Chemical Ionization Overview - Agilent Technologies 5975 Operation Manual

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Chemical Ionization Theory

Chemical Ionization Overview

Chemical ionization (CI) is a technique for creating ions used in mass
spectrometric analyses. There are significant differences between CI and
electron ionization (EI). This section describes the most common chemical
ionization mechanisms.
In EI, relatively high-energy electrons (70 eV) collide with molecules of the
sample to be analyzed. These collisions produce (primarily) positive ions.
Upon ionization, the molecules of a given substance fragment in fairly
predictable patterns. EI is a direct process; energy is transferred by collision
from electrons to the sample molecules.
For CI, in addition to the sample and carrier gas, large amounts of reagent gas
are introduced into the ionization chamber. Since there is so much more
reagent gas than sample, most of the emitted electrons collide with reagent gas
molecules, forming reagent ions. These reagent-gas ions react with each other
in primary and secondary reaction processes that establish an equilibrium.
They also react in various ways with sample molecules to form sample ions. CI
ion formation involves much lower energy and is much more "gentle" than
electron ionization. Since CI results in much less fragmentation, CI spectra
usually show high abundance of the molecular ion. For this reason, CI is often
used to determine the molecular weights of sample compounds.
Methane is the most common CI reagent gas. It yields certain characteristic
ionization patterns. Other reagent gases yield different patterns and may
result in better sensitivity for some samples. Common alternative reagent
gases are isobutane and ammonia. Carbon dioxide is often used in negative CI.
Less common reagent gases are carbon dioxide, hydrogen, Freon,
trimethylsilane, nitric oxide, and methylamine. Different ionization reactions
occur with each reagent gas.
Ammonia is toxic and corrosive. Use of ammonia requires special maintenance and
WARNING
safety precautions.
Water contamination in reagent gases will decrease CI sensitivity
dramatically. A large peak at m/z 19 (H 3 0 + ) in positive CI is a diagnostic
symptom of water contamination. In high enough concentrations, especially
when combined with calibrant, water contamination will result in a heavily
176 5975 Series MSD Operation Manual for MassHunter