Full Length ArticleSpace charge compensation in air by counterion flow in 3D printed electrode structure
Graphical abstract
Introduction
The widespread adoption of electrospray ionization [1] as well as other ambient ionization techniques [2,3] as ion sources for MS has been paralleled by an increased interest in the physics of ions and charged microdroplets under ambient conditions. Surprising effects, such as strong ambient ion and microdroplet focusing [[4], [5], [6], [7], [8], [9], [10], [11], [12]] have been observed. For instance, ions passing through an aperture, such as a grid, have been observed to focus into spots up to 10x smaller than the grid apertures [6,9,13]. Subsequent work has shown that ions can be transported in the air while maintaining focus using shaped electrodes or electrode arrays. Stacked apertures have also been used to create well-focused beams of ions from ESI sources [10,14,15]. In contrast to conventional, vacuum-based ion manipulation, where the ion motion is dictated by the mass-to-charge (m/z) of the ion and in which a combination of DC and radiofrequency (RF) fields is often employed to control position and velocity, the motion of an ion at ambient pressure is related to its ion mobility and is easily controlled through the use of DC-only potentials [8,16]. Similar to in-vacuum ion optics, focusing devices and ion guides are now being developed to manipulate ions at high, i.e., ambient pressures [[4], [5], [6],8].
Ions and droplets under ambient conditions are, due to collisions with background gases, quickly thermalized and move with low kinetic energies [8,16]. Under these low-velocity conditions, space charge effects become particularly pronounced [17], leading to a loss of ion current entering the mass spectrometer [18,19]. The expansion of the dimensions of a beam of ions due to space charge can be described by the equation:where z is the axial displacement, r0 is the initial beam radius, V is the acceleration potential, m is the ion mass, I is the ion current, r is the beam waist at time t and unit charge on the ion is assumed [20,21].
Efforts to increase the ion signal at a mass spectrometer through multiplexing ion sources have been hampered by the effects of diverging electrical potentials and space charge from the droplet plumes produced by multiple emitters [[22], [23], [24]]. While electrodes can be used to shape ion beams from ESI emitters [4,5,7,8,11,25], merging these beams is still a challenge [22,26]. Practical methods for reducing space charge require either lowering the current produced by the emitter or spatially separating the emitters [22,27,28]. Both of these tactics are detrimental to ion signal and signal density. A third approach, the neutralization of space charge by a counterion beam, was first proposed in the 1920s by Kingdon and Langmuir [29,30]. Since their initial studies, which focused on atomic ions created from the ionization of mercury vapors or rare gas ions generated by filament-produced electrons in sealed tubes, this methodology has been used to control electron and atomic ion beams in accelerators [17,[31], [32], [33], [34], [35], [36]] as well as to enhance the sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) and intense laser ion sources [[37], [38], [39]]. The ionization of background gas by uranium (U+) ions to give associated gaseous anions was also used to compensate for space charge in calutrons for the Manhattan project [40]. However, all these results have utilized atomic ions and most have utilized high energies and low pressures, making them unsuited for the focusing and transmission of atmospheric pressure ions. Moreover, none of them have dealt with molecular ions, the subject of central interest in much of modern mass spectrometry.
In this work, a beam of ions produced via nanoelectrospray ionization (nESI), guided through a curved 3D-printed atmospheric pressure ion guide, was exposed to a beam of ions of opposite polarity produced through atmospheric pressure chemical ionization (APCI) in the same device. The spatial distribution of the ion beam and the absolute intensity of the ion signal were measured using an IonCCD™ detector [41,42] when ions of both polarities were introduced into the ion guide.
Section snippets
Electrode design
Electrode designs were generated using AutoCAD software and imported as stereolithography (.stl) files into Simplify 3D, a slicing software, to break up the model into “layers”. The layers were then imported as G-code to a MendelMax 3 3D printer. The electrodes used in this experiment were printed using carbon-doped polylactic acid (PLA) plastic while non-conductive portions of the device were printed from standard PLA plastic. A schematic view of the nested cylindrical device is shown in Fig. 1
Ion focusing
The SIMION predicted paths of positive (red) and negative (blue) ions are shown in Fig. 2A and B. For these simulations, the nESI emitter was held at 6 kV, while both APCI needles were held at −1.5 kV. The negative ions originated near the two APCI needles while the positive ions originate at the nESI emitter. The negative ions do not travel in the middle of the device, but instead move along the periphery of the device, in opposition to the positive ions that are centered throughout the turn.
Conclusions
This work demonstrates the usefulness of space charge neutralization by a counterion flow on a beam of low energy (<10 eV translational kinetic energy) ions at atmospheric pressure prior to entering a mass spectrometer. Counterion-induced focusing is shown through the use of a spatially resolved ion detector. It is observed that the ion distribution for a mixture of TAA's doubles in peak intensity and becomes three times narrower upon introduction of negative ions using either the inside or
Author statement
Brett Marsh: Conceptualization, Methodology, Formal Analysis, Investigation, Writing-Original Draft.
Saquib Rahman: Conceptualization, Methodology, Formal Analysis, Investigation, Writing-Original Draft.
Victoria Benkowski: Investigation, Writing-Original Draft.
Shane Tichy: Supervision, Writing-Review and Editing.
R. Graham Cooks: Supervision, Writing-Review and Editing, Conceptualization, Project Administration, Funding Acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully acknowledge the support of Agilent Technologies Inc. through gift #4517. Robert L. Schrader and Dylan T. Holden are thanked for helpful comments.
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Cited by (1)
Characterization and optimization of a rapid, automated 3D-printed cone spray ionization-mass spectrometry (3D-PCSI-MS) methodology
2022, International Journal of Mass SpectrometryCitation Excerpt :Due to the ability to create rapid prototypes, offer open-source designs, and increase the reproducibility of the design, 3D-printing has become increasingly useful for analytical chemistry [26–31]. Commercial and 3D-printed components have been combined to create custom ambient ionization sources and other apparatus, including a low temperature plasma source [29], an ion mobility spectrometer [32,33], ion manipulation devices [34–36], a microfluidic holder for PSI [37], and PSI cartridges [38]. Open source access allows these designs to be easily tailored or customized to the analysis.
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These authors contributed equally to this work.