Electrospray and Atomization
The electrostatic spraying of liquids is a well established technique [1] that offers several advantages over other methods of atomization. It appears in such diverse fields as spray painting, rocket propulsion, high intensity ion sources from liquid meta
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P.K. Watson
Chapter 14
ELECTROSPRAY AND ATOMIZATION
14.1 Introduction The electrostatic spraying of liquids is a well established technique [1] that offers several advantages over other methods of atomization. It appears in such diverse fields as spray painting, rocket propulsion, high intensity ion sources from liquid metals, etc. In many of these sources the liquid, dispersed from a high voltage capillary, forms a cone of semiangle -49.3°, as discussed by Taylor [2]. In this configuration the electrostatic forces are balanced by the surface tension of the liquid, and the spray arises from the tip of the cone where the field is a maximum and the equilibrium cannot be maintained. This technique has been brought into scientific prominence in the past few years with the emergence circa 1988 of electrospray as a method for producing molecular ions in the gas phase from macromolecules in solution. These ions are required in order to extend the molecular weight range of mass spectrometers (indispensable tools for the chemist) so as to include large, polar organic molecules of interest in biology and medicine. A useful survey of this topic and its historical background is to be found in a review paper by Fenn et al. [3]. There were in effect two distinct problems that needed to be solved in this field, and it is a remarkable fact that the resolution of the first of these problems brought with it the answer to the second: (i) The technique of mass spectrometry requires as a preliminary step, the transfer of neutral molecules into the vapor phase, by evaporation, and their transformation into molecular ions in vacuo; the subsequent trajectories of these ions in a mass analyzer provides a measure of the normalized mass-to-charge ratio m/z. But macromolecules such as protein, for example, cannot be vaporized without extensive or catastrophic decomposition, so classical methods of ionization cannot be used. Moreover, even if one could transform macromolecules of say 105 daltons (units of molecular weight) into the corresponding singly charged ions, one would then run into the second problem:
A. Castellanos (ed.), Electrohydrodynamics © Springer-Verlag Wien 1998
Conduction, Instabilities and Breakdown in Liquid Dielectrics
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(ii) Mass spectrometers are limited in the rnlz range they are able to analyze. For example, a typical quadruple analyzer can resolve singly charged ions of up to 1500 daltons but not beyond that, and molecules of interest to the biochemist are one or two orders of magnitude larger than this (for example insulin -5730 daltons, lysozyme -14,300 daltons, bovine albumen dimer -133,000 daltons). In their 1984 paper reporting on early experiments on electrospray mass spectrometry, Fenn and coworkers [4] called attention to the tendency of this spray technique to produce multiply charged ions, and pointed out that if one could promote multiple charging that would reduce rnlz and thus extend the effective mass range of the mass analyzer. The fact that this was subsequently accomplished is a major reason
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