using the analysis of electromagnetic radiation (light) to determine trace elements and their concentrations in a sample. In the atomic spectrometry techniques most commonly used for trace element analysis, the sample is decomposed by intense heat into a cloud of hot gases containing free atoms and ions of the element of interest. In general, there are four types of thermal sources normally used in analytical atomic spectrometry to perform this decomposition process: flames, furnaces, direct electrical discharges, and plasmas. Instruments which separate, isolate and measure light by wavelength in this way are called spectrographs or spectrometers.

Atomic absorption spectrometry shines a light of a wavelength characteristic of the element of interest through the hot vapor (usually created by a flame or furnace). Some of this light is then absorbed by the atoms of that element. The amount of light that is absorbed is measured and used to determine the concentration of that element in the sample.
Optical emission spectrometry uses electrical discharges or radio-frequency stimulated plasmas to bring the sample to temperatures high enough to dissassociate the sample into atoms and introduce significant amounts of excitation and ionization through atom-to-atom collisions. Once the atoms or ions are in this excited state, then they decay to lower states by radiating light. The intensity of the light is measured at specific wavelengths and used to determine the elemental concentrations. For example, two popular types of spectrometers use the emission principle:
Rotating disc emission (RDE) spectroscopy
where the electric arc struck between a rod and a rotating disc or between two rotating discs provides the energy source;
Inductively coupled plasma (ICP) spectroscopy
where a high-temperature discharge is generated by flowing a conductive gas (for example, argon) through the magnetic field generated by a radio-frequency load coil that surrounds the tubes carrying the gas. This highly energetic source stimulates the necessary light emission from the elements being analyzed.

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