7
Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion Jean-Jacques Dunyach, Satendra Prasad, Michael Belford 1 Thermo Fisher Scientific, San Jose, CA, USA
Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral DiffusionJean-Jacques Dunyach, Satendra Prasad, Michael Belford 1Thermo Fisher Scientific, San Jose, CA, USA
2 Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion
Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion Jean-Jacques Dunyach1, Satendra Prasad1, Michael Belford1 1 Thermo Fisher Scientific, San Jose, CA
Conclusion The use of a second entrance plate in order to create separate distribution
channels for the FAIMS gas and the desolvation gas helps control flow separation from the curved surfaces, making possible the use of the FAIMS device under a wide range of HPLC conditions
The elliptical shape of the outer electrode aperture creates a confinement in the lateral diffusion of the FAIMS gas along the axis of the inner electrode, which minimizes ion spread within the gap.
The reduced lateral diffusion offers the opportunity to use a shorter set of electrodes, which will reduce the ion residence time under the standard gas flow conditions.
Better control of the carrier gas flow allows for potential operation of the FAIMS device in transmission mode, with no Dispersion Voltage or Compensation Voltage applied and negligible ion losses.
References 1. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2013 ThP018
2. Coanda, H., US Patent n. 1,104,963 “Improvement in Propellers”, 1911, USA
3. Newman, B. G., “The Deflexion of Plane Jets by Adjacent Boundaries-Coanda Effect,” Boundary Layers and Flow Control, edited by Lachmann, G. V., Vol. 1, Pergamon Press, Oxford, 1961, pp. 232-264.
4. Prasad, S.; Belford, M. W.; Dunyach, Jean-Jacques. Control of gas flow in high field asymmetric waveform ion mobility spectrometry. US Patent 8,664,593 B2, March, 4, 2014.
5. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2014 WP763
6. Stokes-Einstein equation of the diffusion coefficient D: D =kT / 6πηa, where a is the radius of the molecule and η is the coefficient of viscosity
Overview Purpose: Characterize the effects of the entrance plate and outer electrode aperture dimensions and shape on the boundary conditions affecting the flow of the carrier gas within a FAIMS device. Optimize the design in order to improve the sampling of ions from the ion source, minimizing their lateral diffusion, residence time, and improving the overall ion transmission efficiency of the FAIMS interface in both separation (On) and transmission (Off) modes.
Methods: Computational Fluid Dynamics (CFD) was first used to simulate the gas flow profile at the ES-FAIMS interface and within the analytical gap of the FAIMS device. Alternatives to the original entrance plate and outer electrode designs were then studied and their effects on the carrier gas flow were characterized. The optimized geometry was then further evaluated using SIMION® software for ion trajectory simulations.
Results: Simulations demonstrate that a careful shaping of the entrance plate and outer electrode apertures of the FAIMS device can have significant effect on the carrier gas flow profile at the entrance as well as within the device, greatly affecting ion motion and residence time as well as allowing improved transmission of the ions. The optimized design minimizes the lateral diffusion of the ions and enables the use of the FAIMS interface in a so called transmission mode, where the asymmetric waveform is turned off and no mobility separation of the ions occurs, eliminating the need for removing the interface when ion mobility separation is not desired.
Introduction As demonstrated in previous work, the entrance conditions and the gas dynamics within the FAIMS device can play an important role in the overall ion transmission efficiency, by affecting both the sampling of the ions from the electrospray ionization source (ESI) and their trajectories within the analytical gap.1
The use of a properly curved surface at the aperture of the outer electrode has been shown to improve ion transmission by preventing the gas stream from hitting the inner electrode, efficiently deflecting the ions and sending them into the analytical gap as the flow stream stays attached to the curved surface, due to the so called Coanda effect.2, 3
Here, gas dynamics in the entrance region as well as within the analytical gap of the device are further studied, using CFD modeling coupled to SIMION™ simulations, in order to better identify the critical design characteristics that influence the gas flow profiles and ion trajectories within the device.
Methods
COMSOL Multiphysics® software, version 4.3, was used to simulate gas flow in the FAIMS device.
Three-dimensional CAD assemblies of different designs of the outer electrode ‘s entrance aperture were imported into Spaceclaim® and fluid volumes were extracted into COMSOL Multiphysics® for meshing and CFD analysis (Figure 1).
The meshing quality on the faces that constitute the modified surface were further refined with extremely fine tetrahedral elements and several boundary layers were added to accurately model the flow separation at the modified surface.
Both laminar and turbulent flow models were applied to calculate the gas flow profiles in the device. Since the Reynold’s number was less than 300, the use of the laminar flow model was preferred over the turbulent model.
From the converged solutions, the x, y, z and pressure matrices were extracted and exported to MATLAB® for post processing which included converting the matrices to SIMION readable potential arrays (PA).
After optimization, the 3D-CAD model of the best design in terms of gas flow characteristics was imported into SIMION using the STL import tools.
The effect of gas dynamics and electric field at the ESI-FAIMS interface on ion sampling was studied using a model Bromochloroacetate (BCA-) anion, and the ions trajectories within the analytical gap were plotted and compared to the gas flow profiles.
Finally, simulated spectra of Reserpine and BCA- were generated to evaluate the transmission efficiency of the FAIMS interface in “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
Results Design Optimization
In previous work, we had established the beneficial effect of curving the entrance orifice of the outer electrode, as the FAIMS gas was directed away from the inner electrode, which allowed for a better introduction of ions into the analytical gap (Figures 2 and 3).4
However, the proposed design was still non-optimal and lacked a proper implementation of a counter current desolvation gas, which was detrimental to its usability at higher LC flow rates (300 µL/min to 1000 µL/min), as increased flow rates required a better upfront desolvation of the ions in order to ensure reproducible separation.
Simply increasing the FAIMS carrier gas flow did not provide satisfactory results as flow streamlines separated from the inner walls of the outer electrode entrance orifice with the increased counter flow, impacting the overall ion transmission (Figures 4 and 5).
COMSOL Multiphysics and SIMION are trademarks of COMSOL Inc. and Scientific Instrument Services, Inc., respectively. MATLAB is a trademark of The Mathworks, Inc. Spaceclaim is a trademark of ANSYS Inc. All other trademarks are the property of Thermo Fisher Scientific and its subsidiaries.
This information is not intended to encourage use of these products in any manners that might infringe the intellectual property rights of others.
PO64099-EN 0614S
FIGURE 8. 3D simulation of the carrier gas flow profile using an outer electrode with an elliptical entrance aperture and showing reduced lateral spread along the inner electrode axis.
FIGURE 12. Simulated Reserpine cation (purple) and BCA- anion (green) transmission through the FAIMS assembly operated in pass through mode (DV=0V, CV range -20 to +20 V and diffusion enabled).
Effect of the entrance electrode aperture shape on carrier gas lateral diffusion
In addition, the effect of the entrance aperture shape on the carrier gas lateral diffusion along the inner electrode axis within the analytical gap was also studied, with the goal of minimizing ion losses when no mobility separation is required, and ions are not controlled by the Dispersion and Compensation fields (Off mode).
To this effect, a FAIMS outer electrode with an elliptical shaped aperture machined with its major axis being parallel to the inner electrode axis (and in which the radii of curvature r1 and r2, as defined in FIG. 6, are such that r1 > r2) was modeled, and compared to the original round bore design.
The results indicate that the portion of gas flowing parallel to the major axis of the aperture experienced less flow separation than the portion flowing parallel to the minor axis, resulting in better confinement of the carrier gas, and a lower lateral dispersion of the gas as it flows around the inner electrode (Figure 8).
FIGURE 6. Detail of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
Entrance plate 1 (Desolvation gas)
Entrance plate 2 (Carrier gas)
Outer Electrode
Inner Electrode
Ion beam Inner electrode Outer
electrode
0
10
20
30
40
50
60
70
80
90
100
-20 -10 0 10 20
inte
nsity
CV
Simulated FAIMS Spectrum
FIGURE 1. 3D-CAD assembly of the FAIMS fluid volume
electrode assembly
fluid volume fluid volume (mesh)
FIGURE 2. 2D cross section of gas flow profile for standard electrode set showing strong directional flow towards inner electrode
FIGURE 3. 2D cross section of gas flow profile for modified electrode set showing gas deflection away from the inner electrode.
FIGURE 4. 3D model of FAIMS gas flow without desolvation flow
FIGURE 5. 3D model of FAIMS gas flow with desolvation flow (note flow separation forming at the aperture)
FIGURE 7. 3D model of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
The impact of this narrower lateral spread of the carrier gas ions trajectories was then characterized using SIMION, and the overall ion beam spread of a model BCA- anion was found to be about 10 mm (Figure 11). This indicates that a reduction of the length of the electrodes from its standard 25 mm to 15 mm might be possible without adverse effect on ion transmission. The use of a shorter electrode, coupled with a smaller analytical gap of 1.5 mm (vs. a standard 2.5 mm) would reduce the overall size of the FAIMS device and decrease the ion residence time by more than a factor 2.5 when compared to the standard device under similar gas flow conditions.5
6.0 mm8.0 mm
Lateral expansion of gas flow
FAIMS analyzer gap (lateral direction)
14.0 mm
FIGURE 9. Lateral expansion of gas flow with round bore entrance aperture
FIGURE 10. Lateral expansion of gas with elliptical entrance aperture
FIGURE 11. Ion trajectories simulation (FAIMS parameters: bi-sinusoidal waveform (DV=4000 V), frequency 0.8 Mhz, CV 17.6V and diffusion enabled).
Operation in transmission mode. Finally, simulated spectrum of Reserpine cation (m/z 609.3) and BCA- (m/z 173) were generated to evaluate the transmission efficiency of the FAIMS interface in transmission or “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
The two ions were chosen due to their wide difference in diffusion coefficients which provides a good test case for the simulations, BCA being a smaller ion with higher diffusion coefficient than Reserpine.6
The simulations indicate that a high transmission efficiency can be achieved with this optimized design (90% for BCA- and 100% for Reserpine), and brings the potential for greater flexibility by eliminating the need to remove the interface when ion mobility separation is not desired.
Future work will involve the characterization of the new device, built based on these simulations results, and the comparison of its analytical performance versus previous generation electrodes.
Modified dual entrance plates concept In order to address this limitation, a new design allowing the use of two separate channels for the control of the FAIMS carrier and desolvation gases was investigated, and is shown in Figure 6.
The shape of the entrance plate controlling the flow of the carrier gas was further modified in order to promote the Coanda effect around the curved surface of the outer electrode aperture.
Simulations indicate that the independent control of each gas channel preserved the beneficial effect of the curved entrance aperture while allowing the use of a counter flow for improved desolvation (Figure 7).
The resulting width of the gas flow through the analytical gap was measured to be 6 mm, versus a width of 14 mm using the standard electrode model with a round bore entrance aperture (Figures 9 and 10).
r1 r2
3Thermo Scientific Poster Note • PN-64099-ASMS-EN-0614S
Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion Jean-Jacques Dunyach1, Satendra Prasad1, Michael Belford1 1 Thermo Fisher Scientific, San Jose, CA
Conclusion The use of a second entrance plate in order to create separate distribution
channels for the FAIMS gas and the desolvation gas helps control flow separation from the curved surfaces, making possible the use of the FAIMS device under a wide range of HPLC conditions
The elliptical shape of the outer electrode aperture creates a confinement in the lateral diffusion of the FAIMS gas along the axis of the inner electrode, which minimizes ion spread within the gap.
The reduced lateral diffusion offers the opportunity to use a shorter set of electrodes, which will reduce the ion residence time under the standard gas flow conditions.
Better control of the carrier gas flow allows for potential operation of the FAIMS device in transmission mode, with no Dispersion Voltage or Compensation Voltage applied and negligible ion losses.
References 1. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2013 ThP018
2. Coanda, H., US Patent n. 1,104,963 “Improvement in Propellers”, 1911, USA
3. Newman, B. G., “The Deflexion of Plane Jets by Adjacent Boundaries-Coanda Effect,” Boundary Layers and Flow Control, edited by Lachmann, G. V., Vol. 1, Pergamon Press, Oxford, 1961, pp. 232-264.
4. Prasad, S.; Belford, M. W.; Dunyach, Jean-Jacques. Control of gas flow in high field asymmetric waveform ion mobility spectrometry. US Patent 8,664,593 B2, March, 4, 2014.
5. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2014 WP763
6. Stokes-Einstein equation of the diffusion coefficient D: D =kT / 6πηa, where a is the radius of the molecule and η is the coefficient of viscosity
Overview Purpose: Characterize the effects of the entrance plate and outer electrode aperture dimensions and shape on the boundary conditions affecting the flow of the carrier gas within a FAIMS device. Optimize the design in order to improve the sampling of ions from the ion source, minimizing their lateral diffusion, residence time, and improving the overall ion transmission efficiency of the FAIMS interface in both separation (On) and transmission (Off) modes.
Methods: Computational Fluid Dynamics (CFD) was first used to simulate the gas flow profile at the ES-FAIMS interface and within the analytical gap of the FAIMS device. Alternatives to the original entrance plate and outer electrode designs were then studied and their effects on the carrier gas flow were characterized. The optimized geometry was then further evaluated using SIMION® software for ion trajectory simulations.
Results: Simulations demonstrate that a careful shaping of the entrance plate and outer electrode apertures of the FAIMS device can have significant effect on the carrier gas flow profile at the entrance as well as within the device, greatly affecting ion motion and residence time as well as allowing improved transmission of the ions. The optimized design minimizes the lateral diffusion of the ions and enables the use of the FAIMS interface in a so called transmission mode, where the asymmetric waveform is turned off and no mobility separation of the ions occurs, eliminating the need for removing the interface when ion mobility separation is not desired.
Introduction As demonstrated in previous work, the entrance conditions and the gas dynamics within the FAIMS device can play an important role in the overall ion transmission efficiency, by affecting both the sampling of the ions from the electrospray ionization source (ESI) and their trajectories within the analytical gap.1
The use of a properly curved surface at the aperture of the outer electrode has been shown to improve ion transmission by preventing the gas stream from hitting the inner electrode, efficiently deflecting the ions and sending them into the analytical gap as the flow stream stays attached to the curved surface, due to the so called Coanda effect.2, 3
Here, gas dynamics in the entrance region as well as within the analytical gap of the device are further studied, using CFD modeling coupled to SIMION™ simulations, in order to better identify the critical design characteristics that influence the gas flow profiles and ion trajectories within the device.
Methods
COMSOL Multiphysics® software, version 4.3, was used to simulate gas flow in the FAIMS device.
Three-dimensional CAD assemblies of different designs of the outer electrode ‘s entrance aperture were imported into Spaceclaim® and fluid volumes were extracted into COMSOL Multiphysics® for meshing and CFD analysis (Figure 1).
The meshing quality on the faces that constitute the modified surface were further refined with extremely fine tetrahedral elements and several boundary layers were added to accurately model the flow separation at the modified surface.
Both laminar and turbulent flow models were applied to calculate the gas flow profiles in the device. Since the Reynold’s number was less than 300, the use of the laminar flow model was preferred over the turbulent model.
From the converged solutions, the x, y, z and pressure matrices were extracted and exported to MATLAB® for post processing which included converting the matrices to SIMION readable potential arrays (PA).
After optimization, the 3D-CAD model of the best design in terms of gas flow characteristics was imported into SIMION using the STL import tools.
The effect of gas dynamics and electric field at the ESI-FAIMS interface on ion sampling was studied using a model Bromochloroacetate (BCA-) anion, and the ions trajectories within the analytical gap were plotted and compared to the gas flow profiles.
Finally, simulated spectra of Reserpine and BCA- were generated to evaluate the transmission efficiency of the FAIMS interface in “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
Results Design Optimization
In previous work, we had established the beneficial effect of curving the entrance orifice of the outer electrode, as the FAIMS gas was directed away from the inner electrode, which allowed for a better introduction of ions into the analytical gap (Figures 2 and 3).4
However, the proposed design was still non-optimal and lacked a proper implementation of a counter current desolvation gas, which was detrimental to its usability at higher LC flow rates (300 µL/min to 1000 µL/min), as increased flow rates required a better upfront desolvation of the ions in order to ensure reproducible separation.
Simply increasing the FAIMS carrier gas flow did not provide satisfactory results as flow streamlines separated from the inner walls of the outer electrode entrance orifice with the increased counter flow, impacting the overall ion transmission (Figures 4 and 5).
COMSOL Multiphysics and SIMION are trademarks of COMSOL Inc. and Scientific Instrument Services, Inc., respectively. MATLAB is a trademark of The Mathworks, Inc. Spaceclaim is a trademark of ANSYS Inc. All other trademarks are the property of Thermo Fisher Scientific and its subsidiaries.
This information is not intended to encourage use of these products in any manners that might infringe the intellectual property rights of others.
PO64099-EN 0614S
FIGURE 8. 3D simulation of the carrier gas flow profile using an outer electrode with an elliptical entrance aperture and showing reduced lateral spread along the inner electrode axis.
FIGURE 12. Simulated Reserpine cation (purple) and BCA- anion (green) transmission through the FAIMS assembly operated in pass through mode (DV=0V, CV range -20 to +20 V and diffusion enabled).
Effect of the entrance electrode aperture shape on carrier gas lateral diffusion
In addition, the effect of the entrance aperture shape on the carrier gas lateral diffusion along the inner electrode axis within the analytical gap was also studied, with the goal of minimizing ion losses when no mobility separation is required, and ions are not controlled by the Dispersion and Compensation fields (Off mode).
To this effect, a FAIMS outer electrode with an elliptical shaped aperture machined with its major axis being parallel to the inner electrode axis (and in which the radii of curvature r1 and r2, as defined in FIG. 6, are such that r1 > r2) was modeled, and compared to the original round bore design.
The results indicate that the portion of gas flowing parallel to the major axis of the aperture experienced less flow separation than the portion flowing parallel to the minor axis, resulting in better confinement of the carrier gas, and a lower lateral dispersion of the gas as it flows around the inner electrode (Figure 8).
FIGURE 6. Detail of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
Entrance plate 1 (Desolvation gas)
Entrance plate 2 (Carrier gas)
Outer Electrode
Inner Electrode
Ion beam Inner electrode Outer
electrode
0
10
20
30
40
50
60
70
80
90
100
-20 -10 0 10 20
inte
nsity
CV
Simulated FAIMS Spectrum
FIGURE 1. 3D-CAD assembly of the FAIMS fluid volume
electrode assembly
fluid volume fluid volume (mesh)
FIGURE 2. 2D cross section of gas flow profile for standard electrode set showing strong directional flow towards inner electrode
FIGURE 3. 2D cross section of gas flow profile for modified electrode set showing gas deflection away from the inner electrode.
FIGURE 4. 3D model of FAIMS gas flow without desolvation flow
FIGURE 5. 3D model of FAIMS gas flow with desolvation flow (note flow separation forming at the aperture)
FIGURE 7. 3D model of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
The impact of this narrower lateral spread of the carrier gas ions trajectories was then characterized using SIMION, and the overall ion beam spread of a model BCA- anion was found to be about 10 mm (Figure 11). This indicates that a reduction of the length of the electrodes from its standard 25 mm to 15 mm might be possible without adverse effect on ion transmission. The use of a shorter electrode, coupled with a smaller analytical gap of 1.5 mm (vs. a standard 2.5 mm) would reduce the overall size of the FAIMS device and decrease the ion residence time by more than a factor 2.5 when compared to the standard device under similar gas flow conditions.5
6.0 mm8.0 mm
Lateral expansion of gas flow
FAIMS analyzer gap (lateral direction)
14.0 mm
FIGURE 9. Lateral expansion of gas flow with round bore entrance aperture
FIGURE 10. Lateral expansion of gas with elliptical entrance aperture
FIGURE 11. Ion trajectories simulation (FAIMS parameters: bi-sinusoidal waveform (DV=4000 V), frequency 0.8 Mhz, CV 17.6V and diffusion enabled).
Operation in transmission mode. Finally, simulated spectrum of Reserpine cation (m/z 609.3) and BCA- (m/z 173) were generated to evaluate the transmission efficiency of the FAIMS interface in transmission or “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
The two ions were chosen due to their wide difference in diffusion coefficients which provides a good test case for the simulations, BCA being a smaller ion with higher diffusion coefficient than Reserpine.6
The simulations indicate that a high transmission efficiency can be achieved with this optimized design (90% for BCA- and 100% for Reserpine), and brings the potential for greater flexibility by eliminating the need to remove the interface when ion mobility separation is not desired.
Future work will involve the characterization of the new device, built based on these simulations results, and the comparison of its analytical performance versus previous generation electrodes.
Modified dual entrance plates concept In order to address this limitation, a new design allowing the use of two separate channels for the control of the FAIMS carrier and desolvation gases was investigated, and is shown in Figure 6.
The shape of the entrance plate controlling the flow of the carrier gas was further modified in order to promote the Coanda effect around the curved surface of the outer electrode aperture.
Simulations indicate that the independent control of each gas channel preserved the beneficial effect of the curved entrance aperture while allowing the use of a counter flow for improved desolvation (Figure 7).
The resulting width of the gas flow through the analytical gap was measured to be 6 mm, versus a width of 14 mm using the standard electrode model with a round bore entrance aperture (Figures 9 and 10).
r1 r2
4 Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion
Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion Jean-Jacques Dunyach1, Satendra Prasad1, Michael Belford1 1 Thermo Fisher Scientific, San Jose, CA
Conclusion The use of a second entrance plate in order to create separate distribution
channels for the FAIMS gas and the desolvation gas helps control flow separation from the curved surfaces, making possible the use of the FAIMS device under a wide range of HPLC conditions
The elliptical shape of the outer electrode aperture creates a confinement in the lateral diffusion of the FAIMS gas along the axis of the inner electrode, which minimizes ion spread within the gap.
The reduced lateral diffusion offers the opportunity to use a shorter set of electrodes, which will reduce the ion residence time under the standard gas flow conditions.
Better control of the carrier gas flow allows for potential operation of the FAIMS device in transmission mode, with no Dispersion Voltage or Compensation Voltage applied and negligible ion losses.
References 1. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2013 ThP018
2. Coanda, H., US Patent n. 1,104,963 “Improvement in Propellers”, 1911, USA
3. Newman, B. G., “The Deflexion of Plane Jets by Adjacent Boundaries-Coanda Effect,” Boundary Layers and Flow Control, edited by Lachmann, G. V., Vol. 1, Pergamon Press, Oxford, 1961, pp. 232-264.
4. Prasad, S.; Belford, M. W.; Dunyach, Jean-Jacques. Control of gas flow in high field asymmetric waveform ion mobility spectrometry. US Patent 8,664,593 B2, March, 4, 2014.
5. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2014 WP763
6. Stokes-Einstein equation of the diffusion coefficient D: D =kT / 6πηa, where a is the radius of the molecule and η is the coefficient of viscosity
Overview Purpose: Characterize the effects of the entrance plate and outer electrode aperture dimensions and shape on the boundary conditions affecting the flow of the carrier gas within a FAIMS device. Optimize the design in order to improve the sampling of ions from the ion source, minimizing their lateral diffusion, residence time, and improving the overall ion transmission efficiency of the FAIMS interface in both separation (On) and transmission (Off) modes.
Methods: Computational Fluid Dynamics (CFD) was first used to simulate the gas flow profile at the ES-FAIMS interface and within the analytical gap of the FAIMS device. Alternatives to the original entrance plate and outer electrode designs were then studied and their effects on the carrier gas flow were characterized. The optimized geometry was then further evaluated using SIMION® software for ion trajectory simulations.
Results: Simulations demonstrate that a careful shaping of the entrance plate and outer electrode apertures of the FAIMS device can have significant effect on the carrier gas flow profile at the entrance as well as within the device, greatly affecting ion motion and residence time as well as allowing improved transmission of the ions. The optimized design minimizes the lateral diffusion of the ions and enables the use of the FAIMS interface in a so called transmission mode, where the asymmetric waveform is turned off and no mobility separation of the ions occurs, eliminating the need for removing the interface when ion mobility separation is not desired.
Introduction As demonstrated in previous work, the entrance conditions and the gas dynamics within the FAIMS device can play an important role in the overall ion transmission efficiency, by affecting both the sampling of the ions from the electrospray ionization source (ESI) and their trajectories within the analytical gap.1
The use of a properly curved surface at the aperture of the outer electrode has been shown to improve ion transmission by preventing the gas stream from hitting the inner electrode, efficiently deflecting the ions and sending them into the analytical gap as the flow stream stays attached to the curved surface, due to the so called Coanda effect.2, 3
Here, gas dynamics in the entrance region as well as within the analytical gap of the device are further studied, using CFD modeling coupled to SIMION™ simulations, in order to better identify the critical design characteristics that influence the gas flow profiles and ion trajectories within the device.
Methods
COMSOL Multiphysics® software, version 4.3, was used to simulate gas flow in the FAIMS device.
Three-dimensional CAD assemblies of different designs of the outer electrode ‘s entrance aperture were imported into Spaceclaim® and fluid volumes were extracted into COMSOL Multiphysics® for meshing and CFD analysis (Figure 1).
The meshing quality on the faces that constitute the modified surface were further refined with extremely fine tetrahedral elements and several boundary layers were added to accurately model the flow separation at the modified surface.
Both laminar and turbulent flow models were applied to calculate the gas flow profiles in the device. Since the Reynold’s number was less than 300, the use of the laminar flow model was preferred over the turbulent model.
From the converged solutions, the x, y, z and pressure matrices were extracted and exported to MATLAB® for post processing which included converting the matrices to SIMION readable potential arrays (PA).
After optimization, the 3D-CAD model of the best design in terms of gas flow characteristics was imported into SIMION using the STL import tools.
The effect of gas dynamics and electric field at the ESI-FAIMS interface on ion sampling was studied using a model Bromochloroacetate (BCA-) anion, and the ions trajectories within the analytical gap were plotted and compared to the gas flow profiles.
Finally, simulated spectra of Reserpine and BCA- were generated to evaluate the transmission efficiency of the FAIMS interface in “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
Results Design Optimization
In previous work, we had established the beneficial effect of curving the entrance orifice of the outer electrode, as the FAIMS gas was directed away from the inner electrode, which allowed for a better introduction of ions into the analytical gap (Figures 2 and 3).4
However, the proposed design was still non-optimal and lacked a proper implementation of a counter current desolvation gas, which was detrimental to its usability at higher LC flow rates (300 µL/min to 1000 µL/min), as increased flow rates required a better upfront desolvation of the ions in order to ensure reproducible separation.
Simply increasing the FAIMS carrier gas flow did not provide satisfactory results as flow streamlines separated from the inner walls of the outer electrode entrance orifice with the increased counter flow, impacting the overall ion transmission (Figures 4 and 5).
COMSOL Multiphysics and SIMION are trademarks of COMSOL Inc. and Scientific Instrument Services, Inc., respectively. MATLAB is a trademark of The Mathworks, Inc. Spaceclaim is a trademark of ANSYS Inc. All other trademarks are the property of Thermo Fisher Scientific and its subsidiaries.
This information is not intended to encourage use of these products in any manners that might infringe the intellectual property rights of others.
PO64099-EN 0614S
FIGURE 8. 3D simulation of the carrier gas flow profile using an outer electrode with an elliptical entrance aperture and showing reduced lateral spread along the inner electrode axis.
FIGURE 12. Simulated Reserpine cation (purple) and BCA- anion (green) transmission through the FAIMS assembly operated in pass through mode (DV=0V, CV range -20 to +20 V and diffusion enabled).
Effect of the entrance electrode aperture shape on carrier gas lateral diffusion
In addition, the effect of the entrance aperture shape on the carrier gas lateral diffusion along the inner electrode axis within the analytical gap was also studied, with the goal of minimizing ion losses when no mobility separation is required, and ions are not controlled by the Dispersion and Compensation fields (Off mode).
To this effect, a FAIMS outer electrode with an elliptical shaped aperture machined with its major axis being parallel to the inner electrode axis (and in which the radii of curvature r1 and r2, as defined in FIG. 6, are such that r1 > r2) was modeled, and compared to the original round bore design.
The results indicate that the portion of gas flowing parallel to the major axis of the aperture experienced less flow separation than the portion flowing parallel to the minor axis, resulting in better confinement of the carrier gas, and a lower lateral dispersion of the gas as it flows around the inner electrode (Figure 8).
FIGURE 6. Detail of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
Entrance plate 1 (Desolvation gas)
Entrance plate 2 (Carrier gas)
Outer Electrode
Inner Electrode
Ion beam Inner electrode Outer
electrode
0
10
20
30
40
50
60
70
80
90
100
-20 -10 0 10 20
inte
nsity
CV
Simulated FAIMS Spectrum
FIGURE 1. 3D-CAD assembly of the FAIMS fluid volume
electrode assembly
fluid volume fluid volume (mesh)
FIGURE 2. 2D cross section of gas flow profile for standard electrode set showing strong directional flow towards inner electrode
FIGURE 3. 2D cross section of gas flow profile for modified electrode set showing gas deflection away from the inner electrode.
FIGURE 4. 3D model of FAIMS gas flow without desolvation flow
FIGURE 5. 3D model of FAIMS gas flow with desolvation flow (note flow separation forming at the aperture)
FIGURE 7. 3D model of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
The impact of this narrower lateral spread of the carrier gas ions trajectories was then characterized using SIMION, and the overall ion beam spread of a model BCA- anion was found to be about 10 mm (Figure 11). This indicates that a reduction of the length of the electrodes from its standard 25 mm to 15 mm might be possible without adverse effect on ion transmission. The use of a shorter electrode, coupled with a smaller analytical gap of 1.5 mm (vs. a standard 2.5 mm) would reduce the overall size of the FAIMS device and decrease the ion residence time by more than a factor 2.5 when compared to the standard device under similar gas flow conditions.5
6.0 mm8.0 mm
Lateral expansion of gas flow
FAIMS analyzer gap (lateral direction)
14.0 mm
FIGURE 9. Lateral expansion of gas flow with round bore entrance aperture
FIGURE 10. Lateral expansion of gas with elliptical entrance aperture
FIGURE 11. Ion trajectories simulation (FAIMS parameters: bi-sinusoidal waveform (DV=4000 V), frequency 0.8 Mhz, CV 17.6V and diffusion enabled).
Operation in transmission mode. Finally, simulated spectrum of Reserpine cation (m/z 609.3) and BCA- (m/z 173) were generated to evaluate the transmission efficiency of the FAIMS interface in transmission or “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
The two ions were chosen due to their wide difference in diffusion coefficients which provides a good test case for the simulations, BCA being a smaller ion with higher diffusion coefficient than Reserpine.6
The simulations indicate that a high transmission efficiency can be achieved with this optimized design (90% for BCA- and 100% for Reserpine), and brings the potential for greater flexibility by eliminating the need to remove the interface when ion mobility separation is not desired.
Future work will involve the characterization of the new device, built based on these simulations results, and the comparison of its analytical performance versus previous generation electrodes.
Modified dual entrance plates concept In order to address this limitation, a new design allowing the use of two separate channels for the control of the FAIMS carrier and desolvation gases was investigated, and is shown in Figure 6.
The shape of the entrance plate controlling the flow of the carrier gas was further modified in order to promote the Coanda effect around the curved surface of the outer electrode aperture.
Simulations indicate that the independent control of each gas channel preserved the beneficial effect of the curved entrance aperture while allowing the use of a counter flow for improved desolvation (Figure 7).
The resulting width of the gas flow through the analytical gap was measured to be 6 mm, versus a width of 14 mm using the standard electrode model with a round bore entrance aperture (Figures 9 and 10).
r1 r2
5Thermo Scientific Poster Note • PN-64099-ASMS-EN-0614S
Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion Jean-Jacques Dunyach1, Satendra Prasad1, Michael Belford1 1 Thermo Fisher Scientific, San Jose, CA
Conclusion The use of a second entrance plate in order to create separate distribution
channels for the FAIMS gas and the desolvation gas helps control flow separation from the curved surfaces, making possible the use of the FAIMS device under a wide range of HPLC conditions
The elliptical shape of the outer electrode aperture creates a confinement in the lateral diffusion of the FAIMS gas along the axis of the inner electrode, which minimizes ion spread within the gap.
The reduced lateral diffusion offers the opportunity to use a shorter set of electrodes, which will reduce the ion residence time under the standard gas flow conditions.
Better control of the carrier gas flow allows for potential operation of the FAIMS device in transmission mode, with no Dispersion Voltage or Compensation Voltage applied and negligible ion losses.
References 1. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2013 ThP018
2. Coanda, H., US Patent n. 1,104,963 “Improvement in Propellers”, 1911, USA
3. Newman, B. G., “The Deflexion of Plane Jets by Adjacent Boundaries-Coanda Effect,” Boundary Layers and Flow Control, edited by Lachmann, G. V., Vol. 1, Pergamon Press, Oxford, 1961, pp. 232-264.
4. Prasad, S.; Belford, M. W.; Dunyach, Jean-Jacques. Control of gas flow in high field asymmetric waveform ion mobility spectrometry. US Patent 8,664,593 B2, March, 4, 2014.
5. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2014 WP763
6. Stokes-Einstein equation of the diffusion coefficient D: D =kT / 6πηa, where a is the radius of the molecule and η is the coefficient of viscosity
Overview Purpose: Characterize the effects of the entrance plate and outer electrode aperture dimensions and shape on the boundary conditions affecting the flow of the carrier gas within a FAIMS device. Optimize the design in order to improve the sampling of ions from the ion source, minimizing their lateral diffusion, residence time, and improving the overall ion transmission efficiency of the FAIMS interface in both separation (On) and transmission (Off) modes.
Methods: Computational Fluid Dynamics (CFD) was first used to simulate the gas flow profile at the ES-FAIMS interface and within the analytical gap of the FAIMS device. Alternatives to the original entrance plate and outer electrode designs were then studied and their effects on the carrier gas flow were characterized. The optimized geometry was then further evaluated using SIMION® software for ion trajectory simulations.
Results: Simulations demonstrate that a careful shaping of the entrance plate and outer electrode apertures of the FAIMS device can have significant effect on the carrier gas flow profile at the entrance as well as within the device, greatly affecting ion motion and residence time as well as allowing improved transmission of the ions. The optimized design minimizes the lateral diffusion of the ions and enables the use of the FAIMS interface in a so called transmission mode, where the asymmetric waveform is turned off and no mobility separation of the ions occurs, eliminating the need for removing the interface when ion mobility separation is not desired.
Introduction As demonstrated in previous work, the entrance conditions and the gas dynamics within the FAIMS device can play an important role in the overall ion transmission efficiency, by affecting both the sampling of the ions from the electrospray ionization source (ESI) and their trajectories within the analytical gap.1
The use of a properly curved surface at the aperture of the outer electrode has been shown to improve ion transmission by preventing the gas stream from hitting the inner electrode, efficiently deflecting the ions and sending them into the analytical gap as the flow stream stays attached to the curved surface, due to the so called Coanda effect.2, 3
Here, gas dynamics in the entrance region as well as within the analytical gap of the device are further studied, using CFD modeling coupled to SIMION™ simulations, in order to better identify the critical design characteristics that influence the gas flow profiles and ion trajectories within the device.
Methods
COMSOL Multiphysics® software, version 4.3, was used to simulate gas flow in the FAIMS device.
Three-dimensional CAD assemblies of different designs of the outer electrode ‘s entrance aperture were imported into Spaceclaim® and fluid volumes were extracted into COMSOL Multiphysics® for meshing and CFD analysis (Figure 1).
The meshing quality on the faces that constitute the modified surface were further refined with extremely fine tetrahedral elements and several boundary layers were added to accurately model the flow separation at the modified surface.
Both laminar and turbulent flow models were applied to calculate the gas flow profiles in the device. Since the Reynold’s number was less than 300, the use of the laminar flow model was preferred over the turbulent model.
From the converged solutions, the x, y, z and pressure matrices were extracted and exported to MATLAB® for post processing which included converting the matrices to SIMION readable potential arrays (PA).
After optimization, the 3D-CAD model of the best design in terms of gas flow characteristics was imported into SIMION using the STL import tools.
The effect of gas dynamics and electric field at the ESI-FAIMS interface on ion sampling was studied using a model Bromochloroacetate (BCA-) anion, and the ions trajectories within the analytical gap were plotted and compared to the gas flow profiles.
Finally, simulated spectra of Reserpine and BCA- were generated to evaluate the transmission efficiency of the FAIMS interface in “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
Results Design Optimization
In previous work, we had established the beneficial effect of curving the entrance orifice of the outer electrode, as the FAIMS gas was directed away from the inner electrode, which allowed for a better introduction of ions into the analytical gap (Figures 2 and 3).4
However, the proposed design was still non-optimal and lacked a proper implementation of a counter current desolvation gas, which was detrimental to its usability at higher LC flow rates (300 µL/min to 1000 µL/min), as increased flow rates required a better upfront desolvation of the ions in order to ensure reproducible separation.
Simply increasing the FAIMS carrier gas flow did not provide satisfactory results as flow streamlines separated from the inner walls of the outer electrode entrance orifice with the increased counter flow, impacting the overall ion transmission (Figures 4 and 5).
COMSOL Multiphysics and SIMION are trademarks of COMSOL Inc. and Scientific Instrument Services, Inc., respectively. MATLAB is a trademark of The Mathworks, Inc. Spaceclaim is a trademark of ANSYS Inc. All other trademarks are the property of Thermo Fisher Scientific and its subsidiaries.
This information is not intended to encourage use of these products in any manners that might infringe the intellectual property rights of others.
PO64099-EN 0614S
FIGURE 8. 3D simulation of the carrier gas flow profile using an outer electrode with an elliptical entrance aperture and showing reduced lateral spread along the inner electrode axis.
FIGURE 12. Simulated Reserpine cation (purple) and BCA- anion (green) transmission through the FAIMS assembly operated in pass through mode (DV=0V, CV range -20 to +20 V and diffusion enabled).
Effect of the entrance electrode aperture shape on carrier gas lateral diffusion
In addition, the effect of the entrance aperture shape on the carrier gas lateral diffusion along the inner electrode axis within the analytical gap was also studied, with the goal of minimizing ion losses when no mobility separation is required, and ions are not controlled by the Dispersion and Compensation fields (Off mode).
To this effect, a FAIMS outer electrode with an elliptical shaped aperture machined with its major axis being parallel to the inner electrode axis (and in which the radii of curvature r1 and r2, as defined in FIG. 6, are such that r1 > r2) was modeled, and compared to the original round bore design.
The results indicate that the portion of gas flowing parallel to the major axis of the aperture experienced less flow separation than the portion flowing parallel to the minor axis, resulting in better confinement of the carrier gas, and a lower lateral dispersion of the gas as it flows around the inner electrode (Figure 8).
FIGURE 6. Detail of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
Entrance plate 1 (Desolvation gas)
Entrance plate 2 (Carrier gas)
Outer Electrode
Inner Electrode
Ion beam Inner electrode Outer
electrode
0
10
20
30
40
50
60
70
80
90
100
-20 -10 0 10 20
inte
nsity
CV
Simulated FAIMS Spectrum
FIGURE 1. 3D-CAD assembly of the FAIMS fluid volume
electrode assembly
fluid volume fluid volume (mesh)
FIGURE 2. 2D cross section of gas flow profile for standard electrode set showing strong directional flow towards inner electrode
FIGURE 3. 2D cross section of gas flow profile for modified electrode set showing gas deflection away from the inner electrode.
FIGURE 4. 3D model of FAIMS gas flow without desolvation flow
FIGURE 5. 3D model of FAIMS gas flow with desolvation flow (note flow separation forming at the aperture)
FIGURE 7. 3D model of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
The impact of this narrower lateral spread of the carrier gas ions trajectories was then characterized using SIMION, and the overall ion beam spread of a model BCA- anion was found to be about 10 mm (Figure 11). This indicates that a reduction of the length of the electrodes from its standard 25 mm to 15 mm might be possible without adverse effect on ion transmission. The use of a shorter electrode, coupled with a smaller analytical gap of 1.5 mm (vs. a standard 2.5 mm) would reduce the overall size of the FAIMS device and decrease the ion residence time by more than a factor 2.5 when compared to the standard device under similar gas flow conditions.5
6.0 mm8.0 mm
Lateral expansion of gas flow
FAIMS analyzer gap (lateral direction)
14.0 mm
FIGURE 9. Lateral expansion of gas flow with round bore entrance aperture
FIGURE 10. Lateral expansion of gas with elliptical entrance aperture
FIGURE 11. Ion trajectories simulation (FAIMS parameters: bi-sinusoidal waveform (DV=4000 V), frequency 0.8 Mhz, CV 17.6V and diffusion enabled).
Operation in transmission mode. Finally, simulated spectrum of Reserpine cation (m/z 609.3) and BCA- (m/z 173) were generated to evaluate the transmission efficiency of the FAIMS interface in transmission or “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
The two ions were chosen due to their wide difference in diffusion coefficients which provides a good test case for the simulations, BCA being a smaller ion with higher diffusion coefficient than Reserpine.6
The simulations indicate that a high transmission efficiency can be achieved with this optimized design (90% for BCA- and 100% for Reserpine), and brings the potential for greater flexibility by eliminating the need to remove the interface when ion mobility separation is not desired.
Future work will involve the characterization of the new device, built based on these simulations results, and the comparison of its analytical performance versus previous generation electrodes.
Modified dual entrance plates concept In order to address this limitation, a new design allowing the use of two separate channels for the control of the FAIMS carrier and desolvation gases was investigated, and is shown in Figure 6.
The shape of the entrance plate controlling the flow of the carrier gas was further modified in order to promote the Coanda effect around the curved surface of the outer electrode aperture.
Simulations indicate that the independent control of each gas channel preserved the beneficial effect of the curved entrance aperture while allowing the use of a counter flow for improved desolvation (Figure 7).
The resulting width of the gas flow through the analytical gap was measured to be 6 mm, versus a width of 14 mm using the standard electrode model with a round bore entrance aperture (Figures 9 and 10).
r1 r2
6 Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion
Effect of Electrode Geometry on FAIMS Gas Flow Focusing and Lateral Diffusion Jean-Jacques Dunyach1, Satendra Prasad1, Michael Belford1 1 Thermo Fisher Scientific, San Jose, CA
Conclusion The use of a second entrance plate in order to create separate distribution
channels for the FAIMS gas and the desolvation gas helps control flow separation from the curved surfaces, making possible the use of the FAIMS device under a wide range of HPLC conditions
The elliptical shape of the outer electrode aperture creates a confinement in the lateral diffusion of the FAIMS gas along the axis of the inner electrode, which minimizes ion spread within the gap.
The reduced lateral diffusion offers the opportunity to use a shorter set of electrodes, which will reduce the ion residence time under the standard gas flow conditions.
Better control of the carrier gas flow allows for potential operation of the FAIMS device in transmission mode, with no Dispersion Voltage or Compensation Voltage applied and negligible ion losses.
References 1. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2013 ThP018
2. Coanda, H., US Patent n. 1,104,963 “Improvement in Propellers”, 1911, USA
3. Newman, B. G., “The Deflexion of Plane Jets by Adjacent Boundaries-Coanda Effect,” Boundary Layers and Flow Control, edited by Lachmann, G. V., Vol. 1, Pergamon Press, Oxford, 1961, pp. 232-264.
4. Prasad, S.; Belford, M. W.; Dunyach, Jean-Jacques. Control of gas flow in high field asymmetric waveform ion mobility spectrometry. US Patent 8,664,593 B2, March, 4, 2014.
5. Belford, M.; Prasad, S.; Dunyach, J. – ASMS 2014 WP763
6. Stokes-Einstein equation of the diffusion coefficient D: D =kT / 6πηa, where a is the radius of the molecule and η is the coefficient of viscosity
Overview Purpose: Characterize the effects of the entrance plate and outer electrode aperture dimensions and shape on the boundary conditions affecting the flow of the carrier gas within a FAIMS device. Optimize the design in order to improve the sampling of ions from the ion source, minimizing their lateral diffusion, residence time, and improving the overall ion transmission efficiency of the FAIMS interface in both separation (On) and transmission (Off) modes.
Methods: Computational Fluid Dynamics (CFD) was first used to simulate the gas flow profile at the ES-FAIMS interface and within the analytical gap of the FAIMS device. Alternatives to the original entrance plate and outer electrode designs were then studied and their effects on the carrier gas flow were characterized. The optimized geometry was then further evaluated using SIMION® software for ion trajectory simulations.
Results: Simulations demonstrate that a careful shaping of the entrance plate and outer electrode apertures of the FAIMS device can have significant effect on the carrier gas flow profile at the entrance as well as within the device, greatly affecting ion motion and residence time as well as allowing improved transmission of the ions. The optimized design minimizes the lateral diffusion of the ions and enables the use of the FAIMS interface in a so called transmission mode, where the asymmetric waveform is turned off and no mobility separation of the ions occurs, eliminating the need for removing the interface when ion mobility separation is not desired.
Introduction As demonstrated in previous work, the entrance conditions and the gas dynamics within the FAIMS device can play an important role in the overall ion transmission efficiency, by affecting both the sampling of the ions from the electrospray ionization source (ESI) and their trajectories within the analytical gap.1
The use of a properly curved surface at the aperture of the outer electrode has been shown to improve ion transmission by preventing the gas stream from hitting the inner electrode, efficiently deflecting the ions and sending them into the analytical gap as the flow stream stays attached to the curved surface, due to the so called Coanda effect.2, 3
Here, gas dynamics in the entrance region as well as within the analytical gap of the device are further studied, using CFD modeling coupled to SIMION™ simulations, in order to better identify the critical design characteristics that influence the gas flow profiles and ion trajectories within the device.
Methods
COMSOL Multiphysics® software, version 4.3, was used to simulate gas flow in the FAIMS device.
Three-dimensional CAD assemblies of different designs of the outer electrode ‘s entrance aperture were imported into Spaceclaim® and fluid volumes were extracted into COMSOL Multiphysics® for meshing and CFD analysis (Figure 1).
The meshing quality on the faces that constitute the modified surface were further refined with extremely fine tetrahedral elements and several boundary layers were added to accurately model the flow separation at the modified surface.
Both laminar and turbulent flow models were applied to calculate the gas flow profiles in the device. Since the Reynold’s number was less than 300, the use of the laminar flow model was preferred over the turbulent model.
From the converged solutions, the x, y, z and pressure matrices were extracted and exported to MATLAB® for post processing which included converting the matrices to SIMION readable potential arrays (PA).
After optimization, the 3D-CAD model of the best design in terms of gas flow characteristics was imported into SIMION using the STL import tools.
The effect of gas dynamics and electric field at the ESI-FAIMS interface on ion sampling was studied using a model Bromochloroacetate (BCA-) anion, and the ions trajectories within the analytical gap were plotted and compared to the gas flow profiles.
Finally, simulated spectra of Reserpine and BCA- were generated to evaluate the transmission efficiency of the FAIMS interface in “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
Results Design Optimization
In previous work, we had established the beneficial effect of curving the entrance orifice of the outer electrode, as the FAIMS gas was directed away from the inner electrode, which allowed for a better introduction of ions into the analytical gap (Figures 2 and 3).4
However, the proposed design was still non-optimal and lacked a proper implementation of a counter current desolvation gas, which was detrimental to its usability at higher LC flow rates (300 µL/min to 1000 µL/min), as increased flow rates required a better upfront desolvation of the ions in order to ensure reproducible separation.
Simply increasing the FAIMS carrier gas flow did not provide satisfactory results as flow streamlines separated from the inner walls of the outer electrode entrance orifice with the increased counter flow, impacting the overall ion transmission (Figures 4 and 5).
COMSOL Multiphysics and SIMION are trademarks of COMSOL Inc. and Scientific Instrument Services, Inc., respectively. MATLAB is a trademark of The Mathworks, Inc. Spaceclaim is a trademark of ANSYS Inc. All other trademarks are the property of Thermo Fisher Scientific and its subsidiaries.
This information is not intended to encourage use of these products in any manners that might infringe the intellectual property rights of others.
PO64099-EN 0614S
FIGURE 8. 3D simulation of the carrier gas flow profile using an outer electrode with an elliptical entrance aperture and showing reduced lateral spread along the inner electrode axis.
FIGURE 12. Simulated Reserpine cation (purple) and BCA- anion (green) transmission through the FAIMS assembly operated in pass through mode (DV=0V, CV range -20 to +20 V and diffusion enabled).
Effect of the entrance electrode aperture shape on carrier gas lateral diffusion
In addition, the effect of the entrance aperture shape on the carrier gas lateral diffusion along the inner electrode axis within the analytical gap was also studied, with the goal of minimizing ion losses when no mobility separation is required, and ions are not controlled by the Dispersion and Compensation fields (Off mode).
To this effect, a FAIMS outer electrode with an elliptical shaped aperture machined with its major axis being parallel to the inner electrode axis (and in which the radii of curvature r1 and r2, as defined in FIG. 6, are such that r1 > r2) was modeled, and compared to the original round bore design.
The results indicate that the portion of gas flowing parallel to the major axis of the aperture experienced less flow separation than the portion flowing parallel to the minor axis, resulting in better confinement of the carrier gas, and a lower lateral dispersion of the gas as it flows around the inner electrode (Figure 8).
FIGURE 6. Detail of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
Entrance plate 1 (Desolvation gas)
Entrance plate 2 (Carrier gas)
Outer Electrode
Inner Electrode
Ion beam Inner electrode Outer
electrode
0
10
20
30
40
50
60
70
80
90
100
-20 -10 0 10 20
inte
nsity
CV
Simulated FAIMS Spectrum
FIGURE 1. 3D-CAD assembly of the FAIMS fluid volume
electrode assembly
fluid volume fluid volume (mesh)
FIGURE 2. 2D cross section of gas flow profile for standard electrode set showing strong directional flow towards inner electrode
FIGURE 3. 2D cross section of gas flow profile for modified electrode set showing gas deflection away from the inner electrode.
FIGURE 4. 3D model of FAIMS gas flow without desolvation flow
FIGURE 5. 3D model of FAIMS gas flow with desolvation flow (note flow separation forming at the aperture)
FIGURE 7. 3D model of the dual entrance plate concept with discrete FAIMS carrier gas and desolvation gas channels.
The impact of this narrower lateral spread of the carrier gas ions trajectories was then characterized using SIMION, and the overall ion beam spread of a model BCA- anion was found to be about 10 mm (Figure 11). This indicates that a reduction of the length of the electrodes from its standard 25 mm to 15 mm might be possible without adverse effect on ion transmission. The use of a shorter electrode, coupled with a smaller analytical gap of 1.5 mm (vs. a standard 2.5 mm) would reduce the overall size of the FAIMS device and decrease the ion residence time by more than a factor 2.5 when compared to the standard device under similar gas flow conditions.5
6.0 mm8.0 mm
Lateral expansion of gas flow
FAIMS analyzer gap (lateral direction)
14.0 mm
FIGURE 9. Lateral expansion of gas flow with round bore entrance aperture
FIGURE 10. Lateral expansion of gas with elliptical entrance aperture
FIGURE 11. Ion trajectories simulation (FAIMS parameters: bi-sinusoidal waveform (DV=4000 V), frequency 0.8 Mhz, CV 17.6V and diffusion enabled).
Operation in transmission mode. Finally, simulated spectrum of Reserpine cation (m/z 609.3) and BCA- (m/z 173) were generated to evaluate the transmission efficiency of the FAIMS interface in transmission or “OFF” mode, where no voltages are applied to the electrodes and where ions are only guided by the carrier gas.
The two ions were chosen due to their wide difference in diffusion coefficients which provides a good test case for the simulations, BCA being a smaller ion with higher diffusion coefficient than Reserpine.6
The simulations indicate that a high transmission efficiency can be achieved with this optimized design (90% for BCA- and 100% for Reserpine), and brings the potential for greater flexibility by eliminating the need to remove the interface when ion mobility separation is not desired.
Future work will involve the characterization of the new device, built based on these simulations results, and the comparison of its analytical performance versus previous generation electrodes.
Modified dual entrance plates concept In order to address this limitation, a new design allowing the use of two separate channels for the control of the FAIMS carrier and desolvation gases was investigated, and is shown in Figure 6.
The shape of the entrance plate controlling the flow of the carrier gas was further modified in order to promote the Coanda effect around the curved surface of the outer electrode aperture.
Simulations indicate that the independent control of each gas channel preserved the beneficial effect of the curved entrance aperture while allowing the use of a counter flow for improved desolvation (Figure 7).
The resulting width of the gas flow through the analytical gap was measured to be 6 mm, versus a width of 14 mm using the standard electrode model with a round bore entrance aperture (Figures 9 and 10).
r1 r2
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PN-64099-EN 0614S
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