Ion Transport and Focal Properties of an Ellipsoidal Electrode Operated at Atmospheric Pressure

Introduction

Most frequently, ions are transported from ambient pressure and manipulated under low pressure conditions. While a vacuum environment is necessary to make precise measurements of an ion’s mass-to charge ratio, the ability to effectively control ion trajectories and spatially manipulate ions without the use of vacuum systems is of great interest in a number of different fields. Modifications to surfaces made using low energy molecular ion beams [1-9] are of particular note. Such modifications include cases in which the ion/surface interaction occurs at atmospheric pressure [10, 11]. Applications include the chemical functionalization (derivatization) of surfaces [12, 13] and the preparation of thin films [14, 15]. While thin film preparation typically uses exposure to highly controlled yet poorly characterized plasmas [16], polymer film deposition using ion beam conditioning has become increasingly common [17]. Ambient ionization, particularly in the field of analytical mass spectrometry relies on the ionization and subsequent transfer of ions to a vacuum system for analysis. This field focuses on the analysis of samples via mass spectrometry in their native state, with little-to-no sample preparation. A wide variety of ambient ionization methods have been developed and include spray, laser, and plasma techniques which are used to generate representative ions [18, 19]. The growth of interest in gas-phase ion chemistry under ambient environment raises obvious concerns regarding ion transport and focusing at atmospheric pressure. An understanding of factors which contribute to the efficiency by which ions are transported has come both empirically (e.g. the transport over several meters of ions generated by desorption electrospray ionization, and their delivery to a mass analyzer [20]) as well as through fluid dynamics simulations [21]. These simulations have confirmed that once laminar flow is established in a transport tube, modest suction will move typical organic ions long distances through air without significant losses.

© Springer International Publishing AG 2017

Z. Baird, Manipulation and Characterization of Electrosprayed Ions

Under Ambient Conditions, Springer Theses, DOI 10.1007/978-3-319-49869-0_2

The issue of ion focusing in air is has importance beyond the ambient ionization methods. In particular, all forms of spray ionization, including electrospray ionization (ESI), yield droplets, the fission of which results in a highly dispersed spray plume in which the ion concentration decreases rapidly with distance from the source [22]. This undesirable effect is compounded by the fact that the droplets undergo further fission and desolvation before producing gas-phase ions that can be analyzed by a mass spectrometer. Increased distances of travel are needed for more effective desolvation [23, 24]. On the other hand, the small sampling orifices (generally 1 mm or less, inner diameter) needed for vacuum compatibility greatly restrict the fraction of ions that may be sampled from the spray plume. Because of these factors, ion collection efficiency is low, typically a small fraction of the ions produced (often <0.1 %) by the ionization source [22, 25].

Multipole ion guides based on collisional focusing through the application of radio frequency (RF) fields have been utilized to increase transport efficiency at lower pressures (0.1-10 mtorr) but the same effect is not observed at atmospheric pressure [26, 27]. Another popular approach is through the use of electrodynamic ion funnels. Ion funnels are composed of stacked ring electrodes of decreasing diameter to which DC and RF potentials are applied. In some cases these have improved sensitivity by more than 10 fold when operated in the first differentially pumped regions of a mass spectrometer. However, the ion funnel is only effective in the pressure range of 0.1-30 torr and is a mechanically complex device [28]. Other methods utilizing stacked ring electrodes have also been demonstrated as a means of increasing ion transmission; however their function is limited to intermediate pressures (ca. 1 mtorr—1 torr). Such methods include the traveling wave ion guide which uses electrodynamic potentials to confine ions radially along with a superimposed voltage pulse to transport ions axially, and the periodic focusing ion guide, which uses only DC potentials to provide periodic ion focusing [29, 30]. As the majority of ion loss takes place at the atmospheric pressure interface of a mass spectrometer, improvement in transport from ambient pressure to the first differentially pumped region is needed to improve sensitivity significantly.

Herein is described the use of a simple elliptical electrode to which only DC potentials are applied to facilitate the efficient transport and focusing of ions at atmospheric pressure. An ellipsoidal shape was chosen to be compatible with the intended future use of an array of spray tips as a means of increasing ion currents. The ellipsoidal shape provides symmetry in the sprayer orientation as each sprayer can be angled to spray towards the same point while experiencing equivalent potentials imposed by the focusing electrode structure. Interest is concentrated on ions produced by ion sources which have low solvent flow rates, typified by nanoESI. The focal properties of the electrode system is explored through the use of a detector that operates at ambient pressure. Quantitative measurements of ion transfer efficiency are made using ionized dyes which, after soft landing onto a surface, can be rinsed off and quantified spectrophotometrically. When interfaced to the atmospheric pressure inlet of a mass spectrometer, the ion optical system described here is shown to increase ion transport efficiency by a factor of 100 over distances of several centimeters.

 
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