Learn about Atomic Absorption Spectroscopy in 5 minutes

 

Learn about atomic absorption spectroscopy in 5 minutes. This article discusses the AAS (atomic absorption spectroscopy) principle and Instrumentation of AAS. This article will help students comprehend AAS (atomic absorption spectroscopy) on their exam.

Spectroscopy

Spectroscopy, study of the absorption and emission of light and other radiation by matter, as related to the dependence of these processes on the wavelength of the radiation.

Absorption spectroscopy

If electromagnetic radiation of a certain range of wavelengths passes through the analyzed substance, the radiation of certain wavelengths is absorbed by the substance. The study of this is called absorption spectroscopy. The absorbed wavelength characterizes a certain specific functional group of the compound or the compound itself.

Atomic Absorption Spectroscopy

Atomic absorption spectroscopy 

• This method includes the study of absorption and radiation by neutral atoms in the vaporous state.
• In AAS, when a solution containing a mixture of metallic species is introduced into a flame, the solvent is evaporated and vapors of the metallic species are obtained, and then the absorption of radiation by the atomic pairs is measured at a selected characteristic wavelength each individual.
• This technique is also called absorption flame photometry because all applications of analyte atomic absorption involve spraying a sample solution into a flame.
• It can analyze over 62 elements.
• It also determines the metal concentration in the sample.
• In this periodic table, elements observable via atomic absorption are shown in pink.

Periodic table of elements which are detected by atomic absorption spectroscopy 

Principle of atomic absorption spectroscopy

• When a solution containing a mixture of metallic substances is introduced into the flame, the solvent is evaporated and vapors of the metallic substances are obtained, the vapor is atomized, and then the absorption of radiation by the unexcited atoms in the atomic vapor is measured at a selected wavelength that is characteristic of each individual element.
• Flame gases are considered to be a medium containing free, unexcited atoms capable of absorbing radiation from an external source.
• This occurs   when   the   radiation corresponds   exactly   to   the   energy required   for a   transition   of   test element from ground   electron state   to   the   excited   state.
• The   unabsorbed   radiation   passes through a monochromator that isolates the exciting spectral lines and   into   a   photodetector.  
• Absorption   is   measured   by   the difference   in transition signal   in   absence   and   presence of test element.

    Instrumentation of atomic absorption spectroscopy

Instrumentation of atomic absorption spectroscopy

For instrumentation, flame, flameless, and graphite furnaces are available in atomic absorption apparatus. Any atomic absorption spectroscopy device has the following types of components:

• Nebulizer
• Atomizer
• Light source (Hollow cathode lamp)
• Monochromator
• Detector
• Recorder

Nebulizer

• Aspirate liquid samples at a controlled rate.
• Create a fine aerosol spray to introduce into the flame.
• Thoroughly mix the aerosol, fuel and oxidizer for introduction into the flame.

Atomizer

• The elements to be analysed must be in the atomic state.
• Atomization is the division of particles into individual molecules and the breaking of molecules into atoms.
• This is accomplished by treating the analyte to high temperatures in a flame or graphite furnace.

Types of atomizers

Flame atomizer

• Oxidant gas and fuel gas must be mixed together to create a flame.
• Air-acetylene or nitrous oxide-acetylene flames are often used.
• Liquid or solution samples are usually used in flame atomizers.

Graphite tube atomizer

• The sample is vaporized using a furnace coated with graphite.
• In GFAAS, the samples are stored in a small tube coated with graphite, which can then be heated to vaporize and atomize the analyte.
•  The graphite tubes are heated using a high voltage source.

Light source

• Hollow cathode lamps are the most common source of radiation in AAS.
• A hollow cathode discharge lamp with a tungsten anode and a cylindrical analytical metal cathode in a glass tube with a filling gas such as Ne (neon) or Ar (argon).
• Each element has its own unique lamp that must be used for this analysis.

Monochromator

• This is a very important part of the AA spectrometer. It is used to separate all thousands of rows.
• A monochromator is used to select the wavelength of light that is absorbed by the sample while excluding other wavelengths.
• The selection of a specific light allows the determination of the selected element in the presence of others.

Detector

• The detector can change the monochromator's light into an improved on simplified electrical signal.
• In general, we used a photomultiplier as a detector in an atomic absorption spectroscopy instrument. The detector can be tuned to respond to a specific wavelength or frequency.
• Electrical signal processing is provided by a signal amplifier. The signal can be displayed for reading or further fed to the data station for printing in the required format.

Recorder

• The recorder can receive electrical signals from the detector and convert them into a readable response.
• Today, in the instrumentation of atomic absorption spectroscopy, we used a computer system with appropriate software to recode the signals coming from the detector.   

Calibration Curve

• A calibration curve is used for estimating an element's unknown concentration in a solution. The device is calibrated using several solutions of known concentrations.
•  The absorbance of each known solution is measured and then a concentration vs. absorbance curve is plotted.

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