Formation of the Electrospray Plume

To understand the formation of a continuous spray, consider the surface of an electrolytic solution when exiting an electrically conducting capillary held at high electric potential with reference to a nearby counter electrode. In practice, the spray capillary has about 75 pm inner diameter and is kept at 3-4 kV with reference to the sampling orifice at approx. 1 cm distance. At the open end of the capillary the emerging liquid is thus exposed to an electric field of about 10 V m  

The electric field causes charge separation in the electrolytic solution and finally deformation of the memscus into a cone. The phenomenon of cone formation has been discovered by Zeleny [39] and first theoretically described by Taylor [40], hence it was termed Taylor cone. 

The process of Taylor cone formation starts when the spherical surface forms an oval under the influence of increasing field strength. In turn, a sharper curvature of the oval increases the field strength. The Taylor cone forms as soon as the critical electric field strength is reached and starts ejecting a fine jet of liquid from its apex towards the counter electrode when surface tension is overcome by the 

The ESI process overall represents an electrolytic flow cell where the connection from the spray capillary to the counter electrode is created by charge transportation via the electrically charged aerosol (Fig. 12.17) [93-95]. In positive-ion mode, neutralization at the sampling orifice is effected by electrons from the high voltage power supply, which in turn originate from oxidation of anions on the inner wall of the spray capillary. In negative-ion mode, reduction of cations will occur in place of oxidation [61]. 

In rare cases, the electrolytic processes during electrospray can cause molecular ion, M , formation by electrolytic oxidation [95]. This is more probable if nonpolar molecules of very low ionization energy are precluded from other ionization pathways such as protonation or cationization. Electrolytic M ion formation normally requires nonprotic solvents, low liquid flow for increased reaction time, and preferably metal spray capillaries rather than fused silica capillaries. ESI may also result in changes of the oxidation state of metal ions, e.g., Ag Ag , 

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