Atomic Absorption Spectroscopy

Atomic absorption spectroscopy (AA or AAS) is one of the commonest instrumental methods for analyzing metals and some metalloids.

Atomic absorption spectroscopy (AAS)  (Source: Wikipedia)

Atomic absorption is a process involving the absorption of light by free atoms of an element at a wavelength specific to that element. In various phenomena of Atomic Spectroscopy (emission, absorption and fluorescence), energy is supplied to the atoms in the form of thermal, electromagnetic, chemical or electrical energy which is converted to light energy by various atomic and electronic processes before measurement. Atomic Absorption Spectrometry is beneficial not just for the identification but also for quantitative determination of the many elements present in samples. The technique is specific and sensitive in that individual elements in each sample can be dependably identified in small amounts.

Atomic absorption spectroscopy is a spectro analytical procedure for the quantitative determination of chemical elements using the absorption of optical radiation by free atoms in the gaseous state.

It detects the concentration of elements present in the sample.

History

This technique was introduced for analytical purpose by Walsh and Alkemade, under the designation Atomic absorption spectroscopy. He was a CSIRO scientist. It is found to be superior to other techniques as it can be used to determine elements from trace to large quantities.

Principle

This technique involves the study of absorption of radiation by neutral atoms in the gaseous state. Thus in AAS, the sample is first converted into an atomic vapor, and absorption of atomic vapor is measured at a selected wavelength, which is characteristics of each element.

So, when a light of a particular wavelength is allowed to pass through a flame having atoms of the metallic species, part of that light will be absorbed and the absorption will be directly proportional to the density of atoms present in the flame.

As the number of atoms in the light pass increase, the amount of light adsorbed also increases. By measuring the amount of light absorbed a quantitative determination of the amount of analyte element present can be made.

The principle is based on the Beer- Lambert’s law. The absorption of radiation by the free atoms is proportional to their concentration.

Absorbance = log10 Io/It=KCL

Where,
Io = Intensity of incident radiation
It = Intensity of transmitted radiation
C = Concentration of analyte
K = Constant
L = Path length

Instrumentation

• Radiation source
• Sample insertion
• Monochromator
• Photomultiplier tube
• Recording system

Radiation Source

• Electrode less discharge lamp
• Hollow cathode lamp
• Deuterium lamp

The common source of light is a Hollow Cathode Lamp (HCL). This contains a tungsten anode and a cylindrical hollow cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas e.g., neon or argon at a pressure of between 1 N/m2 and 5 N/m2 .

Sample Insertion

• Solid Sample

Introduction methods are laser stabilization methods in which high-intensity beam fall on the sample and they sublimate in small particles. With the help of inert gas, it reaches the atomizer.

• Liquid Sample

This method nebulizer is used sample concerts into tiny droplets and mix with fuel and gas and goes in the atomizer.

Atomizer

Nebulization is the mechanism by which the sample solution is introduced as fine spray into the flame. Nebulization refers to the dispersion of a liquid into particles by a rapidly moving gas, liquid stream or by mechanical means. This process is immediately followed by atomization, wherein high energy like that of a flame converts molecules into atoms.

It forms atoms, flame atomizers are used most of the time.

In the atomizer five things happen:

• Dissolvation
• Volatilization
• Dissociation
• Ionization
• Excitation

Monochromator

It is important that the instrument be capable of providing a narrow band width to separate the line chosen for determination from other undesirable lines. Usually used devices are gratings or prisms.

It separate the special line of interest from other spectral lines with the different wavelength emitted by the hollow cathode lamp.

There are two types of AAS:

• Single Beam

The instrument represents a fully functional single beam AAS. It is called single beam because all measurements are based on the intensity of a single beam of light in a single optical path.

• Double Beam

There are two beam one is sample beam which is directed through the sample cell and another is the reference beam which is directed around the sample.

Detector

Photomultipliers are commonly used as detectors. In some instruments additional filters and detectors are used to compensate for the fluctuations in the output of the source. The output of photomultiplier is amplified which helps in source modification.

Photo Multiplier Tube

Photo multiplier tube is generally used in spectroscopy. It consists of an evacuated tube containing one photocathode and 9 to 16 electrodes known as dynodes. Photocathode is negatively charged and then a photon hits the photocathode. Emission of electrodes takes place due to photovoltaic effect. When incident radiation falls on the metal surface of cathode and get emitted which are attracted towards the first dynode. These with are attracted by the second dynode and with are emitted by second dynode. Hence, the process is repeated at all the dynodes. At the end of dynode chain and anode is present which acts as collector of electrons. The current flowing from anode is directly proportional to photo electron flux generated by photo cathode.

Recording System

Here we can see the graph or spectrum and analyze the given sample.

Applications

Atomic absorption spectrometry has many uses in different areas of chemistry. The different methods of Atomic absorption spectrometry are very powerful for analysis elements in a solution. The instruments are simple and easy to operate. They are useful when few elements have to be determined in a large number of samples, as is the case in clinical or food analysis. Atomic absorption spectroscopy methods are of great importance compared with other methods of elemental analysis.

• Detection of trace elements
• Biological sample profiling
• Determination of mode of poison, heavy metals poisoning.