Atomic emission spectroscopy (AES), optical emission spectroscopy (OES), and flame photometry are three different names for the same spectroscopic technique. It is a type of atomic spectroscopy based on the electronic transition between different energy levels. The significance of AES lies in its simple operation, least expensive instrumentation, and its ability to support both qualitative as well as quantitative spectroscopic analysis.
Continue reading for more insightful information on atomic emission spectroscopy.
What is atomic emission spectroscopy (AES)
Atomic emission spectroscopy is primarily based on the interaction of atomic electrons with thermal energy provided by a flame source. Electrons absorb this thermal energy and undergo excitation from a lower to a higher energy level. The electron is relatively unstable at its higher energy level. So, it immediately descends back to the lower energy level, emitting radiant energy equal to the energy difference (∆ E) between the two levels. This radiant energy corresponds to a specific electromagnetic frequency or wavelength. The radiant wavelength is detected as different flame colors and recorded to determine the identity and the concentration of targeted metal atoms.
What elements atomic emission spectroscopy measures
Atomic emission spectroscopy is specifically important for the analysis of low ionization potential alkali or alkaline earth metals such as lithium (Li), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), etc. Different metals heated under a flame gives off a different color to the flame. For instance, the flame appears yellow due to Na metal, pink when it contains Li while a red-colored flame can be seen in the presence of Sr.
Historical perspective of atomic emission spectroscopy
The foundation of AES was laid back in the 1550s when different flame colors were used in the smelting of ores. It was then in 1666 that Issac Newton used a flame source to separate white light into its characteristic colors and called it a spectrum. Gustav Kirchhoff, later on, recognized that each metallic element has its own characteristic spectrum. However, it was actually in the 1930s that AES was accepted as a spectroscopic analytical technique and the atomic emission spectrometer became commercially available.
Let us have a look at how the different components of the spectrometer function to carry out atomic emission spectroscopy.
What is the working principle of atomic emission spectroscopy
- The sample is introduced in a solution form.
- It is then aspirated and converted into a fine mist by the nebulizer.
2. Mixing chamber
- The sample droplets are consequently carried into the mixing chamber with the help of an inert carrier gas.
- Here it is mixed with the methane fuel.
- The flame source lies at the heart of atomic emission spectroscopy. The sample-fuel mixture is sprayed onto the flame.
- High flame temperature (1100-2300 °C) leads to solvent evaporation from the sample-fuel mixture that leaves behind a solid residue.
- The solid particles are further vaporized into gaseous atoms.
- Electrons present in the gaseous atoms absorb thermal energy from the flame and undergo excitation from the ground state to a higher energy level.
- A subsequent de-excitation of these atomic electrons emits radiant energy of a characteristic wavelength. For example, 589 nm wavelength radiations emitted by Na metal atoms which give a bright yellow color to the flame.
- The photon energy of the emitted radiation is equal to the energy difference (∆ E) of the two levels between which electronic transition occurred.
- In addition to using the flame as the energy source, modern atomic emission spectrometers also use plasma, or an electrical arc as prospective energy sources.
4. Lens and monochromator
- The different metal atoms present in the sample may get excited by absorbing thermal energy, but the emission spectrum is collected for only the targeted metal atom.
- The radiations emitted by different metal atoms pass through the lens. However, the monochromator ensures the passage of only a characteristic wavelength (for e.g., 589 nm) which ultimately reaches the detector.
5. Detector and recorder
- The detector (photomultiplier tubes) receives radiant energy and converts it into an electrical signal. The recorder then records the final outcome as an emission spectrum.
- The detector response can additionally be used to plot a calibration curve. This straight-line plot helps in the quantitative analysis of the targeted metal atoms i.e., for determining the number of atoms of a specific element present in the sample.
- Photographic plates can also be used for quantitative analysis in AES. Greater the intensity of emitted radiation, the higher the analyte concentration.
What is atomic emission spectroscopy used for
• Atomic emission spectroscopy is very important for trace metal analysis in biological fluids and in environmental samples (soil, rivers, drinking water).
• Metallic tracings can be detected in injured skin.
• Metallic deficiencies such as Ca deficiency in living organisms can also be detected through AES of urine or blood samples.
• Impurities present in alloys can be readily detected and quantified via atomic emission spectroscopy.
Advantages of atomic emission spectroscopy
The advantages of atomic emission spectroscopy include:
• High sensitivity
• Accurate quantification
• Minutely low concentrations can be quickly detected in ppb (parts per billion)
• Cost-effective method
• The instrument is easy to operate
Limitations of atomic emission spectroscopy
- Restricted to the analysis of metal atoms
- Destructive method
- Extensive sample preparation. The solvent chosen for solution preparation should completely solubilize all sample components without affecting the integrity of the targeted metal atoms
- Time-consuming process overall
- A common drawback of the flame excitation source is that even small temperature changes can significantly affect the number of excited atoms. This undesirably interferes in spectroscopic analysis.
Differences between AAS and AES
Atomic absorption spectroscopy (AAS) is known as the twin-sister of atomic emission spectroscopy. There are many similarities between the basic principle and the functioning of the two techniques. However, a few differences also exist, as highlighted in the table below.
|Electronic excitation in AAS is based on radiant energy absorption of a characteristic wavelength||Electronic excitation in AES occurs via thermal energy absorption|
|The radiant energy is supplied by a hollow cathode lamp||The thermal energy is provided by a flame source|
|The detector records the intensity of a specific wavelength absorbed by the targeted metal atoms||The detector records the intensity of a specific wavelength emitted by the targeted metal atoms|
|The dark bands in the absorption spectra are representative of absorbed radiation||The colored bands in the emission spectra represent the emitted radiation|
|AAS specifically measures high ionization potential metals (Cu, Pb, Hg)||AES specifically measures low ionization potential metals (Na, Ca, Mg)|
|Relatively more expensive because a specific lamp is needed for each different metalThe thermal energy is provided by a flame source||Relatively less expensive than AAS because a common flame source is required for all the metal atoms|
|A decrease in flame intensity followed by radiant energy absorption is the key to AAS||Vibrantly colored flames are a key feature of AES|
Whatsoever, both AAS and AES are highly specific spectroscopic techniques with minimum chances of spectral interference. Both are examples of atomic spectroscopy as opposed to UV-Vis or IR spectroscopy which are the principal examples of molecular spectroscopy.
Check out this video tutorial to revise all the concepts you learned about atomic emission spectroscopy in this article.
You may also like to read some other useful articles from our spectroscopic series :
1. Hollas, J. M. (2002). Basic Atomic and Molecular Spectroscopy RSC.
2. Perring, L. and M. Basic-Dvorzak (2002). “Determination of total tin in canned food using inductively coupled plasma atomic emission spectroscopy.” Analytical and Bioanalytical Chemistry 374(2): 235-243.