Introduction
'JOOJHBO-52
____________________________________________________________ Ionization Techniques
_______________________ Finnigan LTQ Hardware Manual ______________________
1-9
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You can use APCI in positive or negative 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. An exception 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, in general, fewer negative ions are produced than positive ions,
negative ion polarity is sometimes the mode of choice. This is because the
negative ion polarity mode sometimes generates less chemical noise than does
the positive mode. Thus, selectivity might be better in the negative ion mode
than in the positive ion mode.
Figure 1-4. APCI process in the positive ion polarity mode
ION
TRANSFER
CAPILLARY
PRESSURE
= 1 TORR
+10 V
CORONA
DISCHARGE
NEEDLE
(+3 TO +5 kV)
VAPORIZER
TUBE
(500°C)
ATMOSPHERIC
PRESSURE
REGION
ION
SWEEP
CONE
SHEATH
GAS TUBE
(OUTER)
AUXILIARY
GAS
SAMPLE
TUBE
(INNER)
S S
S
S
S
S S
S
S
M
M
(M+H)
H O
+
+
3
e
-
e
-
+++
++
+
N +
2
N +
2
N e
+ -
2
N e
+ -
2
H O
2
M
M
M
M
M
CORONA
DISCHARGE
NEEDLE
(+3 TO +5 kV)
CORONA
DISCHARGE
REGION
(PLASMA)
H20+
H20+
H20+
CHEMICAL
IONIZATION
H20+
H20
3
