Page 102 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

Page 85

In the absence of analyte atoms, water and sample matrix components, the predominant species will be
Ar, Ar and e , although the others are important when considering analyte ionization mechanisms. The
+
-
RF energy used to sustain the plasma is only coupled into the outer region of the plasma, so these
species are primarily formed in this region and are thermally transferred to the centre. This is known as
the skin effect. The depth to which the RF energy will couple into the plasma gas is called the skin
depth, and is determined by the frequency of the RF energy and the nature of the plasma gas. One
desirable consequence of the skin effect is that the ICP is much more energetic in the outer region,
which makes it optically thin. This means that emission from the centre will not be re-absorbed by
unexcited atoms in the outer regions, such as occurs with flames. This lack of self-absorption means
that ICP-AES has a large linear dynamic range.

The various species present in the plasma will circulate between the skin and the central region.
Additionally, when the sample is introduced through the axial channel, the centre will contain a flow of
cooler gas containing the analyte and species derived from the sample matrix and water. The region
between the central channel and the plasma is known as the boundary region. The ICP can be divided
axially into a number of other regions shown in Fig. 4.6. The region within the copper load coil is
termed the initial radiation zone (IRZ) and can be identified by aspirating a solution of 1000 µg ml
-1
yttrium into the plasma, the intense red atomic Y emission indicating the IRZ. The third region is
located between 10 and 20 mm above the load coil and is called the normal analytical zone (NAZ).
This is the zone which is normally observed for routine analytical determinations.

It is obvious from the foregoing the ICP exhibits a large degree of spatial inhomogeneity. In addition,
the ICP is not in thermal equilibrium (TE) because the various collisional processes which occur in
the plasma such as ionization, recombination, excitation and de-excitation are not in equilibrium.
However, the ICP is thought to approach thermal equilibrium, a condition termed local thermal
equilibrium (LTE). One consequence of this is that the ICP cannot be characterized by a single
equilibrium temperature, with the ionization temperature (T ), gas temperature (T ), excitation
ion
gas
temperature (T ) and rotational temperature (T ) all having different values. It may be confusing
exc
rot
when one is used to regarding temperature as having a single value, until one remembers that it is
impossible to measure the temperature of the ICP using everyday means (such as a thermometer)
because it is simply too hot. Hence we must resort to using spectrometric methods of temperature
measurement, and the temperatures mentioned above simply refer to the particular method that was
used to measure it, and because they do not agree it is one indication that the ICP is not in thermal
equilibrium.