Tuesday, May 17, 2016

Spectrophotometry - Atomic Absorption Spectrophotometry

Atomic absorption spectroscopy (AAS) is similar to flame photometry with the difference that it measures the absorption of a beam of monochromatic light by the atoms in the flame. This technique was first introduced by Alan Walsh in Australia in 1954. We will discuss the principle, instrumentation and applications one by one.

The basic principle behind the AAS is that the free atoms normally remain in the ground state which are capable of absorbing the energy of their own specific resonance wavelength. If light of the resonance wavelength is passed through the flame containing the atoms (in sample), then part of the light will be absorbed. The atoms absorb UV or visible light and make the transitions to higher energy levels. The absorption will be directly proportional to the number of atoms in the ground state in the flame.

The major difference in the instrumentation of AAS and flame spectrophotometry is the presence of a radiation source (a particular resonance wavelength cannot be isolated from the continuous source using a prism or diffraction gratings). So, for this purpose, a hollow cathode lamp is used.

Light Source: (Hollow Cathode Discharge Lamp): It contains a tungsten anode and cathode (as can be seen in the diagram on the right) is a hollow cylindrical tube which is lined by the element to be determined. These are sealed in the glass tube filled with an inert gas like neon or argon at a low pressure. At the end of the cylinder is a window, made up of quartz or pyrex, transparent to the emitted radiation. Each element in question will thus emit monochromatic radiation characteristic of the emission spectrum of that particular element involved. So, each element has its own unique lamp which must be used for the analysis.

Nebulizer: It creates a fine spray of the sample for the introduction in the flame. The aerosol and the fuel and oxidant are mixed thoroughly for the introduction into the flame.

Atomizer: The elements which needs to be analysed needs to be in the atomic state. Here comes the role of atomizer. It breaks down the molecules into the atoms by exposing the analyte to high temperatures in a flame of graphite furnace (as explained in previous post, here).

Monochromator: A monochromator is used to select the specific wavelength of light which is absorbed by the sample and to exclude other wavelengths. The selection of the specific wavelength allows the determination of the element.

Detector: The light selected by the monochromator is directed onto the detector that typically is a photomultiplier tube that converts the light signal to electrical signal proportional to the light intensity.

Applications of Atomic Absorption Spectrometry
  • It is highly sensitive technique and can measure upto parts per billion of a gram (ugdm-3)
  • It is used to detect the presence of metals as impurity or in alloys.
  • The minute levels of the metals could be detected in biological samples like copper in the brain tissues.
  • The quantity of elements can be determined be agricultural and food products.
  • It can also be used to determine the impurity in the environmental water sources like in the ocean water, river and stream water, waste water, sludge and suspensions.