The theory of electromagnetic spectrum, absorption,

1. UV-VISIBLE SPECTROSCOPY Brief review of electromagnetic spectrum and absorption of radiation. The Chromophore concept, absorption law and limitation. By ARIJIT CHAKRABORTY M-PHARM (PHARMACOLOGY)

2. The theory of Electromagnetic spectrum, absorption, radiation

3. Electromagnetic spectrum: The arrangement of all types of electromagnetic radiations in order of their increasing wavelengths or decreasing frequencies is known as complete electromagnetic spectrum . If we arrange all types of electromagnetic radiations in order of their increasing wavelengths, then the portion above the visible region is called Infra-red while that below it is the ultra-violet region.

4. THE ELECTROMAGNETIC SPECTRUM

5. In a electromagnetic spectrum, we may note that : Visible and ultra-violet radiation cover the wavelength range from 200-800 mµ. The absorption of radiation in this region causes the excitation of π electron in a conjugated or an unconjugated system. The Infra-red radiations which cover the wavelength range from 0.8 to 2.5 µ constitute near Infra-red region and that from 15 to 25 µ is called far Infra-red region. The most useful region for Infra-red spectroscopy is 2.5 to 15 µ.

6. NMR spectroscopy provides a complete insight into the environment and arrangement of atoms within a molecule. For this technique, radiation of longest wavelength range, i.e. , Radio waves are useful. Due to the different environmental effects, different magnetic nuclei ( say protons, N 15 , C 13 , F 19 , P 31 atoms etc.) will feel the applied magnetic field differently. Hence, absorptions at different field strengths will correspond to different sets of protons or magnetic nuclei.

7. CHROMOPHORE CONCEPT

8. Chromophore: It is define as isolated covalently bonded group that shows a characteristic absorption in the ultraviolet or the visible region. There are two types of chromophores : Chromophore in which the group contains electrons and they undergo n–› π * transition. Such chromophores are ethylene's, acetylenes etc. Chromophore which contain both π electrons and n (non-bonding) electrons. Such chromophores undergo two types of transition i.e ., π–›π* and n–› π* .

9. There are some points: Spectrum consisting of a band near 300 mµ may contain two or three conjugated unites. Absorption bands near 300 mµ with very low intensity, € max 10-100 are due to n–› π* transitions of the carbonyl group. Simple conjugated chromophores such as dines have high € max values, i.e. , from 10,000 to 20,000. The absorption with € max value between 1000 to 10,000 shows an aerometric system.

10. ABSORPTION LAW AND LIMITATION

11. Lambert’s Law : The intensity of the transmitted light passing through a homogeneous medium decreases geometrically as the thickness of the layer increases arithmetically. Beer’s Law : Each molecule of solute absorbs the same fraction of light incident upon it, regardless of concentration in a non-absorbing medium . Beer’s law does not hold over the entire concentration range, but in very dilute solutions, as in uv-spectroscopy, the deviations are small.

12. BEER’S LAW: LIMITATIONS The linearity of the Beer-Lambert law is limited by chemical and instrumental factors. Causes of nonlinearity include: 1. Deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity . 2. Scattering of light due to particulates in the sample.

13. 3. Changes in refractive index at high analyte concentration. 4. Shifts in chemical equilibrium as a function of concentration. 5. Non-monochromatic radiation, deviations can be minimized by using a relatively flat part of the absorption spectrum such as the maximum of an absorption band.

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