Introduction
Ionization Techniques
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______________________ Finnigan LTQ Hardware Manual _______________________
Atmospheric Pressure Photoionization
Atmospheric pressure photoionization (APPI) is also a soft ionization
technique. In APPI an ion is generated from a molecule when it interacts with
a photon from the Syagen PhotoMate
TM
light source. APPI generates
molecular ions for molecules that have an ionization potential below the
photon energy of the light being emitted by the light source.
In APPI, ions are produced and analyzed as follows:
1. The nozzle sprays the sample solution into a fine mist of droplets.
2. The droplets are vaporized in a high temperature tube (the vaporizer).
3. The analyte molecule interacts with the light from the PhotoMate light
source. The analyte molecule M is ionized to a molecular ion M
+
if the
ionization potential of the analyte is less than the photon energy h
ν
:
M + h
ν →
M
+
4. In the presence of protic solvents, the analyte ion may extract a hydrogen
to form an MH
+
ion:
M
+
+ S
→
MH
+
+ S[-H]
5. The analyte ions pass through the ion transfer capillary, enter the MS
detector, and are analyzed.
Molecules including steroids, basic-drug entities, and pesticides have
ionization potentials below the threshold and protonated molecules are
generated in the LC/MS experiment. APPI reduces fragmentation because
only a small amount of energy is deposited in the molecule. Molecules such as
the nitrogen sheath, sweep, and auxiliary gas and the simple solvents used for
LC/MS are not ionized because their ionization potentials are greater than the
photon energy. The result is selective ionization of analyte vs. background.
See Figure 1-5 and Figure 1-6.
