In APCI sources the ionization is supported by a corona discharge needle placed
directly into the aerosol (see Fig. 4.24). First, mainly nitrogen and water molecules
are transformed to ions in the aerosol. Subsequently, the analytes get ionized by
these primary ions as a chemical ionization. Also this ionization is a soft one,
comparable to ESI sources, but its application covers more low molecular and less
polar analytes.
Instantly after ionization either by ESI or by APCI the ions are transferred into the
mass spectrometer towards vacuum conditions preventing secondary reactions and
recombination of ions.
A very special feature of ionization at normal pressure is realized in so-called
desorption electrospray systems DESI (see Fig. 4.25). Here, the surface of samples
gets directly ionized by an electrospray and subsequently analyzed by a mass
spectrometer. Depending on the devices surface analyses can be performed in
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
to mass
analyser
corona
discharge
electrode
skimmer
drying gas
heater
heater
dissolved analytes from LC
N2
e -
e -
e -
e -
e -
e -
e -
e -
e -
e - e -
e -
e -
N2
+
+
+
+
analyte
solvent
Fig. 4.24 Scheme of an APCI source
to mass
analyser
vacuum
charged
solvent
spray
analyte ion
desorption
surface
Fig. 4.25 Principal scheme of a DESI device
64
4 Instrumental Analysis