Atomic Emission Spectroscopy (AES): An Overview

By Vasco Medeiros, BScReviewed by Emily Henderson, B.Sc.

Atomic emission is a phenomenon that comes about through the excitation of an atom to a higher energy state. Once this atom has reached this higher energy state, it will become unstable and emit a characteristic amount of energy as it descends to its ground state. By studying the emission of this light via spectroscopy, the researcher can ascertain the qualitative (type of atom) and quantitative (how much is present in the sample) properties of a given species. A classic example is the “flame test,” where different metals can be heated using a flame source, resulting in different colors.

Image Credit: Sodel Vladyslav/Shutterstock.com

The AES Apparatus

The underlying hull of the AES is made from the following:

• An excitation source
• A sample holder
• A monochromator
• A detector

The excitation source can be comprised of one of four things. First, a flame will convert the target analyte (whether it be in solution, a concentrated liquid, or a solid) into a gaseous medium. Secondly, it could be a plasma source, achieving the same effect. Third, the excitation source could be a spark or arc, which uses a pulse or a persistent electrical discharge (respectively) to vaporize the analyte. Finally, a 100 to 800mJ laser can provide the same effect.

Once atomized and the phase change reaction achieves a low enough Gibbs free energy, the analyte will travel from the sample holder through a spray chamber (with an inert carious gas like N₂ or He) to reach the monochromator. Its function is to segregate the emission signatures into an interpretable pattern and can take the form of a prism or a grating sheet. This refracted pattern will ultimately fall on a detector. The two most prevalent detectors in the market are photomultiplier tubes (sensitive enough to distinguish ultraviolet, visible, or IR light) and photographic plates (used for quantitative studies).

The Measuring of the AES Phenomenon

The ∆E (change in energy) released from an excited species is characteristic of the atom in question. This is because different atoms will absorb differing amounts of energy. The photons released from a homogenous gas sample via relaxation will result in discreet photon yields when shined through a prism or grating, giving bands of light rather than a full rainbow.

Each band of light will represent a different energy level, confirming Niels Bohr’s atomic model, which claims that electrons can transition from one orbit to another so long as they emit/absorb a fixed quanta of light.

Variations in the AES Methodology

Many different takes on the AES experiment have been ushered since its humble beginnings in the 1870s with the simple flame test. The most advanced methods are inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). Though both methods allow for multiplex analysis, ICP-MS is typically a more expensive operation. On the other hand, ICP-MS delivers assay results in the parts per trillion range, so long as the total dissolved content of the sample is less than 0.2%. This makes it ideal for food safety, water supply safety, petrochemical analysis, and more.

Companies That Provide Atomic Emission Spectroscopy

Some time has passed since AES’s initial discovery, and in that time, many companies have adopted and marketed this technique, using different variations of the typical AES instrument. Some popular companies include HITACHI, and its EDX spectrometer from the AZTEC series, JIEBO instruments and its Optical Spectrometer JB-1000, and Agilent technologies, and its Optical spectrometer 4210 MP-AES series.

Applications of (AES)

AES is often used in the estimation of trace elements within pharmaceutical samples, environmental samples, and herbal analysis because of its high sensitivity. Unlike

atomic absorption spectroscopy (AAS), atomic force spectroscopy and its other derivatives AES can be employed to measure heavy metals. This makes it the ideal assaying technique for toxic metals and trace elements within biological fluids. Transitioning from medicine, environmental scientists have also employed this technique to identify lead, cadmium, and arsenic in environmental samples in the ppt (parts-per-trillion).

A paper produced by Jakubeniene M et al. has highlighted the possibility of detecting metallic tracings in injured skin, a useful application for forensics investigations of fatalities caused by electrocution. By applying bare aluminum wire, zinc-plated steel, and tin-lead coating on pig skin, we find reliable indicators that can decern the type of wire that can cause the electric marking, despite the background content of all metals within the alloy.

This technique has also found recent applications in quantifying metallic impurities within supposedly “pure uranium compounds.” When using pure uranium for nuclear energy, its purity does have some variability, and an enriched rod can possess trace elements. However, it can be inferred that the purer the uranium, the cleaner and more productive it will be in delivering energy.

To achieve this, scientists use nitric acid and tributyl phosphate for dissolving and extractions, respectively. Once run through additional processes, the aqueous solution can be analyzed using AES to obtain the chemical makeup of the heterogeneous mixture of uranium. This technique will only grow more popular as the transition from crude oil to solar, hydraulic, and nuclear fuels becomes more popularized.

Further Reading

• All Analytical Chemistry Content
• What is Analytical Chemistry?
• Trending Technologies: A Comprehensive Guide to Spectrophotometry
• Global Market Overview: Spectrometry
• Global Market Overview: Electron Microscopy