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Concept
Inductively coupled plasma
Summary
An inductively coupled plasma (ICP) or transformer coupled plasma (TCP) is a type of plasma source in which the energy is supplied by electric currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields.
There are three types of ICP geometries: planar (Fig. 3 (a)), cylindrical (Fig. 3 (b)), and half-toroidal (Fig. 3 (c)).
In planar geometry, the electrode is a length of flat metal wound like a spiral (or coil). In cylindrical geometry, it is like a helical spring. In half-toroidal geometry, it is a toroidal solenoid cut along its main diameter to two equal halves.
When a time-varying electric current is passed through the coil, it creates a time-varying magnetic field around it, with flux
where r is the distance to the center of coil (and of the quartz tube).
According to the Faraday–Lenz's law of induction, this creates azimuthal electromotive force in the rarefied gas:
which corresponds to electric field strengths of
leading to the formation of the electron trajectories providing a plasma generation. The dependence on r suggests that the gas ion motion is most intense in the outer region of the flame, where the temperature is the greatest. In the real torch, the flame is cooled from the outside by the cooling gas, so the hottest outer part is at thermal equilibrium. There temperature reaches 5 000–6 000 K. For more rigorous description, see Hamilton–Jacobi equation in electromagnetic fields.
The frequency of alternating current used in the RLC circuit which contains the coil is usually 27–41 MHz. To induce plasma, a spark is produced at the electrodes at the gas outlet. Argon is one example of a commonly used rarefied gas. The high temperature of the plasma allows the determination of many elements, and in addition, for about 60 elements degree of ionization in the torch exceeds 90%. The ICP torch consumes c. 1250–1550 W of power, but this depends on the elemental composition of the sample (due to different ionization energies).
Official source
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