Atomic emission spectroscopy

Atomic emission spectroscopy (AES) is a method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line in the emission spectrum gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element. The sample may be excited by various methods.

Inductively coupled plasma atomic emission spectrometer

Flame
A flame during the assessment of calcium ions in a flame photometer

The sample of a material (analyte) is brought into the flame as a gas, sprayed solution, or directly inserted into the flame by use of a small loop of wire, usually platinum. The heat from the flame evaporates the solvent and breaks intramolecular bonds to create free atoms. The thermal energy also excites the atoms into excited electronic states that subsequently emit light when they return to the ground electronic state. Each element emits light at a characteristic wavelength, which is dispersed by a grating or prism and detected in the spectrometer.

Sodium atomic ions emitting light in a flame displays a brilliantly bright yellow emission at 588.9950 and 589.5924 nanometers wavelength.

A frequent application of the emission measurement with the flame is the regulation of alkali metals for pharmaceutical analytics.[1]

Inductively coupled plasma
Main article: Inductively coupled plasma atomic emission spectroscopy
Inductively coupled plasma atomic emission source

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) uses an inductively coupled plasma to produce excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element.[2][3]

Advantages of ICP-AES are the excellent limit of detection and linear dynamic range, multi-element capability, low chemical interference and a stable and reproducible signal. Disadvantages are spectral interferences (many emission lines), cost and operating expense and the fact that samples typically must be in a liquid solution. Inductively coupled plasma (ICP) source of the emission consists of an induction coil and plasma. An induction coil is a coil of wire that has an alternating current flowing through it. This current induces a magnetic field inside the coil, coupling a great deal of energy to plasma contained in a quartz tube inside the coil. Plasma is a collection of charged particles (cations and electrons) capable, by virtue of their charge, of interacting with a magnetic field. The plasmas used in atomic emissions are formed by ionizing a flowing stream of argon gas. Plasma's high-temperature results from resistive heating as the charged particles move through the gas. Because plasmas operate at much higher temperatures than flames, they provide better atomization and a higher population of excited states. The predominant form of sample matrix in ICP-AES today is a liquid sample: acidified water or solids digested into aqueous forms. Liquid samples are pumped into the nebulizer and sample chamber via a peristaltic pump. Then the samples pass through a nebulizer that creates a fine mist of liquid particles. Larger water droplets condense on the sides of the spray chamber and are removed via the drain, while finer water droplets move with the argon flow and enter the plasma. With plasma emission, it is possible to analyze solid samples directly. These procedures include incorporating electrothermal vaporization, laser and spark ablation, and glow-discharge vaporization.

Spark and arc

Spark or arc atomic emission spectroscopy is used for the analysis of metallic elements in solid samples. For non-conductive materials, the sample is ground with graphite powder to make it conductive. In traditional arc spectroscopy methods, a sample of the solid was commonly ground up and destroyed during analysis. An electric arc or spark is passed through the sample, heating it to a high temperature to excite the atoms within it. The excited analyte atoms emit light at characteristic wavelengths that can be dispersed with a monochromator and detected. In the past, the spark or arc conditions were typically not well controlled, the analysis for the elements in the sample were qualitative. However, modern spark sources with controlled discharges can be considered quantitative. Both qualitative and quantitative spark analysis are widely used for production quality control in foundry and metal casting facilities.

See also

References
  1. Stáhlavská A (April 1973). "[The use of spectrum analytical methods in drug analysis. 1. Determination of alkaline metals using emission flame photometry]". Pharmazie (in German). 28 (4): 238–9. PMID 4716605.
  2. Stefánsson A, Gunnarsson I, Giroud N (2007). "New methods for the direct determination of dissolved inorganic, organic and total carbon in natural waters by Reagent-Free Ion Chromatography and inductively coupled plasma atomic emission spectrometry". Anal. Chim. Acta. 582 (1): 69–74. doi:10.1016/j.aca.2006.09.001. PMID 17386476.
  3. Mermet, J. M. (2005). "Is it still possible, necessary and beneficial to perform research in ICP-atomic emission spectrometry?". J. Anal. At. Spectrom. 20: 11–16. doi:10.1039/b416511j.|url=http://www.rsc.org/publishing/journals/JA/article.asp?doi=b416511j%7Cformat=%7Caccessdate=2007-08-31

Bibliography
  • Reynolds, R. J.; Thompson, K. C. (1978). Atomic absorption, fluorescence, and flame emission spectroscopy: a practical approach. New York: Wiley. ISBN 0-470-26478-0.
  • Uden, Peter C. (1992). Element-specific chromatographic detection by atomic emission spectroscopy. Columbus, OH: American Chemical Society. ISBN 0-8412-2174-X.
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