Plasma Emission Spectroscopy

Plasmas consist of ions, atoms, and charged particles and comprise the fourth state of matter in addition to solids, liquids, and gases. Plasmas are generated from ordinary matter by heating it to high temperatures. Nuclear fusion supplies the energy needed to generate the plasmas seen in the sun and other stars. Electrical discharges through the earth’s atmosphere give the plasma we know as lightning. An electron beam excites the plasma seen inside of neon signs. High-power lasers blasting away at materials also create plasmas. As a result of the high energy of plasmas, the atoms and ions present are in excited electronic states. When these species relax, they emit light as such:

 

plasma emission spectra
Part of the Balmer series plasma emission lines of the element hydrogen.

 

The wavelengths emitted are unique to each atom or ion present. In plasma emission spectroscopy, this light is examined using a spectrograph and a detector. The wavelengths seen in the spectrum of a plasma indicate what atoms are present. The peaks are very narrow and the use of high resolution can be used to distinguish lines that are close to each other. The intensity of the light emitted by the plasma is proportional to the concentration of the emitting species, hence quantitative analysis of plasmas is possible.

The different types of plasma emission spectroscopy can be categorized by how the plasma is generated. Spectra of nuclear fusion plasmas are used to ascertain the chemical species present and other properties. Arc-spark emissions are mini-lightning strikes that are used to vaporize samples to determine their elemental composition. This technique is frequently used for metals analysis. Laser-induced breakdown spectroscopy (LIBS) uses a blast of laser light to vaporize and excite a sample, giving a plasma, and is used on solids, liquids, and gases.

The high temperatures generated at the beginning of many plasma processes give a continuum emission that masks the sharp lines from individual atomic species. The plasma quickly cools, allowing the sharp lines to be visible. Use of an intensified CCD camera is frequently necessary to gate out the continuum emission. Plasmas normally give off lots of photons, so high-sensitivity detectors are not usually required.

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