Chemical Ionization Overview - HP 5973 MSD Hardware Manual

 Chemical Ionization Theory
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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
that is 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 collisionally 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.
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Water contamination in reagent gases may decrease CI sensitivity dramatically. A
large peak at m/z 19 (H
contamination. In high enough concentrations, especially when combined with
calibrant, water contamination will result in a heavily contaminated ion source.
Water contamination is most common immediately after new reagent gas tubing or
reagent gas cylinders are connected. This contamination will often decrease if the
reagent gas is allowed to flow for a few hours, purging the system.
22
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0 ) in positive CI is a diagnostic symptom of water
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