Principle of UV - Vis Spectroscopy

Spectroscopy and UV-Vis Absorption Variant

Spectroscopy is known as the measurement and interpretation of electromagnetic radiation emitted or absorbed, when ions, atoms or molecules of a sample move from one energy state to another energy state.

Ultraviolet-Visible spectroscopy UV-Vis spectroscopy is a type of absorption spectroscopy, which utilizes the radiation in the UV range and adjacent visible range of the electromagnetic radiation spectrum. The absorption wavelength mainly ranges from 100 - 700 nm. 

Principle of UV-Vis Spectroscopy

• Spectroscopy is defined as the interaction of light with matter.
• When light is absorbed by matter, there will be an increase in the energy content of molecules or atoms.
• The absorption spectrum of a material is dependent on the molecular and atomic composition of the material.
• The frequency of light absorbed by the material depends on the energy difference between two energy states of molecules.
• When radiation is absorbed, there will be excitation of electrons from the ground state to a higher energy state.
• The absorption leads to the formation of an absorption line, which along with the other lines, form an absorption spectrum.
• The incident light of the spectrophotometer is in the range of visible and UV spectra of the electromagnetic radiation. 
• When a photon having sufficient energy reaches an object, the object receives and absorbs the energy, which allows the electron to attain a higher energy state.
• The amount of radiation or photons absorbed results in the formation of the absorption spectrum, which will be measured in terms of absorbance.
• The absorbance of a compound depends on the number of excited electrons from the ground state, which is dependent on the concentration or number of molecules in the sample.
• The absorbance of radiation by a compound produces a distinct spectrum, which is helpful in serving as a marker or identifier of the compound.
• The easier the electrons get excited, the longer the wavelength of the light, the compound can absorb.
• Four possible types of interactions or transitions can be observed. They are (π–π*, n–π*, σ–σ*, and n–σ*). The order of the interaction according to the energy is as follows: σ–σ* > n–σ* > π–π* > n–π*.
• The spectrum arises from electron transition in a molecule from a lower level to a higher level.
• In the visible range, the energy of the radiation is 36-72 kcal/mol, but in the UV range, the energy of radiation is higher than 143 kcal/mol.
• A spectrophotometer is able to record the amount of absorption by a sample at different wavelengths in the UV and visible wavelength range.
• The plot of absorbance (A) versus wavelength (ƛ) is called a spectrum. 
• Transitions between electronic energy levels are responsible for the absorption of electromagnetic radiation in the 200-900 nm region of the spectrum.
• During excitation, the atoms in the bond of molecules merge to form molecular orbitals, which are occupied by electrons of different energy levels. The electrons get excited from the highest occupied molecular orbital (HOMO) to the lowest occupied molecular orbital (LOMO). The resulting spot is called the excited state or antibonding state.
• σ–σ*  transition is a transition that needs larger energy to take place. An example is methane, having a single bond C-H. and exhibits an absorption maximum at 125 nm. 
• n–σ*  transition requires lesser energy than the σ–σ* transition. Absorption peaks occur below 200 nm, Saturated compounds having atoms with unshared electrons undergo this transition. Examples are water and ethanol.
• π–π* and n–π* transitions require unsaturated functional groups to occur. Alkene and alkynes are examples of π–π* transition and carbonyl compounds are examples of  n–π* transition. 

Absorbance Laws

There are two absorbance laws, related to the principle of uv-vis spectroscopy, namely, Beer law and Lambert law.

Beer Law

The intensity of a beam of monochromatic light decreases exponentially with increase in concentration of absorbing substance. But the law is applicable to dilute solutions only.

Lambert Law

When a beam of light passes through a transparent medium, the rate of decrease in intensity with thickness of medium is directly proportional to the intensity of light. The degree of the interactions is dependent on the contraction, there is deviation from the linear relationship between contraction and absorbance. 

Limitation in Laws

• Scattering and reflection can modify the absorption reported.
• Reaction with the solvent
• High concentration affects charge distribution, the average distance between ion decreasing, making particles close to each other.
• Presence of stray light

Conclusion

The difference in energies between the ground state and excited state of an electron is always equal to the amount of radiation absorbed by it. The energy absorbed appears as heat in the solution. Thus, the above paragraphs have shed light on knowledge in the principle of UV-Vis spectroscopy.