Figure B-1 Ion Evaporation
Item
Description
1
Droplet contains ions of both polarities with one polarity being predominant.
2
As the solvent evaporates, the electrical field increases and the ions move
to the surface.
3
At some critical field value, ions are emitted from the droplets.
4
Nonvolatile residue remains as a dry particle.
If the droplet contains excess ions and enough solvent evaporates from the droplet, a critical
field is reached at which ions are emitted from the surface. Eventually, all of the solvent will
evaporate from the droplet, leaving a dry particle consisting of the nonvolatile components of the
sample solution.
Because the solvation energies for most organic molecules are unknown, the sensitivities of any
given organic ion to ion evaporation are difficult to predict. The importance of solvation energy
is evident because surfactants that concentrate at the surface of a liquid can be detected very
sensitively.
APCI Mode
The basis for past incompatibilities in linking liquid chromatography with mass spectrometry
arose from difficulties converting relatively nonvolatile molecules in solution in a liquid into a
molecular gas without inducing excessive decomposition. The APCI probe process of gently
nebulizing the sample to finely dispersed small droplets in a heated ceramic tube results in the
rapid vaporization of the sample so that the sample molecules are not decomposed.
The following figure shows the reaction flow of the APCI process for reactant positive ions, the
proton hydrates, H
3
O
+
[H
2
O]
n
.
Principles of Operation — Ion Source
System User Guide
RUO-IDV-05-6475-C
SCIEX 3200 Systems
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