Atomic Emission Spectroscopy:
Atomic emission spectroscopy pertains to electronic transitions in atoms which use and an excitation source like flames sparks. Emission spectroscopy is related to atoms. Emission spectroscopy is concerned with the characteristic radiation produced when atoms are excited. They emit radiation in the form of discrete wavelengths of light, called spectral lines while returning to the lower energy states.
The source vaporizes the sample and causes the electronic excitation of elementary particles in the gas. Excited molecules in the gas phase emit band spectra. Thus, a molecule in an excited state of energy, E2 undergoes a transition to a state of lower energy E1 and a photon of energy hѵ is emitted where
E2 ─ E1 = hѵ
In each electronic state, a molecule may exist in a number of vibrational and rotational states of different energies.
Types of Spectrum
Spectrum may be classified ad;
1. Continuous Emission Spectrum.
2. Line or Atomic Emission Spectrum
The approximate wavelengths of visible radiation of different colors can observed in table 24.4
Atomic or Line Emission Spectrum
When an element is vaporized in a flame, or in an electric are or in a discharge tube, it emits a light of characteristic color. The resolution of the ray of this light produces colored lines separated by dark space. This type of separation is called a line emission spectrum.
Each element has its own characteristic color by which it can be identified i.e.,
Na gives yellow coloring bunsen flame.
Sr gives red color in the bunsan flame.
K gives a violet color in the bunsan flame.
Similarly in a discharged tube;
Ne glows with orange-red color.
He glows with orange-pink color.
H2 glows with orange-red and blue color.
Cℓ2 glows orange green color.
The line in the spectrum of an element is not haphazardly distributed but they occur in a group or series. In series, the separation between them decreases regularly as their wavelength decreases. At a certain limiting value, the spectrum becomes continuous.
Advantages of Emission Spectroscopy:
Emission method is extremely important in the analysis
- This technique is highly specific
- This method is extremely sensitive. With this technique, all metallic elements can be detected even if they are present in very low concentration.
- Even metalloids have been identified by this technique.
- This analysis can be performed either in a solid or liquid state with almost equal convenience.
- The technique requires minimum sample preparation as a sample can be directly introduced into the spark.
- The technique provides results very rapidly. if automated, the time required is just 30 sec to one minute.
- This method has been used for a wide variety of samples like metals, alloys, paints, geological specimens, environmental and biological samples.
Disadvantages of Emission Spectroscopy:
- The equipment is costly and wide experience is required for its successful handling and interpretation of spectra.
- The recording is done on a photographic plate which takes some time to develop, print and interpret the result.
- Radiation intensities are not always reproducible.
- Relative error exceeds 1 to 2%.
- The accuracy and precision are not high.
Emission spectroscopy has been employed in determining the impurities of Ni, Mn, Cr, Si, Aℓ, Mg, As Sn, Co, V, Pb, Bi, P and Mo in iron and steel in metallurgical processes.
Alloys of Zn, Cu, Pb, Aℓ, Mg, and Sn have been analyzed.
Lubricant oils have been analyzed for Ni, Fe, Cr, Mn, Si, Aℓ, and so on. If the concentration of metal in lubricating oil has increased during use, it indicates excessive wear and tear need for an engine overhaul.
In petroleum, industry oil is analyzed for V, Ni, Fe the presence of which makes fuel poor.
Solid samples and animal tissues have been analyzed for several elements including K, Na, Ca, Zn, Ni, Fe, and Mg, etc.
Emission spectroscopy has been used to detect 40 elements in plants and soil. Thus a metal deficiency in pants and oil can be diagnosed.
The following materials have been analyzed by emission spectroscopy:
- Trace and major constituents in ceramics.
- Traces of Co, Ni, Mo, and V in Graphite.
- Trace metal impurities in analytical reagents.
- Trace of Ca, Cu, Zn in blood.
- Zinc in pancreatic tissues.