Environmental Engineering Reference
In-Depth Information
Fig. 8.1 Schematic of the
ESI MS coupling
interface
gas
counterplate
front plate
extractor
syringe
ESI capillary
skimmer
-12V
200V
900V
3-6kV
eliminating re-solvation of formed macromolecular ions. This discovery was
closely followed by results from a Russian research group (Aleksandrov et al.) [
7
].
In ESI, basically, the liquid containing the analyte of interest is pumped through
a metal capillary, which has an open end with a sharply pointed tip (Fig.
8.1
). A
particular case of high sensitivity represents the nano-electrospray ion sources
developed by the groups of Mann [
8
] and Smith [
9
].
Since all droplets contain the same electrical charge, at the very end of the cone,
they emerge into a fine spray called ESI plume. Depending on the polarity of the
applied electric field, the charges may be positive or negative. The droplets are
usually less than 10
m across and contain both solvent and analyte molecules. The
charged droplets move across the electric field existing between capillary and
counter-electrode and, under a curtain gas flow, the solvent molecules evaporate
from the droplet. According to Dole
μ
s CRM, as the droplet size decreases while the
total charge on the droplet is constant, the charge surface density increases until the
droplet
'
s surface tension is exceeded by the repulsive electric forces. At this critical
point, the droplet explodes into smaller, still highly charged droplets. This process,
called
Rayleigh explosion
, repeats itself until the analyte molecule is stripped of all
solvent molecules, and is left as a multiply charged ion.
The tip is attached to a voltage supply and its end faces a counter-electrode plate.
As the voltage is increased, the liquid becomes charged and due to charge-repulsion
effect, it expands out of the capillary tip forming the so-called
Taylor
cone
(Fig.
8.2
).
Application of the field to the liquid results in the formation of an electrical
double layer. The accumulation of charge on the meniscus surface induces charged
droplets spraying. By solvent evaporation and explosion of droplets gas phase ions
are formed.
The number of charges retained by an analyte depends on such factors as the
composition and pH of the electrosprayed solvent as well as the chemical nature of
the sample [
10
]. For small molecules (
'
2,000 Da) ESI typically generates singly,
doubly or triply charged ions, while for large molecules (
<
2,000 Da) the ESI
process typically generates a series of multiply charged species and the resultant
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