For LC/MS systems another ionization approach has been developed. Since high
vacuum cannot be kept in systems with a continuous inflow of liquids around
0.5–1 mL/min (as common in liquid chromatography), the ionization in LC/MS
systems occurs under normal pressure. Two methods are used, the electron spray
ionization (ESI) and the atmospheric pressure chemical ionization (APCI).
As a first step, ESI sources produce an electrospray or aerosol by a nebulizer with
a high voltage that is placed at the end of the liquid chromatograph (see Fig. 4.23).
During the formation of the aerosol the solvent gets evaporated and the drops size
decrease continously. At the end of the process ionized analytes are formed. This
type of ionization is appropriate especially for macromolecular compounds and polar
or even charged molecules. It is a soft ionization, hence fragmentation is not
observed but the intact molecule ions are generated. Noteworthy, this kind of
ionization produces also particles with multiple charges allowing to detect high
molecular weight compounds. Higher charges reduce the m/z values to fit the
normally measured m/z range.
Table 4.2 Typical auxiliary
gases used in CI sources
Auxiliary gas
Proton affinity (kJ/mol)
Methane
527
Isobutane
807
Ammonia
840
Triethylamine
966
± 2 to 6 kV
N 2
N 2
+
+
+
+
+
+
+
+
+
+
prevacuum
high vacuum
to mass
analyser
atmospheric pressure
temperature: 20 – 350 °C
sample
nebulizing gas
Charged analyte
reaches mass
spectrometer
solvent evaporates,
ions move to the surface,
electric field strength
increases
Is a criƟcal field
strength achieved, the
drops burst and ions are
emiƩed
Drop with
analytes and
ions
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
- - -
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
+
+
+
+
Fig. 4.23 Scheme of an ESI source and principals of ion formation in electrosprays
4.2 Mass Spectrometry MS
63
vacuum cannot be kept in systems with a continuous inflow of liquids around
0.5–1 mL/min (as common in liquid chromatography), the ionization in LC/MS
systems occurs under normal pressure. Two methods are used, the electron spray
ionization (ESI) and the atmospheric pressure chemical ionization (APCI).
As a first step, ESI sources produce an electrospray or aerosol by a nebulizer with
a high voltage that is placed at the end of the liquid chromatograph (see Fig. 4.23).
During the formation of the aerosol the solvent gets evaporated and the drops size
decrease continously. At the end of the process ionized analytes are formed. This
type of ionization is appropriate especially for macromolecular compounds and polar
or even charged molecules. It is a soft ionization, hence fragmentation is not
observed but the intact molecule ions are generated. Noteworthy, this kind of
ionization produces also particles with multiple charges allowing to detect high
molecular weight compounds. Higher charges reduce the m/z values to fit the
normally measured m/z range.
Table 4.2 Typical auxiliary
gases used in CI sources
Auxiliary gas
Proton affinity (kJ/mol)
Methane
527
Isobutane
807
Ammonia
840
Triethylamine
966
± 2 to 6 kV
N 2
N 2
+
+
+
+
+
+
+
+
+
+
prevacuum
high vacuum
to mass
analyser
atmospheric pressure
temperature: 20 – 350 °C
sample
nebulizing gas
Charged analyte
reaches mass
spectrometer
solvent evaporates,
ions move to the surface,
electric field strength
increases
Is a criƟcal field
strength achieved, the
drops burst and ions are
emiƩed
Drop with
analytes and
ions
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
- - -
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
+
+
+
+
Fig. 4.23 Scheme of an ESI source and principals of ion formation in electrosprays
4.2 Mass Spectrometry MS
63