Perspective on Electrospray Ionization and Its Rel

Authors: Boguslaw P Pozniak Richard B Cole

Publish Date: 2015/01/27

Volume: 26, Issue: 3, Pages: 369-385

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Abstract

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 electricallyisolated 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 consequencesStudies into electrochemistry/electrospray have dealt with electrochemical issues relating to standard redox cells flow cells homogeneous and heterogeneous reaction kinetics and mass transport These are combined with phenomena pertaining to liquid jet charging and instability plus surface phenomena and gasphase ion production In this Perspective we present observations and opinions based on our own experiments and studies of relevant literature Our discussion will focus on electrochemical phenomena that accompany the liquid charging step of the electrospray processThe electrospray phenomenon and in general electrified liquid jets are topics of interest not only to mass spectrometrists who use them as a means to transfer ions from a liquid into the gas phase but also to users of a large variety of other applications starting from jet printing through metal plating polymer fiber production to aerial spraying of fertilizers 1 Flow phenomena in electrified liquids are studied in a discipline called electrohydrodynamics This belabored field has a large body of literature that developed over time heavy often staggering mathematicsIn the latter part of this Perspective we present a simple mathematical model to not only qualitatively but also quantitatively assess experimental measurements of electrical potential and current within the ES emitter The goal for developing the model was to have mathematical formulas that can describe the obtained potential and current maps extract useful electrochemical parameters and provide confidence that the observed trends in those maps conform to electrochemical principles At the same time the derived mathematical formulas are intended to be simple enough to be handled with minimum computational effort yet precise enough to simulate maps with less error than observed during experimental acquisitions A Supplemental Material section which contains all of the details of mathematical formula derivations accompanies this documentA stable electrospray exists only when all relevant parameters fall within a certain narrow range of values Often a sizeable change of one parameter necessitates a major adjustment of others in order to regain spray stability In this Perspective our intention is to take a fresh look at electrospray phenomena and operating parameters that are pertinent to solution conditions commonly found in analytical applications Our presentation has been made as quantitative as possible while striving to maintain an elementary approach The intention was not to derive a model to compete with fundamental theories developed by researchers in the field of electrohydrodynamics but rather to present a “new view” in electrospray phenomena such that an experimentalist might consider an electrospray mass spectrometer as something more than a “black box” deviceThis Perspective is not intended to be a comprehensive review or a tutorial of electrochemical cell coupling to mass spectrometry It is meant to be a quantitative guide for mass spectrometry experimentalists who deal with electrochemical transformations of the sprayed liquid but have difficulty in exploiting its benefits or fighting its adverse effects This Perspective also seeks to fill the gap between existing advanced theory and everyday analytical lab practice thus a goal here is to facilitate experimental implementation of existing theory to electrospray mass spectrometry practitioners Because of this intentionally limited scope our selection of literature is subjective and many important publications are neither quoted nor discussed We refer the interested reader to a selection of published reviews pertaining to electrospray ionization and/or EC/ESI coupling available elsewhere 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 The focus in the current paper is kept on features of electrochemistry/electrospray that have been largely neglected but might be important for analytical practice especially if one considers how the development of electrochemical derivatization of analytes has become an established and fruitful method 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

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