Decoding Atomic Absorption Spectroscopy: Uncovering Principles and Instrument

Atomic Absorption Spectroscopy(AAS) is a quantitative analytical technique that provides the total metal content of the sample and is almost independent of the molecular form of the metal in the liquid sample. It analyzes the concentration of elements in a liquid sample based on energy absorbed from certain wavelengths of light.

Alan Walsh, a Lancashire-born physicist was the one who worked on determining the small concentrations of metallic elements using spectroscopy. He got this idea in the 1950s while working in his garden. He worked on it for several years and finally convinced the manufacturers to use Atomic Absorption Spectroscopy for metallic determination.

The first commercial AAS was launched in the 1960s and since then this technique has established a robust place in the field of analytical analysis. 

Principle of Atomic Absorption Spectroscopy

AAS is based on the absorption of energy by ground-state atoms in the gaseous state. It states that when a beam of electromagnetic radiation of a particular wavelength is passed through the vaporized atoms present in the flame, then atoms absorb the radiation, and a decrease in the intensity of radiation will be directly proportional to the atoms present in the ground state.

Furthermore, since the electronic structure of every element is unique, the radiation absorbed represents a unique property of each element and it can be measured. 

The amount of energy absorbed by the atoms can be represented by:

Total EM radiations absorbed by the atom = (𝛑𝚎²/mc) * Nf at a particular wavelength

Where,

e = electronic charge
m = mass of the atom
c = velocity of electromagnetic radiation
N = number of atoms
f = oscillator frequency or ability for each atom to absorb frequency

The above equation implies that the absorption of radiation by the atoms is independent of the wavelength of absorption and temperature of the atoms. The radiant energy absorbed by the electrons is directly related to the transition that occurs during this process. 

Instrumentation of Atomic Absorption Spectroscopy

A typical AAS instrument is composed of the following components:

  • Radiation source
  • Chopper
  • Atomizer and Nebulizer
  • Monochromator
  • Detector, amplifier, and recorder

I. Radiation Source

The radiation source emits stable intense radiation of the element to be determined, which is usually a resonance line of the element.  The commonly used sources used in AAS are:

a) Hollow Cathode Lamp

A hollow cathode lamp is a cup, filled with the element to be determined and is called the cathode while tungsten wire is used as an anode. Both the cathode and anode are housed in a tube that is filled with inert gas. The lamp window is made up of glass silica or quartz.

When a potential is applied between the two electrodes, a current in the milliampere range arises, the inert gas is charged at the anode and the charged gas is attracted by high velocity to the cathode. This results in the vapourization of element atoms. 

Excess current may give rise to a self-absorption process wherein the sputtered ground-state atoms absorb some of the emitted radiation. If the source lamp is run below the recommended current, a loss of intensity and a corresponding loss of sensitivity will result.

The inert gas filled in the lamp is primarily responsible for the excitation of the ground-state metal atoms.

b) Electrodeless discharge lamp

An electrodeless discharge lamp is used for volatile elements such as arsenic, germanium, or selenium. This lamp consists of an evacuated tube in which the metal of interest is placed. The tube is then filled with argon gas at low pressure and sealed off.

This sealed tube is placed in a microwave discharge cavity. In the microwave, the gaseous argon is converted into plasma and causes excitation of the metal sealed inside the tube. The emission from the metal is that of its spectrum, including the resonance line.

II. Chopper

The chopper is a rotating wheel that is imposed between the hollow cathode lamp and the flame which breaks the steady light from the lamp into a pulsating light/current. This pulsating current is amplified and recorded which helps in measuring the absorption without any interference from the light emitted by the flame. 

III. Atomizer and Nebulizer

Atomizer is the component of the AAS which reduces the liquid sample to the gaseous state. The common atomizers used are :

A) Flame Atomizer

The flame is used to convert the liquid sample into a gaseous state. A consumption burner is used to produce the flame in which the sample, fuel, and oxidizing gases are passed through separate passages to meet at the opening of the base of the flame. The flame breaks up the liquid sample into droplets which are then evaporated or burnt, leaving the reduced residue to atoms.

b) Carbon Atomizer

The sample, usually of the order of 2-30 microlitres, is loaded onto the carbon atomizer, which is then warmed gently to remove the solvent. The temperature is then increased under controlled conditions to ash the sample and remove most of the organic material present.

Finally, the sample is heated rapidly to very high temperatures to cause atomization. The free atoms are vaporized from the carbon atomizer into the optical light path, where their absorption is measured. 

The method of converting liquid samples into small droplets is known as nebulization. The common method of nebulization is by use of a gas moving at high velocity called pneumatic nebulization.

IV. Monochromator

The Monochromators are placed to select a given absorbing line from spectral lines emitted from the hollow cathode lamp. The most common monochromators utilized in Atomic Absorption Spectroscopy are the prisms and gratings.

V. Detector, Amplifier & Recorder

The detector is the component that catches the desired signals from the monochromator. The most effective and commonly used detector is a photomultiplier tube, where an evacuated envelope contains a photocathode, a series of electrodes called dynodes, and an anode. The photocathode is fixed to the terminal of the power supply.

As soon as a photon strikes the photocathode, an electron is dislodged and the photon is accelerated to dynode 1, resulting in the liberation of two or more electrons from this dynode, then the electrons are accelerated from this dynode, and the electrons are accelerated to dynode 2 resulting in the liberation of more electrons. This way the current is multiplied at each dynode. 

The detected signals are amplified and recorded by the amplifier and recorder respectively. Generally, lock-in amplifiers are preferred which provide a very narrow frequency band pass and help to achieve an excellent signal-to-noise ratio.

Conclusion

Atomic Absorption Spectroscopy is a quantitative technique that analyses the metallic constituents in a given sample. It is particularly useful in determining trace metals in liquids and is almost independent of the molecular form of the metal in the sample.

There is no requirement for sample preparation in this instrumental method, which means that the determination of one element can be made in the presence of many other elements. This is also the reason for its acceptance in various industries.

In forensics, this technique is always welcomed with open arms as it has high sensitivity and accuracy. Further, it facilitates the unique identification of each element.

Suksham Gupta

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