Figure 8.12
An ICP torch.electrons. The induction coil operates at a power level of about 2 kW. A third current of argon (0.5 dm–^3
min–^1 ) entering between the inner and outer tubes has the effect of lifting the plasma clear of the quartz
tube, minimizing damage to the orifice by the high temperature. In this device a sample aerosol may be
injected into a plasma at a temperature of 10 000 K. In this way the sample argon stream drives a tunnel
through the centre of the fireball and a plasma with toroidal geometry results. Thus, the sample passes
through the high temperature region at the centre and is heated with virtually no contact with the plasma
itself. It follows that sample material cannot be trapped within the plasma itself and any consequent
memory effects are avoided. Having emerged from the fireball, the sample stream forms a tail or plume
containing the sample elements free of molecular associations. On cooling, the ionized atoms will relax
to their ground states emitting characteristic radiation with little background or continuum. The exact
point at which relaxation occurs will depend upon the temperature and the ionization energy of the
element in the sample. Thus the optimum alignment for the optical channel of the instrument will vary
from element to element.
It can be seen from the above that the sample stream emerging from the plasma will be rich in free ions
and atoms of the elements from the sample. Thus, the ICP could provide an attractive source for
analytical methods other than those based upon straightforward emission. Instruments using the ICP
source as a basis for atomic fluorescence have been developed.