Atomic Absorption Spectroscopy – Principles and Applications
What is Atomic Absorption Spectroscopy?
AAS, or atomic absorption spectroscopy, is a technique for determining the concentrations of metallic elements in various materials.
History of Atomic Absorption Spectroscopy
Atomic Absorption Spectroscopy was invented by Alan Walsh in the 1950s for the qualitative determination of trace metals in liquids. The technique’s superiority over others is based on the fact that it can determine 50-60 elements without interference from trace to large quantities.
All of these elements, which fail to produce satisfactory results in flame photometry, can be detected here. As a result, it is a successful instrument for detecting and estimating metals and nonmetals, both types of factory pollution.
The technique has also proven to be extremely beneficial for both aqueous and non-aqueous solutions.
Principle of Atomic Absorption Spectroscopy
When a solution containing a mixture of metallic species is introduced into a flame, the solvent evaporates and metallic species vapour is produced. Some metal atoms can be raised to a sufficiently high energy level to emit metal characteristics radiation—a phenomenon used in flame photometry. A large number of metal atoms remain in the non-emitting ground state in this region.
These ground state atoms of a specific element are sensitive to light radiation with their own unique resonance wavelength. As a result, when this wavelength of light passes through a flame, a portion of it is absorbed, and this absorption is proportional to the intensity of atoms in the flame.
Because absorption is proportional to element concentration, the amount of light absorbed is determined in atomic absorption spectroscopy.
Atomic Absorption Spectroscopy has the following Advantages over Flame Photometry
- It is not affected by the spectral interference that occurs in flame emission spectroscopy.
- It is unaffected by flame temperature.
- Traces of one element can be easily determined using the atomic absorption technique in the presence of high concentrations of other elements.
- It has been very successful in the analysis of bronze and copper alloys, as well as in the determination of metals such as platinum and gold.
Atomic Absorption Spectroscopy has the following disadvantages:
The following are some of the disadvantages:
- Because these elements produce oxides in the flame, this technique has not been very successful in estimating V, Si, Mo, Ti, and A1.
- The anion has a noticeable effect on the signal in aqueous solution.
- Each element requires its own lamp to be determined. Attempts are being made to circumvent this problem by employing a continuous source.
Atomic Absorption Spectroscopy Instruments
The instruments used in atomic absorption spectroscopy are as follows:
- Source of Radiation
- Monochromator
- Atomizer
- Hollow Cathode Lamp
- Slits
- Detector
Sources of Radiation
A hydrogen lamp is commonly used as a continuous source of radiation.
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Atomizer
Burners are typically used to break the liquid sample into droplets, which are then allowed to enter the flame. The droplets are then evaporated, leaving the sample element in the residue. Flame is then used to decompose the residue. As a result of this process, the sample is reduced to atoms.
Hollow Cathode Lamp
The radiation source for atomic absorption spectroscopy is a hollow cathode lamp.
It is made up of the following components:
(I) Cathode: is made of or coated with the element to be determined.
(ii) Anode A tungsten, zirconium, or nickel anode is used.
(iii) Depending on the wavelength of the emitted radiation, the window is made of Pyrex glass.
(iv) The lamp contains neon or argon gas.
These gases have distinct line spectra.
These lamps are typically designed for single elements, but multi-element lamps have also been designed for all purposes.
A hollow cathode lamp emits more than one composite line for each element, but the required spectral line can be separated using a monochromator with a low dispersion. The majority of lines are non-absorbing because they involve a transition that is not from the ground state. To provide maximum intensity, the most intense absorption line is chosen.
Inlet and exit slit widths of the monochromator should be narrow to isolate the particular line being used; the requirements depend on:
(1) The focal length
(2) Grating ruling of Monochromator
Monochromator
Since the major focus for the AAS process is Atomic ABSORPTION, the function of the monochromator is to allow the light not to get absorbed by the analyte atoms in the flame to reach the PMT.
Types of Monochromator
Monochromators are typically of two types and they are;
- Gratings
- Prisms.
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Slits or Filters
If the element has a simple line spectrum, filters or slits are used to isolate the required spectral line. Filter photometers are used to measure potassium, sodium, calcium, magnesium, and other elements in samples.
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Detectors
Photomultipliers are commonly used as detectors. To compensate for source fluctuation, some instruments employ two filters and two detectors.
As explained above, atomic absorption spectroscopy (AAS), measures the concentration of metal ions in a sample. It finds use in industries such as petrochemical, pharmaceutical and geological.
When irradiated, an element’s atomic electrons move up to higher energy levels and emit radiation at specific wavelengths. AAS uses this spectrum to identify the element.
Applications of Atomic Absorption Spectroscopy
Atomic absorption spectroscopy has many applications, including food safety, quality control and environmental testing. It is especially useful for liquid samples that contain metallic elements because it has a wider range of concentrations that can be tested than other techniques, and it works accurately with very small sample sizes.
Metals are found throughout the environment and in a wide variety of products, from industrial metals to natural mineral deposits. Their presence is often desirable, but it is also important to ensure that metals are not present in substances at concentrations that could be harmful to human or animal health. For this reason, atomic absorption spectroscopy has been a mainstay for decades in the testing of metals.
AAS is based on the ability of certain atoms to absorb atom-emitting light in their ground electronic state, which differs from the excited electronic state in which they normally emit radiation. The concentration of a particular element in a sample can be determined by measuring the amount of absorbed light, which is proportional to the concentration of the absorbing atoms.
The fundamental components of an AAS are a sample holder, a monochromator and a detector, with a readout device to display the results. A sample holder can be either a flame (flame AAS, or FAAS) or a graphite furnace tube (GFAAS). The atoms in a sample are heated to a point that frees them from solvents and disrupts the formation of salts, but not so hot as to promote an electron transition to an excited state. The resulting free atoms will then absorb electromagnetic radiation at a specific wavelength, and the intensity of this light is measured.
The results of this measurement are plotted against the concentration of an absorbing sample, and the slope of the line corresponds to the concentration of the element in the sample. The calibration of a new sample can be obtained from a standard solution containing the desired metallic element, and the data is used to scale the reading on a readout device.
Uses of Atomic Absorption Spectroscopy
Atomic absorption spectroscopy analyzes the concentration of elements in liquid samples. It requires a light source that matches the narrow bands of light that a particular atom absorbs (such as a hollow cathode lamp), a flame or graphite furnace to vaporize the sample, a monochromator to select the wavelength of the light and a photodetector to measure the energy. From these signals it is possible to determine the parts per million, or ppm, levels of specified metals in the sample.
Scientists have used atomic absorption spectrophotometry for many years to find and test trace amounts of substances. The technology is well-suited to the analysis of environmental, marine and geological samples. It can also help identify contaminated land and soil. It can even be used to determine the origin of a fruit juice or wine based on its chemical composition, such as by identifying calcium, silicon and sodium.
When the light from a hollow cathode lamp passes through the atomized sample, electrons in its vapor phase absorb part of the radiation and are boosted into an excited state. This causes them to emit some of their own radiant energy, which is detected by the detector and analyzed using the principles of Beer’s law.
Researchers can further improve the accuracy of atomic absorption spectroscopy by employing Zimmerman correction to correct for background noise, which can interfere with readings. They can also use line sources, which are designed to irradiate only one specific element, or continuous source lamps, which produce radiation over a wider range of wavelengths. Line source atomic absorption spectrometers are the most popular. They are simpler and more cost effective to operate than continuous source atomic absorption spectrometers.
Devices and Techniques Used in Atomic Absorption Spectroscopy
AAS analyzes the concentration of metal atoms in a sample. Detection of metals in samples is useful for a variety of reasons; they can be essential for the production of certain materials, such as some alloys, or they can be contaminants (poisons). The technique allows for the quick and accurate detection of trace amounts of metals and is used in a variety of laboratories.
The method is based upon the ability of atoms to absorb light. During AAS, a solution of the element being tested is placed into a flame or graphite tube. Heat energy is applied to convert the atoms into free ground state atoms that are then ready for absorption of light with a wavelength specific to the element. Absorption of this light promotes the electrons to an excited state. The more excited the atoms are, the more light they absorb, and thus the more sensitive the machine is.
This is a simple process, but the specialized instrumentation required for it requires a hollow cathode lamp which contains an anode and cathode electrode sealed in a vacuum-sealed tube of noble gas, a monochromator to select the wavelength of light, and a photodetector. Each type of element has its own atomic spectrum, and the narrow bands of wavelengths that can be detected with a monochromator are specific to that element.
The instrument is calibrated using a standard solution of the element being analyzed, with five other standards that have different concentrations of the element for calibration and comparison. This calibration is then used to determine the concentration of the unknown substance in a sample, as well as to produce a calibration curve to allow for future analysis.
Disadvantages of Atomic Absorption Spectroscopy
Atomic absorption spectroscopy (AAS) relies on the fact that each kind of element preferentially absorbs certain frequencies of light. By analyzing the resulting spectrum, one can determine the relative concentration of an element in a sample.
This method of analysis is primarily used to measure trace metals in liquid samples down to the ppm (parts per million) or ppb (parts per billion) levels. Its use is widespread and its reliability makes it an ideal method for pharmaceutical manufacturing.
Nevertheless, there are some disadvantages to using atomic absorption spectroscopy. In particular, a number of different interferences can interfere with the accuracy of measurements. Those interferences can be caused by the presence of nonvolatile impurities, matrix effects, spectral interferences, and chemical interferences.
For example, if the sample is zinc, it can interfere with the results by absorbing or scattering radiation from other elements in the sample. This can be minimized by analyzing a blank that contains the same amount of Zn as the sample and using the appropriate matrix for both the standards and the blank.
Spectral interferences can also occur due to the broad absorption bands of molecular species in the sample and by the radiation-scattering properties of particulates in the matrix. These can be corrected by analyzing a blank that is prepared in the same way as the sample and by calibrating with an external standard whose absorbance closely brackets that of the analyte.
Other interferences can occur if the sample is not prepared properly or if it is analyzed at an inappropriate temperature. For example, the use of an electrothermal atomizer may cause interfering oxides and hydroxides to form in the sample. This can be mitigated by preparing the sample in a solvent with a low boiling point, or by increasing the atomization temperature.
Advancements in Atomic Absorption Spectroscopy
A Lancashire-born physicist, Alan Walsh, was working in his garden on a Sunday morning in the early 1950s when he had a flash of inspiration. His idea would solve a huge analytical chemistry puzzle and change the world. He realized that instead of vaporizing an element and measuring the emission spectra, he could do the opposite: measure its absorption. He figured out how to do this, and by teatime on Monday he had developed atomic absorption spectroscopy (AAS).
AAS works by analyzing the free atoms of an element in a gaseous state created by a flame or graphite furnace atomizer. As each element has its own unique atomic structure, the atoms in the gaseous state will absorb specific wavelengths of light that relate to the element’s properties. This gives a measurable signal that can be used to determine the concentration of the element in the sample. The concentration is determined by comparing the absorption of the sample with the absorption of a series of standards that are known to have a certain concentration.
Today, AAS is an established standard technology for chemical analysis and is used across the globe. The ability to precisely measure very small amounts of elements makes it a powerful tool in the development of new materials and chemicals, as well as for quality control applications in manufacturing, environmental monitoring, medicine and many other industries.
In addition to performance improvements in atomic absorption spectrometry, other recent advances have been focused on making it easier for scientists and technicians to use the technology. For example, the sensitivity of GFAAS has been improved so that toxic metals such as mercury can be detected in low volume samples without sample pretreatment, such as dilution with water. This helps improve safety and efficiency in labs.
