Production and Focusing of Ions in Air
Gas phase ions are commonly produced through the use of electrospray ionization (ESI). In brief, the principle of ESI is based on the principle that upon desolvation, electrosprayed droplets produce ions of organic molecules with very little, if any fragmentation . Current MS and IMS instruments rely heavily on the use of ESI as a method of producing ions from aqueous solutions, especially in the field of proteomics. In additions to these roles, ESI is the basis for a number of ambient ionization sources including: desorption electrospray ionization (DESI), laser ablation electrospray ionization (LAESI), extractive electrospray ionization (EESI), paper spray ionization (PS), etc. . Perhaps one of the most profound caveats of this otherwise robust ion source is the inherent low ionization efficiency. This, in combination with the dispersive nature of an ESI plume typically results in all but a miniscule portion of generated ions lost to the surrounding environment. As a result a very small fraction of ions are available for analysis in MS experiments . The overall effect is a decrease in analytical sensitivity and inefficient product collection in preparative experiments such as ion soft landing [18, 19].
Fig. 3.1 3D printed electrode assembly interfaced with inlet of MS (a); cutaway rendering of assembly with overlaid ion trajectories shown in red (b); Surface plot of electric field magnitude overlaid with electric field streamlines (green traces) originating in a 10 mm diameter sphere centered in Esource, 11 mm distant from the nanoESI spray tip (c). Potentials applied to electrodes are identical to those given in Fig. 3.5a, c
While it is only ions that produce the signals measured in MS and IMS, the presence of adventitious neutrals can produce undesirable effects in both instances. Neutral species place a larger load on the vacuum system (notably evaporating solvent droplets) and may undergo reactions with the ions of interest to produce unexpected species that complicate analysis . Additionally, in the case of ion soft-landing, neutral impingement on the deposition surface negates the highly discriminatory nature of the ion selection prior to the surface collision.
In an attempt to address sensitivity and neutral transmission complications, a curved electrode system was constructed from a conductive polymer using a fused deposition modelling (FDM) 3D printer. The assembly is composed of a cylindrical source electrode region (Esource) with an inner diameter (ID) of 20 mm and a length of 30 mm, proceeded by 3 curved electrodes (En) with an ID of 15 mm and a swept angle of 45° around a 15 mm radius of curvature. All electrodes are separated by 3 mm with spacers printed in either acrylonitrile butadiene styrene (ABS) or polylactic acid (PLA). The design of this electrode assembly is such that ions produced via ESI within the source region are focused into a well-defined spot at the deposition surface. This is achieved through the application of appropriate potentials to separate electrode components. A direct line-of-sight from the ion source to the deposition surface is avoided by the curved design, as such, neutral solvent droplets are blocked from contaminating the deposition surface or the API of a mass spectrometer. The device is shown in Fig. 3.1 interfaced with the inlet of a mass spectrometer and is accompanied by a cutaway rendering which displays an overlay of simulated ion trajectories. The low-cost, wide availability, an ever-expanding selection of materials, and the rapid manner of production makes FDM the ideal manufacturing strategy by which parts such as these are produced. The entire assembly discussed in this chapter was constructed in under 3 h, including spacers and electrode components.