Introduction
Ionization Techniques
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1-8
_______________________ Finnigan LTQ Hardware Manual _______________________
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
−
.
APCI is typically used to analyze small molecules with molecular weights up
to about 2000 u. APCI is a very robust ionization technique. It is not affected
by minor changes in most variables such as changes in buffer or buffer
strength.
Figure 1-4 shows the APCI process in the positive ion polarity mode.
