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chromatograph. Other types, however, require complicated transport interfaces for them to function
satisfactorily.

An early solution to the problem of sample involatility was the use of a solid probe, which was coated
with the sample and inserted into the electron beam of the ionizing source. Subsequently, the probe was
heated, and this increased the range of substances that could be ionized. However, the majority of high
molecular weight or strongly polar materials started to decompose, long before they were sufficiently
volatile to provide a vapor pressure that was adequate to produce ions, particularly parent ions. Many
high molecular weight samples, however, do have a significant vapor pressure before the decomposition
temperature is reached, but it is very low. It follows that if the ionization process could act directly on
the solid surface, sufficient ions should be produced to permit mass spectra to be obtained. The process
is a form of desorption ionization.

In general, desorption ionization is carried out by coating the sample on the surface of a suitable metal,
and then bombarding the sample with particles of high energy. On collision with the sample, some of
the energy is communicated to surface molecules and ejects them from the surface, and some of the
energy is used for their ionization. There are a number of different ways of communicating the energy
to the surface molecules, some of which will now be briefly described.

Secondary Ion Mass Spectrometry (SIMS)

In secondary ion mass spectrometry the excitation beam that produces the ionization consists of a
stream of ions having a kinetic energy in the kiloelectron volt range. These ions are generated in a
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specially designed ion-gun. Those most commonly used are the Ar , 02 and Cs ions. If the sample, or
sample support, is conducting (i.e. a metal) then the charge received by the target can leak away. If,
however, the target is not conducting, an electric charge can build up on the sample surface and
interfere with the focusing of the ions that are produced. This charge build up can be eliminated by
flooding the sample with low-energy electrons from a separate electron source. Abundant molecular
ions are