Page 110 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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(iii) Charge-transfer ionization/excitation—caused by the transfer of charge between ions and atoms,
e.g.

The relative concentrations of excited- and ground-state species are given in Table 4.1.

Most elements are almost completely singly ionized in the argon ICP (a fact which also makes it an
ideal ion source for mass spectrometry), hence the majority of the most sensitive emission lines result
from atomic transition of ionised species, so-called ion lines, with fewer sensitive atom lines. Ion lines
are usually quoted as, e.g., Mn II 257.610 nm and atom lines as, e.g., Cu I 324.754 nm, with the roman
numerals II and I denoting ionic and atomic species, respectively.

4.4.5 Monochromators

The commonest configuration for an ICP-AES instrument is for the monochromator and detector to
view the plasma side-on as shown in Fig. 4.10a. This means that there is an optimum viewing height in
the plasma which yields the maximum signal intensity, lowest background and least interferences, and
it is common practice to optimize this parameter to obtain the best performance from the instrument.
Criteria of merit commonly used include the signal-to-noise ratio (SNR), signal-to-background ratio
(SBR) and net signal intensity. Different elements, and different analytical lines of the same element,
will have different optimum viewing heights so a set of compromise conditions is usually determined
which give satisfactory performance for a selected suite of analytical lines. Emission lines of ionic
species, i.e. ion lines, tend to emit most strongly in the NAZ, whereas emission lines of atomic species,
i.e. atom lines, emit most strongly lower down.

An alternative configuration which is gaining popularity is end-on viewing of the plasma shown in Fig.
4.10b. This has the advantage that an optimum viewing height does not have to be selected because the
analytical signal from the whole length of the central channel is integrated. However, any interferences
present are also summed, so that interference effects are expected to be more severe with this
configuration.

Atomic emission from the plasma is focused on to the entrance slit of the monochromator using a
combination of convex or plano-convex lenses or a concave mirror. The combination of focusing
optics, monochromator and detector is generally referred to as a spectrometer, although the heart of the
device is the monochromator. A monochromator is an instrument that