Plasma atomic confinement

An ICP-OES instrument consists of a sample introduction system, a plasma torch, a plasma power supply and impedance matcher, and an optical measurement system (Figure 1). The sample must be introduced into the plasma in a form that can be effectively vaporized and atomized (small droplets of solution, small particles of solid or vapor). The plasma torch confines the plasma to a diameter of about 18 mm. Atoms and ions produced in the plasma are excited and emit light. The intensity of light emitted at wavelengths characteristic of the particular elements of interest is measured and related to the concentration of each element via calibration curves. 

The difference between conventional and magnetron processes lies largely in the plasma environment. As will be shown later, the plasma is confined to the surface of the cathode by a magnetic field created by permanent magnets located under the target and by an electric field which is situated perpendicularly to the surface. The electrons travel in spiral trajectories and can thus carry out many ionizing collisions with the atoms of the sputter gas. Either the whole vacuum chamber is used as anode or the anode is built of metal or metal bars, which are positioned near to the chath-ode. 

ICP is an emission technique, which means that it does not use a light source. The light measured is the light emitted by the atoms and monoatomic ions in the atomizer. The ICP atomizer is an extremely hot plasma, which is a high-temperature ionized gas composed of electrons and positive ions confined by a magnetic held. The extremely high temperature means that the atoms and monoatomic ions undergo sufficient excitation (and de-excitation) such that relatively intense emission spectra result. The sample is drawn in with a vacuum mechanism that will be described. The intensity of an emission line is measured and related to concentration. 

Figure 15.8—Coupling of a gas chromatograph with an atomic emission spectrophotometer. Effluents from the capillary column are injected into the plasma and decomposed into their elements. Each chromatogram corresponds to the compound containing the element of interest. For a given retention time, indication as to the elements included in a compound can be obtained. The plasma in this example is generated by heating the carrier gas (He) with a microwave generator confined in a cavity at the exit of the column. A diode array detector system can be used for simultaneous detection of many elements (chromatograms courtesy of a Hewlett Packard document).

Spectroscopy of Confined Atomic Systems Effect of Plasma... 

---