much reduced ionization relative to the rest of the plasma. Consequently, they are unable to carry the
arc current, so that the current density in the rest of the plasma is increased. As a result, the temperature
increases from the 6000 K typical of an arc, to 9000 or 10 000 K. A common version of the DCP
(Figure 8.11) uses an inverted V-shaped electrode assembly with the excitation region established at the
intersection of two argon streams. A nebulizer is used to inject the sample at this intersection point.
Superior stability and sensitivity are claimed for this arrangement together with a much improved signal
to noise ratio. A particular shortcoming of the dc source is the dependence of the signal on the surface
of the carbon electrodes. As the plasma operates, the electrodes are eroded so that after 2–3 hours
replacement or reshaping is needed. On the other hand, a distinct advantage is the stability of the DCP
with varying sample matrices, a characteristic which is particularly valuable where the solvent contains
a high concentration of dissolved solids.
Figure 8.11
DC plasma jet.The Inductively Coupled Plasma Torch
A second plasma emission source which has become the most popular, is the inductively coupled
plasma (ICP) torch. Figure 8.12 illustrates a typical source construction, which consists of three
concentric quartz tubes. An aerosol of the sample solution is injected into the plasma through the central
tube in a stream of argon flowing at about 1 dm–^3 min–^1. A higher flow of argon (15 dm–^3 min–^1 ) is
injected between the second and outer tubes. A 'Tesla' coil is used to provide a high-voltage discharge
to ignite the plasma by 'seeding' it with free electrons. The plasma is then sustained and raised to its
operating temperature by induction heating derived from the interaction of the electromagnetic field
from the RF coil, with the free