Atomic Absorption Spectroscopy: Principle of Operation and Results

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Introduction

The science of atomic spectroscopy has yielded three-technique for analytical use: atomic emission, atomic absorption, and atomic fluorescence. In order to understand the relationship of these techniques to each other, it is necessary to have an understanding of the atom itself and the atomic process included in each technique.

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The atom is made up of a nucleus surrounded by electrons. Each has a specific number of electrons which are associated with the atomic nucleus in an orbital structure which is unique to each element. The electrons occupy orbital positions in an orderly and predictable way. The lowest energy, most stable electronic configuration of an atom. If the energy of the right magnitude is applied to an atom, the energy will be absorbed by the atom, and an outer electron will be promoted to a less stable configuration state. As this state is unstable, the atom will immediately and spontaneously return to its ground state configuration. The electron will return to its initial, stable orbital position, and radiant energy equivalent to the amount of energy initially absorbed in the excitation process will be emitted.

History of Atomic Absorption Spectroscopy

The earliest spectroscopy was first described by Marcus Marci von Kronland in 1648 by analyzing sunlight as is passed though water droplets and thus creating a rainbow. Further analysis of sunlight by William Hyde Wollaston led to the discovery of black lines in the spectrum, which in 1820 Sir David Brewster explained as the absorption of light in the sun’s atmosphere.

In order to understand how atomic absorption spectroscopy works, some background information in necessary. Atomic theory began with John Dalton in the 18th century when he proposed the concept of atoms, that all atoms of element are identical, and that atoms of different elements can combine to form molecules. In 1913, Niels BOHR revolutionized atomic theory by proposing quantum numbers, a positively charged nucleus, and electrons orbiting around the nucleus in the what became known as the Bohr model of the atom.

Soon afterward, Louis de Broglie proposed quantized energy of electrons, which is an extremely important concept in AAS. Wolfgang Pauli then elaborated on deBroglie’s theory by stating that no two electrons can share the same four quantum numbers. These landmark discoveries in atomic theory are necessary in understanding the mechanism of AAS.

Atomic absorption process

The quantity of interest in atomic absorption measurement is the amount of light at the resonant wavelength which is absorbed as the light passes through a cloud of atoms. As the number of atoms in the light path increases, the amount of light absorbed increases in a predictable way. By measuring the amount pf light absorbed, a quantitative determination of the amount of analyte element present can be made. The use of special light sources and careful selection of wavelength allow the specific quantitative determination of individual elements in the presence of others.

The atom cloud required for atomic absorption measurements is produced by supplying enough thermal energy to the sample to dissociate the chemical compounds into free atoms. Aspirating a solution of the sample into a flame aligned in the light beam serves this purpose. Under the proper flame conditions, most of the atoms will remain in the ground state form and are capable of absorbing light at the analytical wavelength from a source lamp. The ease and speed at which precise and accurate determinations can be made with this technique have made atomic absorption one of the most popular methods for the determination of metals.

The third field in atomic spectroscopy is atomic fluorescence. This technique incorporates aspects of both atomic absorption and atomic emission. Like atomic absorption, ground state atoms created in a flame are excited by focusing a beam of light into the atomic vapor. Instead of looking at the amount of light absorbed in the process, however, the emission resulting from the decay of the atoms excited by the source light is measured. The intensity of fluorescence increases with increasing atom concentration, providing the basis for quantitative determination.

Principles

The technique uses basically the principle that free atoms (gas) generated in an atomizer can absorb radiation at a specific frequency. Atomic-absorption spectroscopy quantifies the absorption of ground state atoms in the gaseous state. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration. Atomic absorption is a very common technique for detecting metals and metalloids in environmental samples.

Atomic absorption instrumentation

The basic components: 

Every absorption spectrometer must-have components that fulfill the three basic requirements, there must be:

  1. A light source.
  2. A sample cell
  3. A means of specific light measurement.

Light source

Types of light sources in Atomic Absorption Spectroscopy:

1. The Hollow Cathode Lamp: The hollow cathode lamp consists of a glass cylinder filled with an inert gas usually Argon or Neon at low pressure. The cathode is made from metal which is to be determined.. The emission line of the lamp corresponds with the absorption wavelength of the analyte.

2. The Electrodeless Discharge Lamp: The internal electrodeless lamp or induction lamp is a gas discharge lamp in which an electric or magnetic field transfers the power required to generate light from outside the lamp envelope to the gas inside.

Nebulizer:

  • Suck up liquid samples at a controlled rate.
  • Create a fine aerosol spray for introduction into flame.
  • Mix the aerosol and fuel and oxidant thoroughly for introduction into flame.
  • The general term nebulizer refers to an apparatus that converts liquids into a fine mist. … Analytical nebulizers are a special category in that their purpose is to deliver a fine mist to spectrometric instruments for elemental analysis.

Monochromator:

  • This is a very important part in Atomic Absorption Spectrometer. It is used to separate out all of the thousands of lines.
  • A monochromator is used to select specific wavelength of light which is absorbed by the sample, and to exclude other wavelengths.
  • The selection of the specific light allows the determination of the selected element in the presence of others.

Detector

The type of detector found in AAS is the photomultiplier tube – the principle of operation is the emission of electrons upon exposure to radiation. The detector contains a photoemissive cathode and a series of dynodes. The light selected by the monochromator is directed onto a detector that is typically a photomultiplier tube, whose function is to convert the light signal into an electrical signal proportional to the light intensity. The processing of the electrical signals is fulfilled by a signal amplifier. The signal could be displayed for readout, or further fed into a data station printout by the request format.

Calibration Curve

A calibration curve is used to determine the unknown concentration of an element in a solution. The instrument is calibrated using several solutions of known concentrations. The absorbance of each known solution is measured and then a calibration curve of concentration vs absorbance is plotted. The sample solution is fed into the instrument, and the absorbance of the element in this solution is measured. The unknown concentration of the element is then calculated from the calibration curve.

Atomizers:

Flame Atomizers Graphite Tube Atomizers

Atomizer:

Elements to be analyzed needs to be in atomic sate. Atomization is separation of particles into individual molecules and breaking molecules into atoms. This is done by exposing the analyte to high temperatures in a flame or graphite furnace. The atomizers most commonly used nowadays are (spectroscopic) flames and electrothermal (graphite tube) atomizers. Other atomizers, such as glow-discharge atomization, hydride atomization, or cold-vapor atomization, might be used for special purposes.

Types of Atomizers

1)Flame Atomizers:

  • To create a flame, we need to mix oxidant gas and fuel gas.
  • In most of the cases, air-acetylene flame or nitrous oxide-acetylene flame is used.
  • Liquid or dissolved samples are typically used with flame atomizers.
  • Flame Atomic Absorption is a very common technique for detecting metals present in samples. The technique is based on the principle that ground state metals absorb light at a specific wavelength. Metal ions in a solution are converted to an atomic state by means of a flame

2)Graphite Tube Atomizer:

  • Uses a graphite-coated furnace to vaporize the sample.
  • In the Graphite Tube Atomizer AAS sample, samples are deposited in a small graphite-coated tube which can then be heated to vaporize and atomize the analyte.
  • The graphite tubes are heated using a high current power supply.
  • In GFAAS, samples are deposited in small graphite or pyrolytic carbon-coated graphite tube, which can then be heated to vaporize and atomize the analyte. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels.
  • The nature of the graphite tube surface makes graphite furnace analyses susceptible to certain types of nonspectral interferences, especially carbide formation. A number of elements tend to form nonvolatile carbides by in interaction with the graphite surface.

 

Applications

1) Environmental Science.

Environmental pollution remains are important issue for population, economic and political decision factors in all countries. The small the territory of Romania is affected by pollutants from different pollution sources: chemical industries, iron and steel smelters, coal mining and thermal power stations, cement factories, auto traffic, the use of pesticides and fertilizer, so, it is vital to know the elemental concentration of heavy and toxic elements in flora and fauna and to quantify these using analytical methods.

2) Clinical Applications.

The majority of samples analyzed are taken from the main group of biological fluids, such as whole blood, plasma, serum, and urine. Hard and soft tissues, such as bone, fingernails, and hair. Flame-based analysis for the major and minor essential elements, graphite furnace analysis for the trace elements, and vapor analysis for the group of toxic.

3) Food Technology.

Food technology is the application of food science to the selection, preservation, processing, packaging, distribution, and use of safe food. Food Technology is the combination of engineering, food science, hotel management, and home science. Food Technology course is an advanced study of the technology and processing methods used to develop, research, manufacture, produce, preserve, and process food and related substances.

4) Pharmaceuticals.

In drug discovery and testing, most pharmaceutical companies these days develop drugs which are targeted at specific cells in the body. These drugs must be tested for correct activity but more importantly for the absence of any adverse side reactions.

5) Petrochemicals.

Atomic absorption spectrometry (AAS) is a technique of particular utility in the determination of trace elements in petroleum feedstuffs and products. It provides sufficient sensitivity for most trace metal analysis requirements met in the petroleum industry. Lubricating oils, very rotating mechanism in the machinery of all types depend on their use for smooth operation. Like engines and gearboxes used in modern transportation, such as aircraft, ships, cars and lorries and heavy construction equipment. Oils in use so that oil changes can be carried out in time to prevent excessive wear from occurring in the components concerned.

6) Geochemical/ mining.

The primary purpose is to identify geochemical “anomalies”—that is, areas that have unusually high concentrations of one or more elements. Most of the analytical requirement is met by atomic absorption spectrometry (AAS) methods: about 70 percent of all geochemical samples are analyzed by conventional flame techniques, usually for several elements.

7) Bio-monitoring.

Biological monitoring is a way of assessing chemical exposures by measuring the chemical or its breakdown products in a biological sample (usually urine, blood or breath). The most important biological samples used in everyday practice are urine, blood, and occasionally exhaled air. Urine collection is more readily accepted by workers (easy and not invasive).

8) Agriculture. 

Plants may be sampled to monitor nutrient uptake efficiency and also to check for toxic metal accumulation for health reasons. Soil analysis provides a measure of a soil’s potential to supply the necessary nutrients to plants.

9) Nanomaterials.

Nanomaterials are chemical substances or materials that are manufactured and used at a very small scale. Nanomaterial is a material with any external dimension in the nanoscale (size range from approximately 1 – 100 nm) or having an internal structure or surface structure in the nanoscale. Nanomaterials have several advantages over bulk materials such as the huge surface-to-volume ratio, very high porosity, and completely different physiochemical properties.

10)Pathology.

Clinical pathology is a medical specialty that is concerned with the diagnosis of disease based on the laboratory analysis of bodily fluids such as blood and urine, as well as tissues, using the tools of chemistry, clinical microbiology, hematology, and molecular pathology.

Conclusion

While other analytical techniques may offer specific advantages over atomic absorption, it can be seen from the above comparison on that no single technique offers all of the advantages. The versatility, moderate cost, and established methodology of atomic absorption will continue to make it a valuable tool for the laboratory.

Atomic Absorption Spectroscopy is a quantitative method of analysis that is applicable to many metals and a few mom-metals. Almost every metallic element can be determined quantitatively by using the spectral absorption characteristics of atoms. Also, it is a very common technique for detecting and measuring the concentration of metals in the samples. This technique basically uses the principle that free atoms (gas) generated in an atomizer can absorb radiation at a specific frequency. The atoms absorb UV or visible light and make transitions to higher electronic levels, AAS quantifies the absorption of ground-state atoms in the gaseous state.

Reference

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  7. https://www.google.com/search?q=applications+of+aas&rlz=1C1CHBF_enIQ833IQ833&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiTn7aCjO3pAhWGUcAKHdRjBCcQ_AUoAXoECA4QAw&biw=1313&bih=640#imgrc=Auvaq203YN3UqM
  8. https://www.slideshare.net/sharmasuriti/atomic-absorption-spectroscopy-15185397
  9. https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy
  10. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Acids_and_Bases/Buffers#:~:text=A%20buffer%20is%20a%20solution,of%20the%20solution%20relatively%20stable.
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29 April 2022

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