Atomic emission spectroscopy is the study of the electromagnetic radiations emitted from matter when excited by an excitation sources.
Atomic emission spectroscopy is also known as flame photometry because of the use of flame to provide the energy of excitation to atoms introduced into the flame. Atomic emission spectroscopy is based on the measurement of intensity of the light emitted when a metal is introduced into the source. The wavelength of the colour tells us what the element is, and therefore the colour’s intensity tells us what proportion of the element is present.
PRINCIPLE OF ATOMIC EMISSION SPECTROSCOPY
When a liquid sample containing a metallic salt solution is introduced into the source, the process involved in this is very complex, thus the following events take place:
- A solvent is vaporized, leaving particles of the solid salt
- The salt is vaporized or converted into the vaporised state
- A part or all of the gaseous molecules are progressively segregate to give free metals or radicals. These neutral atoms are excited by thermal energy of the radiation source. The excited atoms, which are unstable, quickly emit photons and return to the ground state, eventually reaching the unexcited state. The measurement of the emitted photons i.e. radiations forms the basis of AES.
INSTRUMENTATION OF AES
The instrumentation of atomic emission spectroscopy is same as that of the atomic absorption spectroscopy. The block diagram involves the systematic instrumentation involved in AES:
- Sampling devices and excitation source: there are a number of different types of excitation source used in atomic emission spectroscopy. The sampling device and source depends on the type of sample and the analytical data desired. Not every single source is best for all applications of AES. In general, for solid samples arc excitation is more sensitive, while spark sources are more stable. Plasma sources are the choice for solutions and for gaseous samples; their sensitivity enables trace analyses to be carried out at the parts per billion levels. The various sampling techniques are: electrical discharges, direct – current arcs, alternating current arcs, high voltage alternating- current sparks etc.
2. Mirrors: the radiation from the source/ flame is emitted in all directions in space. Much of the sample radiation is lost, and loss of signal results. In order to minimize the amount of radiation used in the analysis, a mirror is located behind the source or burner to reflect the radiation back to the entrance slit of the monochromator.
3. Slits: the entrance slit cuts out most of the radiation emitted from the surroundings and allows only the radiation from the source. The exit slit is placed after the monochromator and allows only a specific wavelength range to undergo detector. For many purposes it is essential that this wavelength range may be very narrow, that is of the order of a few nano meters.
4. Monochromators: in this monochromator is a simple prism. But in expensive models, gratings are used. Quartz is the material most commonly used for making prisms even though its dispersive power is less than that of the glass.
5. Filters: in some elements, the emission spectrum contains a few lines. In such cases, wide wavelength ranges are going to allowed entering the detector without causing any serious error. When a filter is placed between the excitation source and detector, the radiation of desired wavelength from the sample will be entering the detector and e measured. The remaining undesired wavelength will be absorbed by the filter and not measured. In such a situation, an optical filter may be used in place of the slit and monochromator system. The filter is formed such a material which is transparent over a narrow spectral range.
6. Detectors: the radiation coming from the optical system is allowed to fall on the detector which measures the intensity of radiation falling on it. The detector should be sensitive to radiation of all wavelengths which will be examined. The most commonly used detector in AES are photo multiplier tube, photographic plate.
APPLICATION OF AES:
AES can be used in the following:
- Analysis of ferrous and non-ferrous alloys
- Determination of metal impurities in alloys, metals, reagents and solvents.
- Analysis of metal in geological, environmental, and biological materials.
- Water analysis ICP instruments have been coupled with mass spectrometers to provide powerful analytical techniques.
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