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then the energy needed for dissociation of the organic solvent will be too high for the plasma to be
sustained.

4.5 Other Flame-like Plasma Sources

4.5.1 Microwave Plasmas

The capacitatively coupled microwave plasma is formed by coupling a 2450 MHz magnetron, via a
coaxial waveguide, to metal plates or a torch where the plasma is formed. Considerable problems have
been encountered with this low-cost plasma, particularly from easily ionizable elements which cause
dramatic changes in the excitation temperature in the plasma.

Alternatively, a microwave-induced plasma (MIP) may be formed in a resonant cavity, using a
similar generator. Powers up to 1.5 kW may be achieved, although a maximum of 200 W is more
common. If a small flow of argon (e.g. 300 ml min ) is passed through a small bore (2 mm internal
-1
diameter) quartz tube placed in the cavity, and seeded with electrons, a self-sustaining plasma will
form. Several types of cavity have been used, including the 3/4-wave Broida cavity and the 1/4-wave
Evenson cavity, but the most popular configuration has been the TM 010 Beenakker cavity (Fig. 4.19)
because it is capable of sustaining a helium plasma at atmospheric pressure. More recently, the
surfatron has gained popularity,

Figure 4.19
Schematic diagram of a Beenakker TM 010 microwave cavity.