A novel form of atmospheric pressure ion focusing was demonstrated in which up to 80% of electrosprayed ions from a nanoESI emitter are focused into an annulus with a line width of less than 200 p.m FWHM. The annular focusing source was integrated with a fully 3D printable DT-IMS incorporating concentric ring electrodes operated at ambient pressure with no supplementary gas flow. A separation of electrosprayed TAA cations was demonstrated with a resolving power comparable to traditionally manufactured atmospheric pressure DT-IMS instruments. All mechanical components were manufactured by FDM, making this a low-cost, readily accessible means of constructing an IMS.
The ability to separate an electrosprayed mixture under ambient pressure and temperature may prove to be a valuable tool in the study of accelerated reaction in electrosprayed droplets. Normally, IMS instruments employ a desolvation region when coupled with electrospray sources, thus allowing for studies on the conformation and size of solvent-free ions. In the case of accelerated reactions in electrosprayed droplets [46, 47], it is of interest to study solvated ions and measure the properties of droplets with sizes too small to measure using techniques such as phase-Doppler anemometry (PDA). In such cases, it may be necessary to “freeze” droplets in their current state for analysis, by carrying out IMS separations in solvent-saturated vapors. Because of the inherent low-cost and speed at which the 3D printed instrument can be made, replacing IMS components that may become damaged by long-term and repeated solvent exposure is very economical (the typical cost of FDM feedstock is typically less than $0.05/g). Additionally, a wide variety of thermoplastic feedstocks are available for purchase, and many more in development, thus material can be selected based on the application.
In light of these potential applications, there remains a significant amount of work to be done, towards understanding IMS separations occurring under such unique conditions. A good understanding of the fundamentals of ion migration through these complex media under electric fields is necessary to infer physical properties from the measured data. Moreover, improvements in the separation power and sensitivity of this type of ion mobility drift cell must be made in order to further the usefulness of this design. In particular, transmission efficiency may be improved by taking better advantage of the annular focusing nature of the source region. In the current design, the greatest sensitivity in IMS separations was accomplished with the least amount of annular focusing. This is likely the result of the stainless steel meshes, as ions have a larger probability to pass through the mesh openings when spread over a large area. This effect may be mitigated by replacing the mesh with an electrode containing a single open slit, in the form of a ring. By precisely aligning this slit with the focal region a larger injection efficiency is expected. Furthermore, ions would be restricted to injection only in the most uniform region of the electric field, thus simultaneously improving the resolution of ion separation.