ATOMIC ABSORPTION SPECTROSCOPY (AAS)

AAS is an analytical technique used to determine the concentration of metal atoms/ions in a sample. It is used for detecting metals and metalloids in samples. It used to determine how much of certain elements are in a sample. For e.g. detection of Ca, Mg, Na and K ions in blood, detection of metal elements in food industry , trace metals in soils, lakes, rivers, oceans, drinking water, forensic analysis, detection of Pb in petrol etc. It analyze over 62 elements. First Atomic Absorption Spectrometer – CSIRO scientist Alan Walsh (1954).

An electron is excited from the ground state to higher energy level by absorbing energy (light) at a specific wavelength. In atomic absorption spectroscopy, the wavelength of absorbed light is determined by the type of atom (which element it is) and the energy levels the electrons are moving to. When the specific wavelength of light is provided, the energy (light) is absorbed by the atom. Electrons in the atom move from the ground state to an excited state. The amount of light absorbed is measured and the concentration of the element in the sample can be calculated.

PRINCIPLE OF AAS

AAS uses basically the principle that free atoms (gas) generated in an atomizer can absorb radiation at specific frequency. AAS quantifies the absorption of ground state atoms in the gaseous state. The atom absorb UV or Visible light and make transition to higher electronic energy level. The analyte concentration is determined from the amount of absorption. Concentration measurements are usually determined from a working curve after the instrument with standard of known concentration.

THEORY

In AAS, a solution containing the analyte is introduced into a flame. The flame converts samples into free ground state atoms that can be excited. A lamp emitting light at a wavelength specific to the atoms is passed through the flame, and as the light energy is absorbed, the electrons in the atoms are elevated to an excited state. The Beer-Lambert Law describes the relationship between light absorption and concentration of the element. According to the law, the amount of light absorbed is proportional to the of atoms excited from the ground state in the flame. Atoms absorb light at a definite wavelength depending upon the nature of the chemical elements. For example, sodium is absorbed in 589 nm, uranium is absorbed in 385.5 nm, and potassium is absorbed in 766.5 nm.

1. Light source (Hollow cathode lamp) :

Hollow cathode lamp are the most common radiation source in AAS. It contains a tungsten anode and a hollow cylindrical cathode. These are sealed in a glass tube filled with an inert gas (mainly neon or argon). Each elements has its own unique lamp which must be used for that analysis.

2. Nebulizer :

It suck up liquid samples at controlled rate. Create a fine aerosol spray for introduction into the flame. Mix the aerosol and fuel and oxidant thoroughly for introduction into flame.

3. Atomizer :

Elements to be analyzed needs to in atomic state and this is done by means of atomizer. Atomization is separation of particles into individual molecules and breaking molecules into atoms. The atomizers most commonly used nowadays are

4. Monochromator :

It is very important part in an AAS. It is used to separate out all of the thousands of lines. A monochromator is used to select the specific wavelength of light which is absorbed by the sample and to remove other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others.

5. Detector :

The light selected by the monochromator is directed onto a detector whose function is convert the light signal into an electrical signal. Photomultiplier tube detector is mainly used.The processing of electrical signal is fulfilled by a signal amplifier. The amplified signal is then displayed on read out system or fed into a data station for printout by the requested format.

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

ADVANTAGES AND DISADVANTAGES :

ADVANTAGES	DISADVANTAGES
1. Low cost per analyses.	1. Cannot detect non-metals.
2. Easy to operate.	2. New equipment is quite expensive.
3. High sensitivity (up to ppb detection).	3. More geared towards analysis of liquids.
4. High accuracy.	4. Sample is destroyed.
5. Wide applications.

APPLICATIONS

Determination of small amount of metals (lead, mercury, calcium, magnesium). AAS is widely used in metallurgy, alloys and in inorganic analysis.

Biochemical Analysis : A number of elements present in biological samples can be analyzed by AAS. These include estimated of sodium, calcium, potassium, zinc, iron, lead, mercury, etc.\

Pharmaceutical Analysis : Estimation of zinc in insulin preparation, calcium in calcium salt is done by using AAS.

Sodium, potassium, calcium in saline and ringer solution are estimated by AAS.

Analysis of ash for determining the content of sodium, potassium, calcium, magnesium and iron is done by AAS.