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Introduction
Ionization Techniques
Thermo Scientific
Surveyor MSQ Plus Hardware Manual
7
APCI is a gas phase ionization technique in which the gas phase acidities and basicities of the
analyte and solvent vapor play an important role.
In the positive-ion mode, sample ionization occurs in a series of reactions that start with the
electron-initiated cation formation. Typical examples of primary, secondary, and adduct ion
formation are shown below:
Primary ion formation
e
-
+ N
2
—> N
2
+
• + 2e
-
Secondary ion formation
N
2
+
• + H
2
O —> N
2
+
H
2
O
+
•
H
2
O
+
• + H
2
O —>
H
3
O
+
+ HO•
Proton transfer
H
3
O
+
+ M —>
(M + H)
+
+ H
2
O
In negative-ion mode, (M – H)
-
is typically formed by the abstraction of a proton by OH
-
.
Because the APCI process produces only singly charged ions, its use is limited to small
molecules with molecular weights up to about 2000 u. Because the APCI process takes place
in the gas phase, minor changes in most variables such as changes in buffer or buffer strength
have no effect.
You can use APCI in the positive, negative, or positive-negative switching ion polarity mode.
For most molecules, the positive-ion mode produces a stronger ion current. This is especially
true for molecules with one or more basic nitrogen (or other basic) atoms. Exceptions to the
general rule are molecules with acidic sites such as carboxylic acids and acid alcohols, which
produce more negative ions than positive ions. Although the negative ion polarity mode
generates fewer ions, it also generates less chemical noise than does the positive mode, making
it more selective.
Spectral Characteristics
Like electrospray, APCI is a soft ionization technique and forms singly charged ions, either
the protonated, [M + H]
+
or deprotonated, [M – H]
-
molecule depending on the selected ion
polarity mode. Unlike electrospray however, APCI does not produce multiply charged ions
and so is unsuitable for the analysis of high molecular weight compounds such as proteins or
peptides.
Because APCI uses a heated probe to aid the desolvation process, it is not suitable for
thermally labile (unstable) compounds, which can fragment in the ion source.
