Absorption spectrum

Absorption Spectrum or Absorption spectra

What Is An Absorption Spectrum?

• The absorption spectrum is the spectrum of radiation emitted by a substance that has absorbed the radiation.
• Whenever electromagnetic radiation interacts with atoms, if their electrons at the ground level jump to an immediate higher energy level, absorbing energy will result in a continuous spectrum with dark lines, which is known as the absorption spectrum.
• Different pigments absorb light of different wavelengths.
  For example, chlorophyll effectively absorbs blue and red.
• The absorption spectrum of a pigment is produced by examining, through the pigment and an instrument called a spectroscope, a continuous spectrum of radiation. The energies removed from the continuous spectrum by the absorbing pigment show up as black lines or bands and can be graphed.
• An absorption spectrum is the opposite of an emission spectrum. It is a continuum of radiation passed through a sample that absorbs radiation of certain wavelengths. The missing wavelength which corresponds to the radiation absorbed by the matter leaves dark spaces in the bright continuous spectrum.

What is a spectroscope?

• A spectroscope is an instrument designed for the visual observation of spectra, and an instrument that photographs or maps spectra is a spectrograph.
• It is used to observe the spectrum of electromagnetic radiation emitted by an object.
• A spectroscope gives useful information, but it is only temporary. To capture spectroscopic information permanently, the spectrograph was developed. The working principle of the spectrograph is the same as the spectroscope, but it contains some means to permanently capture an image of the spectrum.
• To know about the working of spectroscope and spectrograph you can read this linked article from Hubblesite – SPECTROSCOPY: READING THE RAINBOW
  
• In brief, a Spectrograph is an instrument used for dispersing light into a spectrum of wavelengths and then photographing that spectrum.

Formation of the Absorption spectrum.

• When light from any source is passed through a chemical solution, then a pattern comprising of dark lines(absence of light) is observed. This pattern is analyzed using the spectroscope. Depending on the nature of the chemical or element, certain radiation is absorbed by the solution or atom when passed through it.
• The presence of a dark line in the pattern shows the absorption of radiation by the solution or atom. The spectrum thus obtained is known as the absorption spectrum.
• The emission spectrum, unlike the absorption spectrum, emits all the colors in an electromagnetic spectrum, whereas few colors in the absorption spectrum may be absent due to the redirection of absorbed photons.

Absorption Spectroscopy

• Branch of science that deals with the study of the absorption of the spectrum are known as absorption spectroscopy.
• Absorption spectroscopy is the spectroscopic technique that measures the absorption of radiation.
• It is used to determine if a particular substance is present in a sample and often also quantifies how much of the substance is present.
• Near-infrared (NIR) and Ultraviolet-visible (UV-VIS) spectroscopy are particularly common in these kinds of analytical applications.
• Near-infrared (NIR) spectroscopy is based on the absorption of electromagnetic (EM) radiation at wavelengths in the range of 780 to 2,500 nm. The light interacts with the sample and the detector measures its transmittance and absorbance.
  
• Ultraviolet-visible (UV/Vis) spectroscopy is based on the absorption of the electromagnetic radiation in the UV/Vis region, with the wavelength ranges of 200–400 nm, called ‘ultraviolet spectroscopy,’ and 400–800 nm, called ‘visible spectroscopy.
  

Application of Absorption Spectroscopy

1. Chemical Analysis

• The numerical nature of absorption spectroscopy makes it a perfect choice for chemical analysis. The absorption spectra of the different compounds can be differentiated from one another using absorption spectroscopy. This is only possible because of the specificity nature of the absorption spectrum.

2. Quantitative Analysis

• UV absorption spectroscopy can be used for the quantitative determination of compounds that absorb UV radiation. This determination is based on Beer’s law which is as follows.
  A = log I0 / It = log 1/ T = – log T = abc = εbc
  
  Where ε is extinction coefficient, c is concentration, and b is the length of the cell that is used in the UV spectrophotometer.
• Other methods for quantitative analysis are as follows.
  a. Calibration curve method.
  b. Simultaneous multicomponent method.
  c. Difference spectrophotometric method.
  d. Derivative spectrophotometric method.

3. Detecting Pollutants

• Absorption spectroscopy is an infrared gas analyzer that is often used for detecting the pollutants present in the air or atmosphere. It also helps to distinguish between pollutants from nitrogen, oxygen, etc.
• Air is fundamental for every living organism on Earth, and the quality of the atmospheric environment is closely related to the activities performed by humans. After the development of modern industrialization and the improvement of human living standards, combustion of fossil fuels and emission of automobile exhaust have caused severe atmospheric pollution, which has raised more concern in recent years. In order to monitor the atmospheric quality Fourier-transform infrared (FTIR) spectroscopy is used.
  
  You can read about it in this linked article.
  Must Read – Rapid Identification of Atmospheric Gaseous Pollutants Using Fourier-Transform Infrared Spectroscopy.

4. Remote Sensing

• Absorption spectroscopy is also used for analytical purposes such as measuring the presence of dangerous elements.
• Satellite remote sensing, covering large and remote areas, is considered useful for detecting and monitoring marine pollution.
  Recommended Read – Detection and Monitoring of Marine Pollution Using Remote Sensing Technologies
  

5. Chemical Kinetics

• Kinetics of reaction can also be studied using UV spectroscopy. The UV radiation is passed through the reaction cell and the absorbance changes can be observed.

• The study of chemical kinetics provides important information on the rate and mechanism of the chemical reactions that occur all around us.
  Check this article to know-how spectroscopy is used to study Chemical Kinetics.
  Recommended Read – Kinetics Measurements with High Sensitivity Spectroscopy
  

Emission vs Absorption Spectrum Difference

The main difference between emission and absorption spectra is that an emission Spectra consist of different colored lines, while, absorption spectra consist of dark-colored lines in their spectrum. Other differences between absorption and emission spectrum are mentioned below.

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