Electrospray, mechanism

Chiarot, P. R. (2008). Characterization of an electrified fluid interface and the electrospray mechanism development of microscale analytical techniques. PhD thesis submitted to the University of Toronto, Toronto, Ontario, Canada. 

Yeo LY, Chang HC (2006) High frequency AC electrosprays mechanisms and applications. In Rahman M, Brebbia CA (eds) Advances in fluid mechanics VI. WIT, Southampton, pp 223-231... 

Yeo LY, Lastochkin D, Wang S-C, Chang H-C (2004) A new ac electrospray mechanism by Maxwell-Wagner polarization and capillary resonance. Phys Rev Lett 92 133902... 

This chapter is written for users of ESI-MS. It presents an account of how it all works. Such understanding is desirable because the observed mass spectra depend on a large number of parameters. These start with a choice of solvent and concentrations of the analyte, choice of additives to the solution that may be beneficial, choice of the flow rates of the solution through the spray capillary, the electrical potentials applied to the spray capillary (also called needle ) and the potentials of ion optical elements that are part of the mass analyzer. Proper choice of these parameters requires not only some understanding of conventional mass spectrometry but also of the electrospray mechanism. In early work on ESI-MS many of these parameters were established by trial and error, but now that a better understanding of the mechanism is at hand more rational choices are possible. The present chapter provides an up to date account of Electrospray. Eor a broader coverage, which is somewhat dated but still relevant, the review by Smith and coworkers is recommended [7]. 

As discussed in Section 1.2.7, the formation of the final charged droplets that lead to gas-phase ions is associated with a very large loss of solvent by evaporation. This leads to a large increase in the concentration of the solutes present in the electrosprayed solution. Sucdi a large increase in concentration will promote bimolecular reactions, and these will have an effect on the observed mass spectrum. We will consider two examples. In the first example bimolecular reactions are shown to take place on native proteins, indicating various levels of protein surface gas-phase chemistry. In the second example, a comparison of biochemical and physical organic applications of ESMS to the determination of equilibrium constants indicates the limitations on such measurements imposed by the electrospray mechanism. 

Yeo LY, Chang H-C (2006) High frequency AC electrosprays Mechanisms and applications. Advances in Fluid Mechanics VI, Skiathos, Greece. WIT Press, Southampton... 

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

We have previously considered the mechanism of electrospray ionization in terms of the charging of droplets containing analyte and the formation of ions as the charge density on the surface of the droplet increases as desolvation progresses. The electrospray system can also be considered as an electrochemical cell in which, in positive-ion mode, an oxidation reaction occurs at the capillary tip and a reduction reaction at the counter electrode (the opposite occurs during the production of negative ions). This allows us to obtain electrospray spectra from some analytes which are not ionized in solution and would otherwise not be amenable to study. In general terms, the compounds that may be studied are therefore as follows ... 

Electrospray is unusual in that it produces almost exclusively multiply charged ions in a variety of different charge states. The way in which the molecular weight of an analyte may be calculated has been derived. In addition, the appearance of an electrospray spectrum may vary considerably with the conditions in the solution from which it has been generated. Eor this reason, the mechanisms leading to the production of ions using this technique have been described at some length. 

Charge-residue mechanism One of the two mechanisms used to account for the production of ions by electrospray ionization. 

An electrospray is generally produced by the application of an electric field to a small flow of liquid from a capillary tube toward a counter electrode. The principles of electrospray as applicable to mass spectrometry and the mechanisms involved have been a subject of intense debate over the last decade and have been addressed even before that. This is evident from the discussions in the 2000 issue of the Journal of Mass Spectrometry (e.g., Mora11), the book by Cole,12 and several reviews.8,10 13 14 Here we present a summary encapsulating the relevant observations and direct the readers to the above articles for a more elaborate account. 

A characteristic feature of ESMS is the detection of multiply charged analytes. Macromolecules, such as proteins have multiple sites where protonation or deprotonation (the two most common charge inducing mechanisms in electrospray—other routes to charge induction include, ionization through adduct formation, through gas-phase reactions, and through electrochemical oxidation or reduction) occur. These are desorbed effectively in ESMS and... 

Kebarle, P. A brief overview of the present status of the mechanisms involved in electrospray mass spectrometry. /. Mass Spectrom. 2000,35,804-817. 

Electrospray (ES) existed long before its application to mass spectrometry (MS). It is a method of considerable importance for the electrostatic dispersion of liquids and creation of aerosols. The interesting history and notable research advances in that field are very well described in Bailey s book Electrostatic Spraying of Liquids. 37 Much of the theory concerning the mechanism of the charged droplet formation was developed by researchers in this area. The latest works can be found in a special issue38 of the Journal of Aerosol Science devoted to ES. 

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