical FAIMS cell in both the full 3D view and 2D slice. The gas stream acquires
an angular distribution which results in an asymmetrical flow through the entrance
plate and the FAIMS gap. This results in the majority of the gas flowing through the
lower channel of the device. This effect was described more rigorously in a previous
poster.2
Results: The location of gas inlets, the gap between electrodes, and electrode
geometry affect the transmission of ions through the FAIMS device.
Introduction
FAIMS (Field Asymmetric Waveform Ion Mobility Spectrometry) separates ions based
on the difference of their mobility in strong and weak electric fields created between
two electrodes using an asymmetric waveform. FAIMS is commonly operated at a field
strength of 20,000 V/cm and uses a mixture of helium and nitrogen carrier gas for
proper balance between sensitivity and selectivity.1 The gas dynamics within the device
play an important role in the overall ions transmission efficiency by affecting both the
sampling of ions from the ESI source and their trajectories within the analytical gap.
Here, gas dynamics between multiple electrode geometries are studied using CFD
modeling coupled to SIMION ® software simulations and the effects of the gas flow
characteristics on ions transmission efficiency are compared.
Although the gas flow profile in the hemi-cylindrical geometry does not come into
contact with the RF electrode, the electrode is now significantly closer to the entrance
plate. This introduces a dependence on the entrance plate voltage and can cause the
incoming ion beam to enter the FAIMS gap with enough momentum to strike the RF
electrode before it can be entrained in the gas flow. This effect is shown in Figure 9.
The ions are focused as they approach the entrance plate but are then accelerated
into the electrode and discharged.
FIGURE 5. Ion flow through dual gas inlet geometry.
FIGURE 2. Gas flow through the benchmark cylindrical FAIMS device.
FIGURE 1. A 3D model of gas flow channel, shown on the left, was used to
simulate gas flow. A 2D slice of the channel, shown on the right, was used to
simulate ion motion.
FAIMS gas
DV = 4000V
CV = 21.2V
DV = 4000V
CV = 21.2V
entrance plate = 150V
entrance plate = 600V
The trajectory of BCA ion was simulated using this flow profile and is shown in Figure 3.
Please note that the black lines illustrate the magnitude of gas flow and do not really
penetrate the entrance plate or the RF electrode. In addition to forcing the majority of
ions below the RF electrode, the asymmetric gas flow results in a portion of the
incoming ion beam striking the entrance plate and the outside of the ground electrode.
In addition, the rate of the gas flowing into the FAIMS cell causes the ion beam to flow
close to the RF electrode where some ions are discharged.
FIGURE 3. Simulation of BCA through benchmark cylindrical electrode set.
Data defining pressure profile was also extracted. The CFD data resolution (number of
rows and columns) was defined by the number of grid units specified by the SIMION
space containing the FAIMS electrode sketch. The grid units were scaled to mm (0.01
mm/grid unit) to overlay FAIMS electrode dimensions between COMSOL and SIMION.
The gas flow and pressure matrices were converted to a potential array (PA) format,
scaled to mm also, and were added to the SIMION workbench individually using the PA
tab.
During simulation, the SIMION-FAIMS-SDS algorithms accessed gas flow and
pressure data through the PA files to calculate the trajectory of an ion. Thus, the motion
of an ion or ion beam in the FAIMS device was determined by gas flow, pressure, RF
waveform and collision based diffusion. Bromochloroacetate anion was used as a
model ion to characterize and compare the benchmark and modified FAIMS systems
for ion transmission and resolution [BCA-, m/z = 173, Ko=1.7 x 10-4 m2.V-1.s-1,
alpha=7.98 x 10-6 Td-2, beta=-3.05 x 10-10Td-4]. The SIMION-FAIMS-SDS module was
used with the following FAIMS separation parameters: bi-sinusoidal waveform,
DV=4000V, frequency = 0.65MHz and diffusion enabled.
FIGURE 12. Gas flow optimized hemi-cylindrical geometry with one gas inlet
and increased gap between entrance plate and RF electrode.
FIGURE 9. BCA ion beam discharging on RF electrode due to too high of a
voltage on the entrance plate.
Methods
COMSOL Multiphysics® software, version 4.2a, was used to simulate gas flow in the
FAIMS device. The FAIMS model was prepared for CFD analysis by first retrosketching a benchmark FAIMS assembly where only the gas flow channel was
sketched. This included meshing the 3D sketch using extra fine tetrahedral elements.
Boundary conditions were defined and known parameters of nitrogen were used to
define the flowing fluid. A fraction of the 3D data set was extracted using the “slice”
function where a 2D plane was inserted at Z=0 mm (center of the mirror axis). The 2D
data set was extracted in an ASCII format with the X and Y component of the total flow
saved as separate data matrices. These models are shown in Figure 1.
The flow path of BCA ions in this modified geometry is shown in Figure 12. The
increased gas flow allows the ions to traverse the wider gap between the entrance
plate and the electrodes. Furthermore, the ions that were previously striking the RF
electrode are now being entrained in the gas flow. This increases the transmission of
the device.
In order to reduce the number of ions lost at the RF electrode, the geometry was
changed to a hemi-cylindrical electrode. The gas flow profile is shown in 3D in Figure 6
and a 2D center slice in Figure 7. With the new geometry, the gas flow is focused
through the center of the gap.
FIGURE 6. 3D gas flow profile of
hemi-cylindrical geometry
Without voltage on the entrance plate, the incoming ion beam would be completely
dispersed and lost at the surface of the entrance plate. However, when a reduced
voltage is placed on the entrance plate, there is some loss at the entrance plate and
RF electrode but ions are incorporated into the gas flow and are detected. This is
shown in Figure 10.
FIGURE 10. BCA ion beam partially passing through hemi-cylindrical geometry
at reduced entrance plate voltage.
Finally, the transmission of the modified electrode geometry was compared to the
benchmark geometry by simulating 400 BCA ions from the source region through each
device over a range of compensation voltages. This is summarized in Figure 13. In
addition to an increase in transmission, the modified geometry transmitted BCA at a
larger compensation voltage while maintaining peak width. Therefore, the resolution of
the device has also been increased.
FIGURE 13. Compensation voltage scan for BCA ions in benchmark and
modified FAIMS geometries.
100
FIGURE 7. 2D gas flow profile of hemicylindrical geometry
transmision (%)
Methods: Data were generated using computational fluid dynamic (CFD) simulation.
The trajectory of BCA ions into the dual gas inlet device is shown in Figure 5. All ions
pass through the entrance plate and into the electrode set due to the improved gas
dynamics in the source region and the gap between the entrance plate and the
grounded electrode. In addition, ions more uniformly flow through each channel.
However, the increased velocity of gas coming into the device causes many ion to
discharge on the RF electrode.
DV = 4000V
CV = 21.2V
RF electrode
entrance plate = 150V
80
benchmark
modified
60
40
20
0
0
entrance plate
ground electrode
A geometry was tested with a second gas inlet located below the ion inlet (shown in
Figure 4). With the mirrored gas inlets, the flow entering the source region is
perpendicular to the entrance plate and the flow into the electrodes is uniform across
both the top and bottom channels.
A SIMION view of the hemi-cylindrical geometry, Figure 8, show further detail of the
gas flow profile. The gas stream is uniformly incorporated within the FAIMS gap and
there is no longer a counter flow of gas entering the source region. Although the
counter flow can be beneficial by desolvating the ion beam coming from a high liquid
flow source (i.e. heated electrospray), it is also dispersive and makes it difficult to
establish a stable signal at low flow rates (i.e. nanospray mode).
To further increase the number of ions being incorporated into the FAIMS gap, the
distance between the entrance plate and the electrodes was optimized. This is shown
in Figure 11. The increase in gap allows the ion beam more time to incorporate into
the gas flow, thus reducing the number of ions discharged on the RF electrode. In
addition, the lower gas inlet was removed to reestablish the downward gas flow into
the device. Finally, the flow through device was increased to maintain the directional
flow into the entrance plate.
FIGURE 8. Hemi-cylindrical geometry.
FIGURE 11. Gas flow through modified hemi-cylindrical geometry.
FIGURE 4. Gas flow through dual gas inlet geometry.
5
10
15
20
compensation voltage (V)
25
30
Conclusion

Simulation of gas flow on the benchmark geometry shows ion loss on the
entrance plate and ground electrode due to the asymmetry of the gas stream.

The dual gas inlet geometry reduced losses at entrance plate but increased
losses at RF electrode.

Optimization of the gap and gas flow rate on the hemi-cylindrical geometry was
necessary to entrain ions in the gas flow.

Simulation shows the optimized hemi-cylindrical geometry allows for increased
transmission and resolution versus the benchmark geometry.
References
RF electrode
RF electrode
1. Purves, R. W.; Guevremont, R. Anal. Chem. 1999, 71, 2346-2357
2. Belford, M.; Dunyach, J.; Prasad, S. – ASMS 2011 MP056
entrance plate
ground electrode
Acknowledgements
entrance plate
ground electrode
The authors would like to acknowledge David Manura from Scientific Instruments for
refinement of the SIMION module and Randy Purves for meaningful discussion..
COMSOL Multiphysics and SIMION are trademarks of COMSOL and Scientific Instrument Services, Inc. ,
respectively.