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free electron. In practice, however, a quantum mechanical treatment reveals that the ionization cross-
section is very small when the kinetic energy of the electron is equal to the ionization energy of the
atom, but increases up to a maximum at about 100 eV, the rate of ionization being directly proportional
to the electron current.

Charge transfer occurs when interactions between atoms and ions allow insufficient time for the
electrons to adjust themselves to the new conditions, and the charge is transferred from the ion to the
neutral atom. This is a complex process which requires that the ionization energies of the two species
are similar.

Some atomic species can undergo excitation to excited states which are relatively long lived, so-called
metastable states, and can therefore undergo several collisions before de-excitation. If the metastable
atom collides with a neutral molecule it can induce ionization of the neutral molecule with consequent
de-excitation of the metastable atom, a process called Penning ionization. The likelihood of this
occurring is greatest when the potential energy of the metastable species is similar to the ionization
energy of the neutral atom.

The various types of ionization processes that occur in the ICP are as follows:

5.3 Ion Sampling.

An important difference between ICP-AES and ICP-MS is the sampling interface. In ICP-AES,
radiation emitted by excited-state atoms and ions is sampled, so the sampling interface—in this case a
lens or mirror—does not need to be in direct physical contact with the plasma. However, in ICP-MS,
ions are physically extracted from the plasma into a mass spectrometer which is required to be at
extremely low pressure, so the sampling interface must be in direct contact with the plasma. The
problem of extracting ions from an extremely hot plasma at atmospheric pressure into a mass
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spectrometer at ca 10 atm is overcome by making use of a series of differentially pumped vacuum
chambers held at consecutively lower pressures. A schematic diagram of the ICP-MS sampling
interface is shown in Fig. 5.3. The ICP is aligned so that the central channel is