Ir spectroscopy instrumentation, b y -dr. umesh kumar sharma and arathy s a

IR- Spectroscopy Instrumentation, by -Dr. Umesh Kumar Sharma and Arathy S A

1. INFRA – RED SPECTROSCOPY INSTRUMENTATION 1 By: Dr. Umesh Kumar Sharma & Arathy .S .A Department Of Pharmaceutics, Mar Dioscorus College Of Pharmacy, Alathara , Sreekaryam , Thiruvananthapuram

2. Contents Instrumentation of Dispersive and Fourier Transform Infra-Red Spectrophotometer. Factors affecting Vibrational Frequencies. Application of Infra-Red Spectrophotometer. 2

3. INSTRUMENTATION The usual optical materials, glass or quartz absorb strongly in the infrared region. The apparatus for measuring infrared spectra is appreciably different from that for the visible and ultraviolet regions. The main parts of an IR spectrometer are: IR radiation sources, Monochromators , Sample cells and sampling of substance, Detectors, 3

4. 4 The sources of IR radiation are : a. Incandescent lamp In the near IR instruments an ordinary incandescent lamp is generally used . This fails in the far IR because it is glass enclosed and has a low spectral emissivity. Incandescent Lamp IR radiation sources : Requirements: A source of radiant energy which provides a means for isolating narrow, frequency bands. Intense enough for detection. Steady. Extend over the desired wavelengths.

5. 5 b. Nernst glower It consists of a hollow rod, composed of rare earth oxides such as zirconia, yttria and thoria . It is non conducting at room temperature and must be heated by external means to bring it to a conducting state. Glower is generally heated to a temperature between 1000 to 1800 c. Maximum radiation is about 7100cm -1 . Disadvantage of Nernst glower is its frequent mechanical failure. Nernst Glower

6. 6 Globar source It is a rod of sintered silicon carbide. It is heated to a temperature between 1300 and 1700 c. It emits maximum radiation at 5200 cm -1 . The main disadvantage is, it is a less intense source than the Nernst glower. Globar Source

7. 7 d . Mercury arc lamp In the far IR region the sources described above lose their effectiveness and special high pressure mercury arc lamps are used . Mercury Arc This device consists of a quartz-jacketed tube containing mercury vapor at a pressure greater than 1 atmosphere.

8. 8 Monochromators : The radiation source emits radiation of various frequencies. The sample in IR spectroscopy absorbs only at certain frequencies, it is therefore becomes necessary to select desired frequencies from the radiation source and reject the radiation of other frequencies. This selection has been achieved by means of monochromators which are mainly of two types- 1. Prism monochromators 2. Grating monochromators

9. 9 Prism monochromator Single pass monochromator :

10. 10 Double pass monochromator :

11. 11 Grating monochromators :

12. Sample cells and sampling of substances: IR spectroscopy is used for characterization of soild , liquid and gas samples. The only common point to the sampling of different phases have to be treated differently. This condition restricts selection to only certain salts like Nacl or KBr . Detectors The various types of detectors used in IR spectroscopy are Bolometers Thermocouple Thermistors Golay cell Photoconductivity cell 12

13. Bolometers: It consist of a thin metal conductor. When radiation falls on this conductor, its temperature changes, as the resistance of a metallic conductor changes with temperature. T he degree of change in resistance is regarded as a measure of the amount of radiation that has fallen on the bolometer. It is made one arm of the W heatstone bridge. When no radiation falls on the bolometers, the bridge remains balanced. If radiation falls on the bolometers , the bridge become unbalanced due to changes in the electrical resistance which causes a current to flow through the galvanometer. 13 BOLOMETER

14. 14 Thermocouple: It is based upon the fact that an electric current will flow when two dissimilar metal wires are connected together at both ends and a temperature differential exits between the two ends . The end exposed to the IR is called the hot junction. In order to increase the energy gathering efficiency it is usually a black body. The other connection the cold junction is thermally insulated and carefully screened from stray light. The electricity which flows is directly proportional to the energy differential between the two connections. THERMOCOUPLE

15. Thermistors: A thermistor is made of a fused mixture of metal oxides. As the temperature of the mixture increases, its electrical resistance decreases. 15 THERMISTORS

16. 16 Golay cell: It consists of a small metal cylinder which is closed by a blackened metal plate at one end and by a flexible metalized diaphragm at the other. After filling the cylinder with xenon, it is sealed. When IR radiation is allowed to fall on the blackened metal plate, it heats the gas which causes it to expand. The signal seen by the phototube is modulated in accordance with the power of the radiant beam incident on the gas cell. Golay cell:

17. 17 Photoconductivity cell: This is a nonthermal detector of greater sensitivity. It consists of a thin layer of lead sulphide supported on glass and enclosed into an evacuated glass envelope. When IR radiation focussed on lead sulphide, its conductance increases and causes more current to flow. PHOTOCONDUCTIVITY CELL

18. FOURIER TRANSFORM SPECTROMETERS In the FT spectrometers, coding is accomplished by splitting the source into two beams. It is used for data processin.g FT spectrometers can be single beam or double beam instruments. This type of instrument is to first obtain a reference interferogram by scanning a reference, coding the data and storing the results in the memory of this instrument computer. 18

19. DISPERSIVE INSTRUMENTS It is double beam recording instrument. The double beam design is less demanding with respect to the performance of sources and detectors. 19

20. FACTORS AFFECTING VIBRATIONAL FREQUENCIES 1. Coupled vibrations and Fermi resonance, There is one stretching absorption frequency for an isolated C-H bond. In the case of methylene (- CH 2 -) group, two absorption occur which correspond to symmetric & asymmetric vibrations. 20 H H H H Asymmetric vibrations always takes place at higher wavelength compared with the symmetric vibrations. These are known as coupled vibrations.

21. 21 In IR spectrum absorption bands are spread over wide of frequencies. It may happen that the energy of an overtone level chances to coincide with the fundamental mode of different vibrations A type of resonance takes place as in the case of coupled pendulums. This type of resonance is known as Fermi resonance 2. Electronic Effects The frequency shifts from the normal position of absorption occurs because of electronic effects which include inductive effect, mesomeric effect, field effect etc. Under the influence of these effects, the force constant or the bond strength changes and its absorption frequency shifts from the normal value.

22. 22 3. Hydrogen Bonding: It gives rise to downward frequency shifts. Stronger the hydrogen bonding greater the absorption shift towards lower wave number from the normal value. APPLICATIONS OF IR Identification of functional group and structure elucidation. Identification of substances. Studying the progress of the reaction. Detection of impurities. Quantitative analysis.

23. REFERENCE: Instrumental analysis, S koog , page no. 488-505. Organic spectroscopy,William Kemp Instrumental methods of chemical analysis, Gurdeep R. Chatwal . 23

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