HP 5973 MSD, Hardware Manual

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24
Chemical Ionization Theory
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Positive CI theory
Positive CI occurs with the same analyzer voltage polarities as EI. For PCI, the
reagent gas is ionized by collision with emitted electrons. The reagent gas ions react
chemically with sample molecules (as proton donors) to form sample ions. PCI ion
formation is more “gentle” than electron ionization, producing less fragmentation.
This reaction usually yields high abundance of the molecular ion, and is therefore
often used for determining molecular weights of samples.
The most common reagent gas is methane. Methane PCI produces ions with almost
any sample molecule. Other reagent gases, such as isobutane or ammonia, are more
selective, and cause even less fragmentation. Because of the high background from
the reagent gas ions, PCI is not especially sensitive, and detection limits are
generally high.
There are four fundamental ionization processes that take place during positive
chemical ionization at ion source pressures in the 0.8 – 2.0 Torr range. These are:
• Proton transfer
• Hydride abstraction
• Addition
• Charge exchange
Depending on the reagent gas used, one or more of these four processes can be used
to explain the ionization products observed in the resulting mass spectra.
EI, methane PCI, and ammonia PCI spectra of methyl stearate are shown opposite.
The simple fragmentation pattern, large abundance of the [MH]
+
ion, and the
presence of the two adduct ions are characteristic of positive chemical ionization
using methane as a reagent gas.
The presence of air or water in the system, especially in the presence of PFDTD
calibrant, quickly contaminates the ion source.