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Sci Tech

What is plasma emission spectroscopy?

M.Jeyakumari, Rajapalayam, T.N.

Spectroscopy is the branch of science (Physics) concerned with production and analysis of spectra (analogous range of radiations in order of wavelength), produced by a spectrophotometer.

In emission spectroscopy, a sample is normally excited by thermal energy. Atoms in the sample absorb thermal energy causing outer orbital electrons to become excited from their ground state to higher orbital energy level. After a short lifetime (10{+-}{+6} to 10{+-}{+9}s), the excited electrons return to the ground state. Simultaneously, electromagnetic radiations (normally in the form of light in the UV-visible region) are emitted. The emitted radiations are analysed by means of spectrograph, which separates various wavelengths.

In Plasma Emission Spectroscopy, the exciting medium/ agent is plasma. The term `Plasma' in Physics means a very hot/ high temperature ionised gas, sometimes described as the fourth state of matter (the other three being solid, liquid and gas). In this method of sample analysis, a `Plasma Source' is the basic component. (Plasma Emission Spectroscopy is similar to Optical Emission Spectroscopy in principles but differs in mechanism. In OES, the sample excitation is done by electric arc or spark mechanism in the temperature range of 3000-5000 degrees Celcius. In PES, the temperature range is 7000-9000 {+o}C.

The sample solution or the analyte solution is supplied to the `Plasma Source' by a `Nebulizer' assembly. On reaching the high temperature (7000-9000 degrres Celsius) plasma, the solution attains this high temperature and all types of molecular bonds break down. Free atoms or ions (electrically charged atoms) are produced, which emit their characteristic spectra. A `Monochromator' assembly is in position after `Plasma Source' for isolation of the desired analyte line (or spectrum). The intensity of an analyte line is directly proportional to the concentration of that particular analyte (element being analysed) in the sample solution. The `detector & read-out' assembly gives us the concentration of the particular element in the sample. Modern instruments being computer controlled, the analysis is very fast and needs little operator skill. (Depending on the technique of production, plasma may be of three classes — viz., D.C. argon plasma, Microwave plasma and Inductively coupled plasma (ICP). The ICP is the most widely used due to mechanical simplicity and operational advantages.

Therefore, plasma Emission Spectroscopy is the method of analysis of various elements in a sample, by passing the sample/ analyte solution through a plasma (high temperature ionised gas) source, thereby exciting the outer orbital electrons of the analyte with simultaneous emission of electromagnetic radiations (in the form of lights), which are analysed by means of a spectrograph, which separates various wavelengths of analytes in the sample.

R.C. Dey, Chennai

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