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  Spectroscopic Techniques, Chemistry tutorial

Introduction

Spectroscopic method of analysis involves the measurements of the intensity and wavelength of radiation that is either absorbed or transmitted. This provides the basis for sensitive methods of detection and quantization. Absorption spectroscopy is most frequently used in the quantization or estimation of molecules and some atoms. Emission spectroscopy covers several techniques that involve the emission of radiation by either atoms or molecules, but varies in the manner in which the emission is induced. This technique of analysis involves the employ of equipment that might be easy and inexpensive, or enormously complex, through design features involving the latest technological development.

Definition of spectroscopy

Spectroscopy is a technique that concerned through the study of the frequencies engaged when electromagnetic radiation relates through matter. 

Interaction of radiation and matter

Radiation is a form of energy that could either be absorbed or transmitted. The interaction of radiation in spectroscopy involves transition between the dissimilar energy levels of atoms or molecules. The other types of interactions these as reflection, refraction and diffraction, are frequently related to the bulky properties of substances rather than to energy levels of precise atoms or molecules. Usually, the absorption or emission of radiation via matter involves the swap of energy. Therefore, to understand the principle of this exchange, it is needed to identify the distribution of energy inside an atom or molecule. The internal energy of a molecule is due to the energy connected by:

  • The electrons
  • Vibrations between atoms
  • The rotation of different groups of atoms inside a molecule

The energy levels can be altered through the absorption or emission of radiant energy. This is since atoms subsist only within a limited number of energy levels. A study of the wavelength or frequency of radiation absorbed or emitted via an atom or molecule will provide information about its identity. This method is recognized as qualitative spectroscopy.

Electromagnetic radiation

Radiation is a form of energy that has both magnetic and electrical properties, therefore termed electromagnetic radiation. The electromagnetic radiation wrapped an extensive range of radiations that are explained or characterized through either wavelength or frequency. 

  • Wavelength (λ) is defined as the distance between the successive peaks that is computed in the unit nanometers (nm).
  • Frequency (μ) of radiation is described as the number of successive peaks passing a following point in 1 second.

The relationship between the 2 parameters is:

  • V α 1/ λ
  • But, the energy (E) is directly proportional to the frequency and inversely proportional to wavelength.

i.e.   E = hv= hc/ λ

  • Where, h is a plank's constant and c is the speed of light

The electromagnetic radiation consists of:

ü  Gamma rays (γ ) in the range of 10-3 to 10-1nm

ü  X-rays in the range of 1 to 103 A0

ü  Ultraviolet radiation in the range of 180 to 380nm

ü  Visible region in the range of 380 to 780nm

ü  Infra red region in the range of 0.78um to 50um

ü  Microwave in the range of 10-2 to 10cm

ü  Radio wave in the range of 107 to 109nm

This can as well be symbolized as bellow:

2448_Range of electromagnetic radiation.jpg

Fig: Range of electromagnetic radiation

Absorption of radiation

Every molecular specie is capable of absorbing its own trait frequencies or wavelength of electromagnetic radiation. This procedure transfer's energy to the molecule or atom and consequences in reduce in the intensity of the incident radiation. Therefore, absorption of radiation could be:

  • Atomic absorption that engages the absorption of electromagnetic radiation through free atomic species. The wavelength that is most powerfully absorbed generally corresponds to an electronic conversion from the ground state to the lowest excited state identified as the resonance line.
  • Molecular absorption that involves the absorption of radiation via molecules in solution. This kind of absorption generally occurs in ultraviolet and observable region of the electromagnetic spectrum. Absorption of radiation in this region reasons conversion of electrons from molecular bonding orbital to the elevated energy molecular ant bonding orbital.

Emission of radiation

When an atom or molecule absorbs electromagnetic radiation, it will consequence in either electronic or vibrational transition from lower to higher energy level. These atom or molecule is said to be in an excited or unbalanced state. If the atom or molecule loses all or part of this energy as radiation, photons of energy will be released that correspond to the dissimilarity between the energy levels engaged. The radiation emitted is of specific frequencies and will show up as a bright line if dispersed as a spectrum. Just like absorption, emission could as well be atomic or molecular.

Atomic emission is displayed via many elements; particularly the metals that after being excited either thermally or electrically emit a discontinuous lines spectrum due to transition ending in the ground state.

Molecular emissions are due to electronic transition inside the molecule but are modified or influenced through bond length. Molecular emissions are more complex than atomic emissions. The radiation emitted consists of broad bands of radiation rather than the narrow lines connected through atomic emission. As well the consequential spectrum is approximately the mirror image of the absorption spectrum of the compound.

Types of spectroscopy

Depend on the nature of the radiation that is being absorbed or emitted; there are different kinds of spectroscopy. Such could be:

ü  Ultraviolet/visible spectroscopy

ü  Infra-red (I R) spectroscopy

ü  Nuclear Magnetic Resonance (NMR) spectroscopy

ü  Mass spectroscopy

ü  X- ray spectroscopy

Apart from the above classification, spectroscopy can as well be largely classified into 3 categories:

  • Electronic spectroscopy
  • Vibrational spectroscopy
  • Rotational spectroscopy

The electronic spectroscopy engages the movement of electrons between their diverse energy levels, as a consequence of absorption or release of electromagnetic radiation. The energy needed for electronic transition comes from ultraviolet or visible region.

Vibrational spectroscopy is concerned through the vibrations of molecules whenever photons of radiation are absorbed. Molecules can vibrate by particular amounts of energy whose values are computed through a quantum number. The energy required for vibration of molecule comes from the infra red region.

Rotational spectroscopy is concerned by the energy that reasons the rotation of a molecule. The energy of the rotational level based on the rotational quantum numbers. The energy needed for the molecular rotation is supplied through the micro wave region of the spectrum.

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