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Introduction
Ionization Techniques
4
Surveyor MSQ Plus Hardware Manual
Thermo Scientific
Mechanism of Ion Desolvation
To produce gas phase ions in ESI, the following sequence of events occurs:
1. The ESI capillary, to which a high voltage is applied, sprays sample solution into a fine
mist of droplets that are electrically charged at their surface.
2. The electrical charge density at the surface of the droplets increases as solvent evaporates
from the droplets until it reaches a critical point, known as the Rayleigh stability limit. At
this critical point, the droplets divide into smaller droplets because the electrostatic
repulsion is greater than the surface tension. The process repeats itself, forming smaller
and smaller droplets.
3. From the very small, highly charged droplets, the force of electrostatic repulsion ejects
sample ions into the gas phase.
4. The charged ESI capillary attracts gas phase ions of opposite charge and repels gas phase
ions of the same charge.
The low vacuum of 1 Torr produced by the forepump draws both ionized molecules repelled
by the charge on the capillary and neutral molecules in the gaseous phase into the MS detector
through the entrance cone.
shows the steps in the formation of gas phase ions from
highly charged droplets.
Figure 2.
Positive ion electrospray mechanism
Spectral Characteristics
In the ESI mode, ionization takes place in the liquid phase. Polar compounds of low
molecular weight (<1000 u) typically form singly charged ions by the loss or gain of a proton.
Basic compounds (for example amines) can form a protonated molecule [M + H]
+
, which can
be analyzed in the positive ion polarity mode to give a peak at an
m/z
value of M + 1, where M
equals the mass of the original molecule. Acidic compounds (for example sulphonic acids) can
form a deprotonated molecule [M – H]
-
, which can be analyzed in the negative ion polarity
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Negative ions
attracted back
to the capillary
Positive ions
ejected from
the surface
Ions moving towards
the surface
Large droplet
Fine spray
Positively charged
cap3 to +5 kV
Step 1
Step 2
Step 3
Step 4
