Results and Discussion
Annular Ion Focusing
The exploration in the use of concentric cylindrical electrodes is the result of experimental observations made during experiments with the ellipsoidal electrode design discussed in Chap. 2 . By placing a series of open cylindrical electrodes inside the elliptical assembly and appropriately adjusting the potentials applied to each component, a condition was established in which an electrosprayed dye was deposited in an annulus with very narrow line-width. Traditional machining of such a device is a fairly complex and costly endeavor, as such, experimental tests of this effect were not performed for some time. With the addition of a 3D printer to the laboratory and the development of methods for constructing suitable ion lenses through FDM methods , the exploration of this annular focusing effect through experiment became a trivial manner.
Initial explorations of electrode configurations were explored with SIMION-SDS simulations to gain a better understanding of the geometries necessary to generate the annular focusing effect. The electrode configurations considered for testing were limited to those that would be most accurately produced by FDM manufacturing methods. Despite the many advantages of additive manufacturing techniques such as FDM, there are limitations that must be considered. Because FDM relies on the sequential deposition of layered materials, there must be a surface on which to extrude subsequent layers. As such, overhanging structures must either be minimized in the design, or a support material printed underneath that is later removed (a strategy which introduces extra dimensional errors). In light of this, only electrode designs that could be manufactured without support material were tested in simulations before device manufacture.
The final electrode design consists of an outer cylinder with a coaxial disc placed flush with the exit plane of the cylinder. Ultimately, this was used as the source region of the IMS and its dimensions are detailed in Fig. 4.1. Simulations of the annular focusing effect were performed in which a potential of 1.5 kV was applied to the outer cylinder and central disc, and ions deposition location was monitored on a grounded, flat electrode located 1.6 mm distant from the opening plane of the focusing electrode. The resultant simulated ion trajectories and final deposition locations of protonated ACN ions that were originated from different points within the focusing electrode are shown in Fig. 4.4.
From the simulations shown in Fig. 4.4 it is evident that the points of origin have an impact on the focal distribution of the ion cloud within the apparatus. This is not problematic in the current embodiment of the system as ions are not generated within the entire volume of the electrode. Instead, ions originate from the tip of the nanoESI emitter, which is placed at a desired location within the cylindrical lens. This setup does not produce a true point-source for the generation of ions due to the initial expansion of the ESI plume and subsequent ionization events from progeny droplets; however, ion origins are still confined to a limited volume within the electrode. An important result of these simulations is the apparent ability to focus ions into an annulus with a line width of approximately 200 pm, even in the case when ions are originated across the entire 15 mm radius of the cylindrical electrode, * 1 cm distant from the central disc. Additionally, ions originating near the center of the electrode, 1 mm from this disc, are also drawn into this annulus rather than colliding with electrode. These points are of important note when considering the placement of a nanoESI emitter within the apparatus for the generation of ions.
These focal effects were experimentally verified by electrospraying ACN from a nanoESI emitter placed within a 3D printed electrode as previously described. Maps of ion intensity at the exit plane of the electrode were reconstructed from IonCCD™ scans as described in Sect. 4.2.3. A range of potentials were applied to the focusing apparatus and the spray emitter to determine the effects of each on the final focus
Fig. 4.4 Trajectories of protonated ACN ions from different origins (left) and radial deposition profiles from different ion origin locations (right). The dashed line denotes the axis of cylindrical symmetry within the simulation environment
and intensity of the ions exiting the focusing electrode. During these experiments the tip of the nanoESI emitter was placed coaxially within the outer cylindrical electrode and the tip was held 10 mm distant from the central disc. This placement was chosen based on the previously discussed simulation results. An illustration of the setup is shown inset of Fig. 4.5b. For imaging experiments, the silver disc (inset Fig. 4.5b) is replaced with an IonCCD™ detector mounted on a computer-controlled moving platform. It is important to note the presence of the 0.7 mm wide filaments connecting the central disc to the outer cylinder. These are necessary for manufacture of the electrodes and to assure uniform potential application to each component (the outer cylinder and coaxial disc). Simulations did not take these into account as the model was simplified to maintain axial symmetry.
In order to better characterize the annular focus, the reconstructed 2D images of ion intensity were subject to additional processing. Briefly, the intensity maps were converted to grayscale images and a threshold was applied at 40% of the maximum image intensity. A scatter plot was generated with each point corresponding to the location of a pixel whose value was above the 40% threshold and the data was then fit to a circle using a Landau-Smith algorithm . The circular fit was then used to center all data from the original reconstructed image and intensities were normalized by subtracting the minimum intensity from each pixel within the original data. A representative centered and normalized image is shown in Fig. 4.5b. A table of position data was then constructed such that the intensity value in each pixel was
Fig. 4.5 Ion intensity map of annularly focused ions following image centering and normalization for an emitter potential of 3.0 kV and focusing potential of 1.5 kV (a), radial distribution of focused ions under different emitter and focusing voltages (b). Potentials applied to nanoESI emitter and focusing electrode are denoted in the legend of (b). Values are given as spray/focusing potential, where spray potential is relative to focusing potential (i.e. in the case of 0.25 kV/1.0, 1.25 kV was applied to emitter and 1.0 kV was applied to focusing electrode). In each case ions were generated by nanoESI of ACN solution
taken as the number of individual points at the corresponding location within the centered image. This table of position information was then transformed to polar coordinates and a histogram of radial deposition locations was constructed for each data set. The radial distribution of ions under different spray and focusing potentials is shown in Fig. 4.5a.
These results clearly illustrate the annular focusing, predicted by simulations. Through a comparison of the simulation results of ion origins, shown in the lower right portion of Fig. 4.4, it is expected that ions are indeed originating entirely from the central portion of the electrode and expansion of the ion plume is not deleterious to ion focusing. A contributing factor may be the initial dispersion of the ion plume into an annulus, thus spreading the generated charge over a larger volume, limiting the effects of space charge.