Mass Spectrometry A Textbook - Department of Mathematics and ...

11.3 Ion Formation 453
field strength is impossible. Instead, at the moment the critical electric field
strength is reached, the Taylor cone instantaneously forms and immediately starts
ejecting a fine jet of liquid from its apex towards the counter electrode. [64] This
mode of operation is termed cone-jet mode. [5] The jet carries a large excess of
ions of one particular charge sign, because it emerges from the point of highest
charge density, i.e., from the cone's tip. However, such a jet cannot remain stable
for an elongated period, but breaks up into small droplets. Due to their charge,
these droplets are driven away from each other by Coulombic repulsion. Overall,
this process causes the generation of a fine spray, and thus gave rise to the term
electrospray (Figs. 11.11 and 11.12).
11.3.2 Disintegration of Charged Droplets
When a micrometer-sized droplet carrying a large excess of ions of one particular
charge sign – some 10 4 charges are a realistic value – evaporates some solvent, the
charge density on its surface is continuously increased. As soon as electrostatic
repulsion exceeds the conservative force of surface tension, disintegration of the
droplet into smaller sub-units will occur. The point at which this occurs is known
as Rayleigh limit. [38] Originally, it has been assumed that the droplets would then
suffer a Coulomb fission (or Coulomb explosion). This process should occur repeatedly
to generate increasingly smaller microdoplets. While the model of a cascading
reduction in size holds valid, more recent work has demonstrated that the
microdroplets do not explode, but eject a series of much smaller microdroplets
from an elongated end (Fig. 11.13). [49,79,80] The ejection from an elongated end
can be explained by deformation of the flying microdroplets, i.e., they have no
Fig. 11.13. Illustration of droplet jet fission. The average number of charges on a droplet,
the radii of the droplets [µm], and the timescale of events are assigned. The inset shows a
drawing of droplet jet fission based on an actual flash microphotograph. Reproduced from
Ref. [49] by permission of the authors.