Decoding Atomic Emission Spectroscopy: Principles and Instruments

Atomic emission spectroscopy (AES) is an analytical technique that is specifically used for elemental-metal analysis. As the name suggests it measures the radiation of discrete wavelength emitted by the atoms of the sample, after being excited by absorbing thermal or electric energy. Atomic emission spectroscopy involves both excitation(absorption of radiation) and de-excitation(emission of radiation) of electrons.

It is also known as Optical Emission Spectroscopy as the radiation shows optical properties during the de-excitation of electrons. Another name for the technique is inductively Coupled Plasma atomic emission spectroscopy(ICP_AES) because it involves the excitation and de-excitation processes for electrons by absorption of radiation.

AES is regarded as the most reliable method for elemental quantitative analysis. If proper precautions are taken, this method can be used for quantitative analysis of about seventy elements at concentration levels as low as 1 ppm.

Principle of Atomic Emission Spectroscopy

When the sample is irradiated by a beam of light at high temperatures, there is excitation of electrons which transfers them from their lower energy level to higher energy level as the radiation energy is absorbed by the atoms. When the excited electrons leave the high-temperature region, they return to the ground states by emission of radiation in the form of discrete wavelength packets.

A characteristic set of wavelengths is emitted by each element or substance which depends on its electronic structure and the study of these wavelengths can reveal the elemental structure of the sample.

The emission of radiation from the atoms produces the emission spectra which can be classified as follows:

  • Continuous Spectra When matter is heated in bulk, there is a production of uninterrupted emission over a considerable wavelength region and by the absence of sharp lines or discrete bands. Since continuous spectra are dependent on high temperatures, therefore, volatile samples cannot be detected by these spectra.
  • Band SpectraIt is also known as molecular spectra as each molecule produces emission bands that are characteristic of that molecule. The bands are the groups of spectral lines arranged so close to each other that they appear like a single band.
  • Line SpectraThese are the spectra we are concerned with. It is called the atomic spectra as it is produced by the substance in its atomic state. Line spectra consist of discrete irregular lines. However, most of the spectral lines fall in the vacuum-ultraviolet region which cannot be studied easily, therefore atomic emission spectroscopy is limited to metals and metalloids only. 

Instrumentation of Atomic Emission Spectroscopy

The basic instrument of AES is composed of the following components:

I. Excitation Source

The excitation source vaporizes the sample and dissociates into atoms. The different types of excitation sources used in AES are:

  • Flames: Flames are inexpensive, stable, and reproducible sources that are capable of handling many analytical problems. It is used for samples that do not require high temperatures to vaporize. The flame’s temperature is dependent on the factors like- the type of fuel and oxidant, the fuel-to-oxidant ratio, the type of burner, and the region in the flame that is focused into the entrance slit of the spectral isolation unit. 
  • Direct Current Arc: Direct current arc is used for samples that are present in very low concentrations. The high temperature generated in the DC arc makes it a very sensitive excitation source. DC arc mainly provides thermal energy rather than electrical energy. The temperature ranges from 4000-8000 degrees Kelvin.
  • Alternating Current Arc: It is a stable, reproducible, and best source for qualitative analysis. The arc is drawn at a distance of 0.5-3 mm and employs a voltage of 1000 volts.
  • Alternating Current Spark: AC sparc is usually used for high-concentration samples. It uses a high voltage of 10-50 KV to produce a spark. 

II. Sample Cell

The two different types of sample cells used in the form of electrodes are:

  • Self-electrodes– These sample cells are used when a sample is a good conductor of electricity and can bear high temperatures.
  • Graphite Electrodes– It is used when the sample is not a good conductor of electricity, it is placed in a small cavity of the cup-shaped graphite electrode.

III. Sample Holders

Sample holders introduce the sample into the electrical discharge. These can be solid sample holders or liquid sample holders. In solid sample holders, the sample is first reduced to powders and loaded into a carbon sample holder.

This sample holder is placed in position as one of the electrodes used in the discharge. Then the sample is vaporized into plasma. Liquid samples can be analyzed by putting them in a porous cup container or a rotating disk electrode. 

IV. Spectrographs 

The spectrographs are responsible for the selection of a monochromatic light of a particular wavelength. It consists of a prism monochromator or grating monochromator and slits. 

Prisms made up of quartz or fused silica are used as a monochromator. Usually, when light passes through a quartz prism, it splits the light into two lines which leads to loss of light’s intensity and produces complications in spectrum interpretation. Therefore two half prisms of Cornu type are used, in which one prism splits the incident ratio and the other recombines the two radiations into one. 

Since gratings give linear dispersion, it is usually preferred over prisms and the resolution of gratings is independent of wavelength and always constant. 

All spectrographs are fitted with entrance and exit slits. Entrance slits keep out stray light and permit only the light from the sample to enter the optical path. Exit slits placed after the monochromator stop all but the desired wavelength range from reaching the detector. The narrow slits should be used to obtain resolution of two lines at similar wavelengths. 

V. Detectors

Detectors catch the signals from the spectrographs and transfer them to the recording system which shows the results in the form of spectral lines. Usually, two types of detectors are employed in this instrument i.e., photographic and photomultiplier tubes. 

In photographic detectors, photographic emulsions are used to register the intensity of spectral lines, which is developed by a suitable developer in the recommended developing time at a particular temperature. A densitometer measures the density of the film and plots it against the logarithm of the exposure.

The photographic emulsion used contains light-sensitive crystals of silver halides suspended in gelatin.  The silver halide is sensitive to only short wavelengths therefore some dyes are also added to it to make it sensitive to longer wavelengths.

Conclusion

Atomic emission spectroscopy stands at the forefront of analytical techniques, providing precise identification and quantification of elements across diverse industries. Its sensitivity, selectivity, and non-destructive nature make it a crucial tool in research and quality control.

As technology advances, the future holds promise for even greater speed and accuracy in elemental analysis. In various fields, from environmental monitoring to forensics, atomic emission spectroscopy remains indispensable, unraveling the atomic composition of substances and contributing to scientific progress and innovation.

Suksham Gupta

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