Ion Focusing at Atmospheric Pressure
The work presented in chapters two through four of this dissertation show examples of different ways in which ions may be manipulated in the air at atmospheric pressures. As discussed in Sects. 1.1 and 1.3, the motion of ions under these conditions is complicated as the result of collisions, droplet desolvation, and space-charge effects. As such, control of the ions’ trajectories requires a non-traditional approach. I have sought to explore a combined tactic of simulations and experiments with rapid prototyping that may be applied to gain an understanding of how unconventional electrode geometries can be used to facilitate control of ion clouds generated by electrospray. These experiments serve only as proof-of-concepts, as more effort must be given to optimize geometries, and gain a better understanding of the phenomena in general. Notably, plastic electrodes produced in this manner have high porosity thus solvent adsorption onto walls and resulting plastic solvation effects must be considered. This is especially important if these electrodes are to be used for long-term applications.
Perhaps the most profound effect discovered by these methods is the annular focusing of electrosprayed ions as discussed in Chap. 4. Compared to the use of the more conventional open-channel electrodes, this design is highly efficient at spatially focusing ions, while blocking the passage of large solvent droplets. In order for the annular focused ions to be detected by MS an efficient method of introducing annular ion clouds into a vacuum system must be developed. Perhaps the most probable method of doing so is by employing an MS inlet in the form of a conical slit, this concept is shown in Fig. 5.1. Experiments with this design must be done to verify that gas flow in this type of vacuum system interface would generate sufficient drag forces to carry ions into the reduced pressure environment. The converging nature of this design, coupled with the large pressure drop across the system makes estimating the flow from first principles or numerically, both challenging approaches. Empirical exploration of this type of inlet system is relatively straightforward, so long as it is possible to effectively manufacture a design of this type.
Fig. 5.1 Concept design of an alternative atmospheric pressure interface for coupling with an annularly focusing ion source a and a cross-section of the concept inlet b