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Absorption Spectroscopy

Spectroscopy is the study of matter using electromagnetic radiation. While this definition is nominally correct, it is rather simple. On this basis, one could argue that everything we know about the universe comes from spectroscopy, since much of we have learned comes from what we see in the world around us. But simply looking at a picture or painting is not usually "spectroscopy", even though the action might involve studying a piece of matter in broad daylight. 



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Spectroscopy is generally a practical subject and is concerned with the emission, absorption and scattering of electromagnetic radiation by molecules or atoms. Electromagnetic radiation includes a wide wavelength range, from radio waves to gamma rays, and the molecules or atoms may be in the liquid, solid phase, gas or, of great importance in surface chemistry, adsorbed on a solid surface. 

The greater the number of molecules capable of absorbing light of a given wavelength, the greater the extent of light absorption. Furthermore, the more effectively a molecule absorbs light of a given wavelength, the greater the extent of light absorption.
In 1665 Newton had started his famous experiments on the dispersion of white light into a range of colors using a triangular glass prism. Early applications were the observation of the emission spectra of various samples in a flame, the origin of flame tests for various elements, and of the sun.

How does Absorption Spectroscopy Work?

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Atomic absorption takes place when the transition of molecules from  lower energy state to higher energy states. The population of lower level determines the degree of absorption. The excited level population is very small compared to the lower energy state or ground level. More absorption lines are produced from the ground states, these are known as resonance lines. The amount of atomic absorption seen using a continuum source, such as is used in molecular absorption spectroscopy, is negligible. 
In this case, the monochromator only has to isolate the line of interest from other lines in the lamp. The atomic absorption and emission lines are overlap each other because these are have same wavelength. The uniqueness of overlaps in the Walsh method is often known as the ' lock and key effect' and is responsible for the very high selectivity enjoyed by atomic absorption spectroscopy.

Types of Absorption Spectroscopy

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Absorption spectroscopy uses the range of electromagnetic spectra in which a substance absorbs the EMR. The technique which deals with the study of absorption of electromagnetic radiation by matter is called absorption spectroscopy. In this spectroscopy, the intensity of a beam of light measured before and after interaction with a sample, that is, intensity of incident and transmitted radiations is compared. 

In atomic absorption spectroscopy, the sample is atomized and then light of particular frequency is passed through the sample's vapor. After calibration, the amount of absorption can be related to the concentrations of various metal ions through the Beer-Lambert law. The method can be automated and is widely used to measure concentrations of ions such as sodium, potassium and calcium in blood and urine. On the basis of the wavelength range of incident beam: The spectroscopy can be classified into:
  • Infrared spectroscopy
  • Near infrared spectroscopy
  • Microwave spectroscopy
  • UV-visible spectroscopy
The absorption of radiations by any sample is governed by the Beer-Lambert law. 

Absorption Spectroscopy Applications

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Absorption spectroscopy is probably one of the most widely used analytical tools in physics, chemistry, and industry. Infrared and far-infrared spectroscopy are commonly used for gas analysis and identification of chemical structures, while visible and ultraviolet spectroscopy are extensively used for quantitative analysis of atoms, ions, and chemical species in solution. 
In addition to the characteristic frequencies absorbed by a given chemical species, another important characteristic is the absorption strength or coefficient. In conventional absorption spectroscopy, a cell containing the material under investigation is placed between a source of continuous radiation and the spectrometer. Radiation from the source coincident with frequencies at which the substance absorbs is removed from the continuum, producing the substance's absorption spectrum. As radiation of a given frequency from a source traverses a homogeneous absorbing medium, under certain conditions it is reduced in intensity by the same fractional amount through each succeeding unit length of path, given by the exponential relationship commonly known as Lambert's law. It is well known that the low brightness of blackbody light source and the attainable dispersion of monochromators impose limits on the resolution and sensitivity that can be achieved by conventional absorption spectroscopy.
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