Spectroscopy,Fundamentals of Spectroscopy and Optical Methods of Analysis

Optical Methods of Analysis -

• Origin of Spectra
• Interaction of radiation with matter
• Fundamental laws of spectroscopy and selection rules
• Validity of Beer-Lambert’s law

WHAT IS SPECTROSCOPY?

• Atoms and molecules interact with electromagnetic radiation (EMR) in a wide variety of ways.
• Atoms and molecules may absorb and/or emit EMR.
• Absorption of EMR stimulates different types of motion in atoms or molecules.
• The patterns of absorption (wavelengths absorbed) and/or emission (wavelengths emitted) are called spectra.
• The field of spectroscopy is concerned with the interpretation of spectra in terms of atomic and molecular structure (and environment).

Fundamentals of Spectroscopy

Spectroscopy deals with interactions of matter and electromagnetic radiations.

There are various forms of electromagnetic radiations such as UV, IR….. Etc.

1. Produced by the oscillation of electric charge and magnetic field residing on the atom
2. Characterized by their λ, ν and wavenumber
3. The energy of electromagnetic radiation, E α ν,E = hν i.e. photon
4.  All radiations travel with the same velocity
5.  When visible light is passed through a prism, it is splitted up into 7 colours (i. e. dispersion of light)
6. The group of electromagnetic radiations can be splitted up into various components for analysis
7. Absorbance and emission spectra
8.  When radiations of certain λ are passed through the substance under analysis, the radiations of certain λ are absorbed by the substance. This is due to the presence of specific functional group
9.  If the light is passed through a prism, a spectrum is obtained which is calledan Absorption Spectra.
11. As a result of energy absorption, atoms/molecules pass from the ground state (low energy) to the excited state (high energy) during the excitation process, when its is quantized.
12. The electromagnetic radiation that is absorbed has energy exactly equal to the difference between the excited and ground states . 

Spectrophotometry: 

A method which uses light source to measure the concentrations i.e. the quantitative measurement of the intensity of electromagnetic radiation at one/more λ with photodetector.  

Absorption of light:

                  The energy of molecule increases.

                                            (Excited State)

Emission of light:

                  The energy of molecule decreases.

                                             (Ground State)

Absorption

When light is absorbed by a sample, the radiant power of the beam is decreased.

                  A = log I0/I = -logT

       I0 :  initial intensity

       I   :  the intensity after passing through thickness

       T  : Transmittance, the fraction of incident light

                which is transmitted.

The obtained spectra are called absorption spectra.