Atomic absorption spectroscopy (AAS)

Atomic absorption spectroscopy (AAS) is one of the earliest spectroscopic techniques available to scientists. It is because of its simple working principle, cost-effectiveness, and reliable results, that atomic absorption spectroscopy is still relevant today. If you are curious to know how it works and what it does, then you are at the right place. We have compiled for you all the important information regarding AAS in this one article.

What is atomic absorption spectroscopy (AAS)

Atomic absorption spectroscopy is based on an electronic excitation at the atomic level. The electrons present in single metal atoms of the targeted sample interact with radiant energy in AAS. The radiant energy is provided by a light source. Electrons present in the atoms absorb radiant energy equal to the energy difference (∆E) between two of its distinct energy levels. As a result of this energy absorption, electrons undergo excitation followed by de-excitation. The absorbed wavelength is characteristic of the tested element and is thus used for identification purposes.

What elements can atomic absorption spectroscopy measure

AAS is mainly used for the analysis of high ionization potential metals such as copper (Cu), lead (Pb), iron (Fe), chromium (Cr), silver (Ag), mercury (Hg), etc. As a general rule of thumb, the higher the absorbance recorded, the greater the concentration of the characteristic metal atoms in the tested sample mixture.

Historical perspective of atomic absorption spectroscopy

Atomic absorption spectroscopy was introduced back in 1802 by  William Hyde Wollaston, an English scientist. He observed dark lines in the sun’s spectrum. Later on, it was in the year 1817  that the German scientist, Joseph von Fraunhofer identified the spectral absorption lines. These spectral lines were thus known as Fraunhofer lines.  However, the first atomic absorption spectrometer became available for commercial use in the 1960s.

Let’s find out how the atomic absorption spectrometer functions.

What is the principle of atomic absorption spectroscopy

The atomic absorption spectrometer consists of the following main components:

1. Nebulizer

• The sample (2-5 mL) is introduced into the spectrometer in the form of a solution.
• The solution is aspirated and converted into a fine mist by the nebulizer.

2. Mixing chamber

• In the mixing chamber, the sample mist is mixed with a fuel such as methane, oxygen, dinitrogen oxide, acetylene, etc.
• The uniformly formed sample-fuel mixture then reaches the flame.

3. Flame

• As the sample-fuel mixture is sprayed onto the flame, the solvent evaporates under high-temperature conditions (2000-3000)° C, leaving behind solid residue.
• The solid residue is further vaporized into a gas.
• The gas molecules are finally atomized to produce constituent atoms.
• Other than flame atomization, graphite furnace or electrothermal atomizers can also be used in atomic absorption spectroscopy. Based on the atomizer used, AAS can be categorized into its sub-types.  

4. The light source

• A hollow cathode lamp is used as a light source in an atomic absorption spectrometer.
• The element e.g., Copper (Cu) to be investigated is placed in the cylindrical hollow cathode against a tungsten anode.
• Both the cathode and the anode are then sealed in a glass tube filled with an inert gas such as Ne or Ar at 1-5 Pa pressure.
• A high potential difference (300-400 V) is applied at this point so that some of the gaseous sample atoms reaching here can be readily ionized.
• These gaseous ions bombard the cathode element and eject a few metal atoms from it in a process called sputtering.
• The electrons present in some of the sputtered metal atoms undergo excitation followed by de-excitation and result in emitting radiations of a characteristic wavelength (e.g., 325 nm for Cu).
• The electrons present in the targeted gaseous atoms absorb this characteristic wavelength. These electrons undergo excitation followed by de-excitation from their ground state to a higher energy level and back. De-excitation releases the energy absorbed in the first instance.
• The spectrometer holds several different lamps, each for a different element. The lamps are placed in a rotating turret so that the lamp of a characteristic wavelength can be quickly selected as per requirement.

5. Monochromator

• A sample may consist of more than one metal atom. Several different metal atoms may absorb radiant energy to undergo excitation followed by de-excitation. Consequently, different wavelength radiations may be transmitted at a time.
• A monochromator is placed to select the transmitted radiation of a specific wavelength only (say 325 nm) and allows it to reach the detector.
• The monochromator also reduces background interference.

6. Detector

• The detector usually a photomultiplier tube generates an electrical signal which is then amplified and recorded.
• An absorption spectrum shows electromagnetic radiations transmitted by the sample atoms as colored bands against a black background.
• The black background corresponds to the absorbed radiations.

• A calibration curve is a straight-line graph of absorbance versus sample concentration. It can be used to determine the unknown concentration of metal atoms in a sample once its absorbance is determined using atomic absorption spectroscopy. 

Why do we need atomic absorption spectroscopy

• Atomic absorption spectroscopy (AAS) is very important for determining the concentration of metal atoms in environmental samples such as fertilizers, soil, and water.
• AAS can be used to keep a check on food contamination.
• A small amount of catalysts and other impurities that become part of a foodstuff or a pharmaceutical drug during its manufacturing can be readily identified using AAS.
• The toxic Pb and Hg atoms present in the output nuclear waste can be detected and quantified using atomic absorption spectroscopy.
• AAS also finds meaningful applications in forensics, geology, materials development, petrochemical refining, etc.

Thus, atomic absorption spectroscopy is extremely important.

Learn more about the historical progress and future applications of AAS here.

Advantages of atomic absorption spectroscopy

Its primary advantages are:

• Easy to operate.
• Allows both qualitative as well as quantitative chemical analysis.
• Offers a high sensitivity and accuracy (0.50-5% variations only) in metallic sample analysis.  
• AAS can determine over 62 metallic elements present in the Periodic Table.  

It is a comparatively less expensive technique as opposed to other spectroscopic techniques that require more delicate equipment such as FTIR spectroscopy and NMR spectroscopy.

Limitations of atomic absorption spectroscopy

• AAS is limited to the analysis of metal atoms specifically high ionization potential metal atoms that can easily transform to their gaseous ions.
• AAS is a destructive technique. The sample must undergo nebulization followed by evaporation and then atomization to be analyzed via atomic absorption spectroscopy.
Here is a video tutorial to keep your urge for learning spectroscopic analysis afresh.
You may also like to read about the twin-sister of atomic absorption spectroscopy i.e., atomic emission spectroscopy.

References

1.Lagalante, A. F. (2004). “Atomic Absorption Spectroscopy: A Tutorial Review.” Applied Spectroscopy Reviews 34(3): 173-189.

2. Palkendo, J. A., J. Kovach and T. A. Betts (2014). “Determination of Wear Metals in Used Motor Oil by Flame Atomic Absorption Spectroscopy.” Journal of Chemical Education 91(4): 579-582.