Samples that are introduced through the APCI probe are sprayed, with the aid of a nebulizer
gas, in the heated ceramic tube. Within the tube, the finely dispersed droplets of sample
and solvent undergo a rapid vaporization with minimal thermal decomposition. The gentle
vaporization preserves the molecular identity of the sample.
The gaseous sample and solvent molecules pass to the ion source housing where the ionization
by APCI is induced by a corona discharge needle connected to the end of the ceramic tube. The
sample molecules are ionized by colliding with the reagent ions that are created by the ionization
of mobile phase solvent molecules. The vaporized solvent molecules ionize to produce the
reagent ions [X+H]
+
in Positive polarity and [X-H]
–
in Negative polarity. Refer to the figure:
. It is these reagent ions that produce stable sample ions when they collide with the sample
molecules.
Figure B-3 Atmospheric Pressure Chemical Ionization
Item
Description
1
Sample
2
Primary ions are created in the vicinity of the corona discharge needle
3
Ionization produces predominantly solvent ions
4
Reagent ions react with sample molecules, forming clusters
5
Curtain plate
6
Interface
x = solvent molecules; M=sample molecules
The sample molecules are ionized through a process of proton transfer in Positive polarity
and by either electron transfer or proton transfer in Negative polarity. The energy for the APCI
ionization process is collision-dominated because of the relatively high atmospheric pressure of
the ion source.
For reverse phase applications, the reagent ions consist of protonated solvent molecules in
Positive polarity and solvated oxygen ions in Negative polarity. With favorable thermodynamics,
the addition of modifiers changes the reagent ion composition. For example, the addition of
acetate buffers or modifiers can make the acetate ion [CH
3
COO]
–
the primary reagent in
Negative polarity. Ammonium modifiers might make protonated ammonia [NH
4
]
+
the primary
reagent in Positive polarity.
Through collisions, an equilibrium distribution of certain ions, such as protonated water cluster
ions, is maintained. The likelihood of premature fragmentation of the sample ions in the ion
source is reduced because of the moderating influence of solvent clusters on the reagent ions
and the relatively high gas pressure in the ion source. As a result, the ionization process yields
primarily molecular product ions for mass analysis in the mass spectrometer.
Principles of Operation — Ion Source
System User Guide
RUO-IDV-05-6475-C
SCIEX 3200 Systems
87/114
