Negative ion mode evolution of potential buildup and mapping of potential gradients within the electrospray emitter

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A VAbstract. The phenomenon of electrospraying of liquids is presented from the perspective of the electrochemistry involved. Basics of current and liquid flow in the capillary and spray tip are discussed, followed by specifics of charging and discharging of the sprayed liquid surface. Fundamental theories and numerical modeling relating electrospray current to solution and spray parameters are described and then compared with our own experimentally obtained data. The method of mapping potentials and currents inside the electrospray capillary by using an inserted electrically-isolated small wire probe electrode is discussed in detail with illustrations from new and published data. Based on these experimentally obtained results, a new mathematical model is derived. The introduced "nonlinear resistor electrospray capillary model" divides the electrospray capillary into small sections, adds their contributions, and then, by transition to infinitely small section thickness, produces analytical formulas that relate current and potential maps to other properties of the electrospraying liquid: primarily conductivity and current density. The presentation of the model is undertaken from an elementary standpoint, and it offers the possibility to obtain quantitative information regarding operating parameters from typical analytical systems subjected to electrospray. The model stresses simplicity and ease of use; examples applying experimental data are shown and some predictions of the model are also presented. The developed nonlinear resistor electrospray capillary model is intended to provide a new quantitative basis for improving the understanding of electrochemical transformations occurring in the electrospray emitter. A supplemental material section gives full derivation of the model and discusses other consequences.

Section: Discussionmentioning

confidence: 99%

“…The use of our wire probe electrode has permitted the acquisition of a great number of differential electrospray emitter potential (DEEP) maps in electrospray capillaries containing a variety of solutions [91,93,94]. Examples are given in Figure 5a and b.…”

Section: Potential and Current Mapsmentioning

confidence: 99%

Perspective on Electrospray Ionization and Its Relation to Electrochemistry

Pozniak

Cole

2015

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“…Collectively, this study provides an empirical foundation for understanding the chemical mechanisms underlying intrasource separation and selective ionization of lipid classes that can potentially be used for global analysis of cellular lipidomes without the need for chromatographic separation. S ince Fenn and colleagues [1,2] initially developed the electrospray ionization (ESI) technique for the mass spectrometric analysis of polar compounds, many theories of the physical processes involved in ion generation during ESI have been proposed and validated in some detail [3][4][5][6][7][8]. The two main stages in the mechanism of ion generation are the production of the charged droplets from the sprayed solution in the presence of high electric fields and the formation of gas-phase ions from the dispersed charged droplets [9].…”

mentioning

confidence: 99%

Factors influencing the electrospray intrasource separation and selective ionization of glycerophospholipids

Han

Yang

et al. 2006

119

The external electric field induces a separation of cations from negative electrolyte ions in the infusate while differential ionization of molecular species that possess differential electrical propensities can be induced in either the positive-or negative-ion mode during the electrospray ionization process. These physical and electrical processes that occur in the electrospray ion source have been used to selectively ionize lipid classes possessing different electrical propensities that are now known as "intrasource separation and selective ionization". However, the chemical principles underlying charge-dependent alterations in ionization efficiencies responsible for the selective ionization of lipid classes are not known with certainty. Herein, we examined the multiple factors that contribute to intrasource separation and selective ionization of lipid classes under optimal instrumental conditions. We demonstrated that many different lipid classes could be selectively ionized in the ion source and that intrasource resolution of distinct molecular constituents was independent of lipid concentration, flow rate, and residual ions under most experimental conditions. Moreover, the presence of alkaline conditions facilitates the selective ionization of many lipid classes through a mechanism independent of the design of the ESI ion source. Collectively, this study provides an empirical foundation for understanding the chemical mechanisms underlying intrasource separation and selective ionization of lipid classes that can potentially be used for global analysis of cellular lipidomes without the need for chromatographic separation. (J Am Soc Mass Spectrom 2006, 17, 264 -274)

“…This characterization implicitly requires that electrochemical processes occur at each electrode to close the current loop. Despite the fact that in certain cases, droplet charging leading to electrospray can be accomplished without performing electrochemistry at electrical connectors [10], in a typical electrospray ion source, electrochemistry is invariably present.In previous publications [11,12] we presented measurements of potential distributions within metal electrospray capillaries using a method that we call "differential electrospray emitter potential" (DEEP) mapping. Construction of these potential maps, along with investigations of the factors that influence their shapes, were conducted in both the positive [11] and negative [12] ion modes.…”

mentioning

confidence: 99%

“…In previous publications [11,12] we presented measurements of potential distributions within metal electrospray capillaries using a method that we call "differential electrospray emitter potential" (DEEP) mapping. Construction of these potential maps, along with investigations of the factors that influence their shapes, were conducted in both the positive [11] and negative [12] ion modes.…”

mentioning

confidence: 99%

Current measurements within the electrospray emitter

Pozniak

Cole

2007

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A movable disc-like wire probe electrode placed inside the electrospray (ES) capillary was used to measure currents flowing within the ES device for the first time. Currents were measured between the wire probe and the ES capillary. Current maps revealing measured current versus wire probe position were generated for a variety of solution conditions in the positive and negative ion modes and are compared to potential maps. The electrospray device was found to subsist on highly stable total currents; this current regulator aspect of the ES device showed remarkable resiliency regardless of the proportion of current produced at the wire probe electrode versus the ES capillary. However, kinks observed in the current and potential maps are attributed to adsorbed air participating in electrochemical reactions, and turbulence in solution flow in the region of the Taylor cone. From differential electrospray emitter potential (DEEP) maps, current maps, and cyclic voltammetry experiments performed at different wire probe locations, evidence is provided for separate regimes of current flow in the bulk solution and in the thin "skin" of highly conductive electrolyte constituting the outer surface (air interface) of the Taylor cone. Current maps reveal that current is drawn more evenly along the length of the ES capillary when solutions are highly conductive, in agreement with previous results for DEEP maps. In less conductive solutions, the area close to the capillary exit contributes more heavily to current production. Evidence that contaminant participation in electrochemical processes occurring within the electrospray device can be largely responsible for production of the excess charge in ES droplets is also provided. These investigations complement previous DEEP mapping studies to further elucidate the details of the electrochemical processes occurring within the electrospray device. 1-3] is a convenient tool to deliver ions from the liquid phase to the gas phase, so that they may subsequently be analyzed by mass spectrometry (MS). The inherent electrochemical nature of the ES device was first expounded by Paul Kebarle and coworkers [4,5], but in many cases, as discussed in an excellent presentation of arguments by de la Mora, Van Berkel, Enke, and Fenn [6], experimentalists are often unaware of the electrochemical processes taking place during the operation of electrospray. In order to use electrospray prudently and efficiently, and to be able to remove chance elements, and extend its reach into new applications, a good grasp of the fundamental electrochemistry that underlies electrospray becomes necessary. Alongside the pursuit of practical applications of this atypical electrochemical cell comes an interest in explaining fundamentals of the electrochemical nature of electrospray [7].The electrospray device-typically consisting of a capillary filled with liquid and a counter-electrode placed at some distance in front of it with a kilovolt potential difference between the two-was discovered to behave in a manner anal...