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Mechanisms of Electrospray Ionization for Mass Spectrometry Analysis Jayo, Roxana Sep 10, 2014

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         MECHANISMS OF ELECTROSPRAY IONIZATION FOR MASS SPECTROMETRY ANALYSIS    Abstract Electrospray ionization mass spectrometry (ESI-MS) is a choice method to accomplish the mass determination of large, non-volatile biomolecules including proteins, peptides, oligonucleotides, carbohydrates, polysaccharides and other compounds which are too large to be vaporized or too fragile to remain intact when other ionization methods are used. Since ions are generated directly from the liquid phase, ESI can be easily coupled to separation techniques such as high performance liquid chromatography (HPLC) and capillary electrophoresis (CE). Although ESI has had an enormous impact on modern mass spectrometry, and has become one of the most important ionization techniques, some parts of the underlying processes have proven to be very difficult to establish. Such is the case of the mechanisms by which gas-phase ions are produced by the charged droplets. Two theories, the Charge Residue Model (CRM) and the Ion Evaporation Mechanism (IEM), have been proposed. Since electrochemistry is considered a fundamental aspect of the operation of the ESI ion source both for the fundamentals of gas-phase ion formation from the charged droplets, and for the analytical utility of the device, there has been considerable efforts in order to elucidate the electrochemical process associated to the operation of this ion source.    The aim of the present report is to provide a concise and fundamental description of the mechanisms involved in the production of isolated ions by ESI, as well as, to present a brief account of the electrolytic nature of ESI.  Keywords:  Electrospray ionization, Mass spectrometry.                 I. THE MECHANICS OF ESI-MS Electrospray ionization refers to the process of generating gaseous ions directly from solution by spraying the sample solution under high electric potential at atmospheric pressure (1). During typical ESI, a dilute solution of analyte dissolved in a polar volatile solvent is pumped through a narrow, stainless steel or silica capillary (50 to 150 μm) at a flow rate of between 0.5 μL/min and 1mL/min and a voltage –either positive or negative– of 2 to 5 kV is applied to the tip of the capillary(2). The mechanism by which the analyte ions are produced in the gas phase from solutions containing dissolved electrolyte ions can be divided into the following steps:  A. Production of charged droplets at the electrospray capillary tip The applied very high electric field at the electrospray capillary tip leads to a partial separation of positive from negative electrolyte ions because the field penetrates partly past the surface of the liquid at the tip of the capillary. The electric field can be approximated by the following equation:    cccc rdrVE 4ln2 , where cr  is the capillary outer radius, cV is the applied electric potential and d is the distance from the capillary tip to the counter-electrode (3).  Under a positive electric field, positive ions will be in excess and will accumulate at the tip of the capillary. The mutual repulsion among the positive ions at the liquid surface leads to the protrusion of the liquid towards the counter electrode, and it is the surface tension of the solvent that holds the liquid (4). As more charges gather on the surface, the Coulombic repulsion overcomes the surface tension and the liquid begins to expand downfield forming the ‘Taylor cone’ (Fig. 1).  Fig. 1. Schematic representation of the ESI process and formation of the ‘Taylor cone’ (2).        When the Taylor cone forms both the electric field intensity and the concentration of excess charge are highest at the cone tip, and as a result a thin surface layer at the cone tip moves towards the counter electrode forming a liquid filament –liquid-jet–, from which a fine spray of charged droplets of a single polarity are emitted (5).  B. Solvent Evaporation from the charged droplets After the charged droplets are dispersed, they undergo solvent evaporation promoted by heat supply from the ambient air, which leads to a decrease of the droplet radius at constant charge. The droplets become smaller until Coulombic repulsion between the charges overcomes the cohesive force of the surface tension; this is defined as the Rayleigh limit and it is expressed by the following equation: 2/130 )(8 Rq  , where q  is the charge on the droplet, 0 the vacuum permitivity, and  the surface tension (1).  Beyond the Rayleigh limit, the droplets experience Coulombic fissions to produce smaller offspring droplets. This fission does not result in uniform offspring droplets and often occurrs with a fine jet thrown from the droplet in a process called ‘droplet jet fission’ (6). The offspring droplets carry off only about 1-2.3 % of the mass and 10-18% of the charges of the parent droplets and the offspring radius is about one-tenth of the parent radius (5). The offspring droplets, on solvent evaporation, themselves undergo Coulombic fissions, which lead to second generation offspring (4). These droplets move through the atmosphere towards the entrance to the mass spectrometer, and generate charged analyte molecules (ions) by one of the two proposed mechanisms (2). C. Gas-Phase Ion Formation The mechanisms by which the small highly charged droplets become gas phase ions is still under debate and two theories has been postulated. The Charge Residue Model (CRM) proposed by Dole assumes that when the solution is dilute enough a series of fissions events will produce small final droplets that bear one or more excess charges but only a single analyte molecule. As the solvent molecules evaporate, the excess charges present will become situated on the sites affording the most stable gas-phase analyte ion (7). However, it was observed that the low concentration condition assumed by Dole is not always met, and the evaporation of solvent at the beginning normally happens much faster than Coulombic fission, so that the residue that is formed contains more than one solute molecule (2).        Iribarne and Thomson suggested another mechanism for the formation of gas-phase ions. The mechanism assumes that solvated ions are emitted directly from the charge droplet  after the radii of the droplets decrease to a certain limit by solvent evaporation. Iribarne and Thomson derived an equation to predict the rate of ion emission from the charged droplets, with a radius limit suggested to be R ≤10 nm (8).  Their explanation is that, with a critical size still larger than the Rayleigh limit, the charge density on the droplet surface is high enough to emit a charged analyte molecule from the surface of the droplet, and this reduces the Coulombic repulsion. This was the so-called ion evaporation mechanism (IEM) which is complementary to the CRM. The basic difference betweeen CRM and IEM is the manner by which the analyte ions are detached from other components in the solutions. Both mechanism have their supporters and are more aplicable in some situations (2). Once the ions are transferred to the gas-phase either by the CRM or the IEM they are ready to be analyzed for mass-to-charge ratio within the mass spectrometer. Normally, a curtain gas that flows counter to the ion flow is used because it helps to completely dry droplets and also prevents solvent and neutrals from entering into the vacuum system of the mass analyzer.    II. THE ELECTROLITYC NATURE OF ESI The electrospray ion source used in ESI-MS is a two electrode system. The first electrode, or   working electrode, is a metal capillary in contact with the solution placed at or upstream of the point at which the charged droplets are generated. The second electrode, or counter electrode is the atmospheric sampling aperture plate (9). Under typical ESI-MS operation conditions, droplets enriched in ions of one polarity are emitted from the capillary, which results in a continuous-electric-current, so the rate of the charge separation determines the average current that flows in the cell (1). This charge-balancing process involves electrochemical reactions of the components of the system and/or one or more species present in solution. Under a high positive voltage (positive ion mode), oxidation reactions occurs at the tip of the capillary while reduction occurs at the counter electrode (2). One possible electrolytic reaction involves the metal atoms from the capillary giving their electrons to the power supply and becoming cations to compensate for the loss of positive charge due to ES. Another possibility is that one or several of the solvent components lose electrons to the power supply to become cations in the solution (1). The electrochemical        reactions could be the discharge of anions or the oxidation of neutral solvent. It is worthwhile to mention that ions that are not present in the solution but produced by the reactions do not have a significant effect on the sample being analyzed, nor do the ions appear in the spectra obtained by MS because of their very low abundance (9). Furthermore the electrochemical process occurring at the counter electrode are not expected to be analitically significant so that species involved in those reactions do not give rise to gas-phase ions.    III. CONCLUSIONS A summary of the stages of the mechanisms which leads to the production of the charged droplets from the solution containing electrolytes has been presented. The origin of the charge of the droplets is due to an excess of one type of  electrolyte ion over another one, which depends on the voltage applied. The evolution of the charged droplets through Coulombic fission and solvent evaporation is known to occur. However, the exact sizes and charges of the offspring droplets produced have not already been possible to determine. It is believed that ionic forms of very large molecules are formed according to CRM. There is still debate on the mechanism of desorption of smaller ions. The occurrence of electrolytic reactions at the emitter electrode of the ES ion source, as normally configured in ESI-MS, is required for the device to sustain the production of charged droplets and gas-phase ions. Under common operating conditions, the electrochemical process rarely have an obvious effect on the identity and relative abundances of the ions in an ESI mass spectrum.   IV. REFERENCES 1. Cole, R. Electrospray Ionization Mass Spectrometry. 1997, New York: John Wiley & Sons, Inc.  2. Cech, N. and Enke, C. Practical implications of some recent studies in Electrospray ionization fundamental. Mass Spectrometry Reviews, 2001, 20, 362-387. 3. Bruins, P. Mechanistic aspects of electrospray ionization. Journal of Chromatography A, 1998, 794, 345-357.  4. Gaskell, S. Electrospray: Principles and Practice. Journal of Mass Spectrometry, 1997, 32, 677-688. 5. Cole, R. Some tenets pertaining to electrospray ionization mass spectrometry. Journal of Mass Spectrometry, 2000, 35, 763-772. 6. Gomez, A. and  Tang, K.Q. Charge and fission of droplets in electrostatic sprays. Physics of Fluids, 1994, 6(1), 404-414. 7. Fenn, J. Electrospray Ionization Mass Spectrometry: How It All Began ?. Journal of Biomolecular Techniques, 2002, 13(3), 101-118. 8. Thomson, B.A and Iribarne, J.V. Field-induced ion evaporation from liquid surfaces at atmospheric-pressure. Journal of Chemical- Physics, 1979, 71 (11), 4451-4463. 9. De la Mora, J.F., Van Berkel, G., Enke, C., Cole, R. , Martinez-Sanchez, M., Fenn, J. Electrochemical processes in electrospray ionization mass spectrometry. Journal of Mass Spectrometry, 2000, 35(8), 804-817.  

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