Environmental Engineering Reference
In-Depth Information
8.1.1 Generation of the Charged Droplets. Electrophoretic
Mechanism
When applied at the tip of ESI capillary containing the analyte solution, the electric
field penetrates the solution and the positive and negative ions will move in
opposite directions until a charge distribution results. It counteracts the imposed
field leading to field-free conditions inside the solution [
10
]. The ions bearing
charge of the same polarity as the capillary are drifted downfield in the solution,
toward the meniscus of the liquid from the tip, while ions of opposite charging are
drifted away from the surface. The reciprocal repulsion between the ions and
capillary surface overcomes the surface tension of the liquid, the surface begins
to expand and the so-called Taylor cone is formed. If the applied field is sufficiently
high, a fine jet emerges from the cone tip, which breaks up into small charged
droplets [
10
].
8.1.2 Droplet Shrinkage by Solvent Evaporation
and Coulomb Fission
The charged droplets shrink because of the solvent evaporation while the charge
remains constant. The decrease of the droplet radius
r
c
at a constant charge q leads
to an increase of the electrostatic repulsion of the charges at the droplet surface until
the droplets reach the Rayleigh stability limit:
q
ʵ
0
˃
q
R
¼
8
π
r
c
ð
8
:
1
Þ
Equation (
8.1
), the mathematical expression of Rayleigh limit, gives the condi-
tions at which the electrostatic repulsion becomes equal to the force of tension
surface
, which keeps the droplet together. The charged droplet becomes unstable
when the radius and charge satisfy Eq. (
8.1
). By this coulombic fission, the extra-
charges are expelled until the ratio charge/volume reaches again the stability limit
[
10
]. The outcome is the formation of very small charged droplets.
˃
8.1.3 Theories Upon Gas-Phase Ion Formation
Two theories have been proposed to account for the formation of gas-phase ions
from very small and highly charged droplets. The first one was proposed by
Dole
[
5
] and called
charged residue model
. It postulates the generation of extremely
small droplets, which contain only one ion. Solvent evaporation from such a droplet
leads to a gas-phase ion. The second theory belongs to
Iribarne
and
Thomson
and it
