Simulated Ion Trajectories
A simulated map of the ion flux at the grounded deposition plate is shown in Fig. 2.4. Each of the images in the figure show the predicted effect of altering the potential applied to the spray emitter. From these results it is evident that altering this potential is likely to have a noticeable effect on the resulting ion beam shape as it exits the electrode. A range of different ion mobilities was explored via simulation, each providing the same resulting intensity plots at the grounded plate. It is important to note that these simulations do not attempt to predict the overall intensity difference under these varied conditions. The total ion current is much greater at higher emitter potentials so it is reasonably expected that the true current at the surface would in fact be much greater in Fig. 2.4c when compared with Fig. 2.4a.
While space charge plays a role in determining the trajectories of ions within the electrode, the methods of incorporating space charge included with SIMION do not effectively model the space charge interaction at ion sources. Because of this, these effects were not included in the simulations of ion motion in the elliptical electrode. Instead, ions were given an initial filled sphere distribution centered at different locations relative to the nanoESI spray tip. This method allows for a qualitative understanding of trajectories ions undergo in the elliptical electrode to aid in the improvement of electrode designs.
The simulated contour plots of ion intensity when the spray emitter is set 1 kV above that of the elliptical electrode appear to mimic the shape of the experimentally mapped ion intensity (to a limited extent) as measured with the IonCCD™ detector. As the spray potential is raised (in relation to the focusing potential), this
Fig. 2.4 Contour plots of simulated ion intensity at the ground plate of the ellipse for sprayer potentials of a 6, b 6.5, and c 7 kV. Ellipse potential was 5 kV in all cases. Ions were given a filled-sphere initial distribution with radius of 1 cm, centered 0.5 cm below the axis of the ellipse, directly below the spray tip (25 mm from opening plane of ellipse). (0, 0) coordinate corresponds to the center of ellipse opening plane
correlation no longer holds, yet the experimentally observed result of a broadened ion beam is apparent. One likely reason for this disagreement between experiment and simulation is that the phenomenon of charged droplet trajectories from a spray tip is not well modeled with the SIMION-SDS model. The effects of droplet evaporation, droplet breakup, and the differing velocities of droplets ejected from the tip in the range of potentials studied are not considered. The use of the SIMION-SDS algorithm did however, allow for a qualitative study and understanding of ion behavior inside the elliptical electrode.