Instrumentation of infrared spectroscopy

brief about instrumentation of IR Spectroscopy

1. Instrumentation of infrared spectroscopy Presented by: Ankith rai Dept: pharmacology Guided by: dr. rajesh

2. Components of ir spectrometer IR SPECTROPHOTOMETRS CONSISTS OF FOLLOWING COMPONENTS Light source Sample holder Monochromator and optic material Detector Recorder ACHARYA & BM REDDY COLLEGE OF PHARMACY 2

3. COMPONENTS OF IR SPECTROPHOTOMETER INFRARED SPECTROPHOTOMETER ACHARYA & BM REDDY COLLEGE OF PHARMACY 3

4. LIGHT SOURCE Near IR region : Tungsten filament, Nichrome wire, Rhodium wire Mid IR region: Nernst glower and Globar source These two are most commonly used IR sources Far IR region: mercury arc lamp and carbon dioxide laser IR sources are hot bodies, which emit continuously throughout the IR region. They require thermal insulation. The radiation source must emit IR radiation, which must be intense , steady and must extend over the desired wavelength ACHARYA & BM REDDY COLLEGE OF PHARMACY 4

5. Nernst glower Nernst glower consist of a rod or hallow tube about 2 cm long and 1mm diameter, made by sintering a mixture of oxides of Cerium, Zirconium, Thorium and Yttrium. Platinum wires are sealed to the ends, and a current is passed through the cylinder. ACHARYA & BM REDDY COLLEGE OF PHARMACY 5

6. Nernst glower is a non conducting at room temperature and must be heated by external means to bring conducting state. It is maintained at higher temperature by making use of electrical heating and can be opened in air, since it is not subject to oxidation ACHARYA & BM REDDY COLLEGE OF PHARMACY 6

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8. Globar source Globar is a silicon carbide rod usually about 5cm in length and 0.5cm in diameter. Water cooling of the electrical contacts is needed. It has an advantage of positive co-efficient of resistance. When heated to temperature between 1300-1700 o c, it strongly emits radiation in IR region. ACHARYA & BM REDDY COLLEGE OF PHARMACY 8

9. The disadvantage is its less intense source than nernst glower. It is more satisfactory because it is worth at wave number longer than 650cm -1 ACHARYA & BM REDDY COLLEGE OF PHARMACY 9

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11. Both Nernst and Globar source provide excellent and easily controlled source for the wavelength band frequency used in analysis. However in far infrared region, both these sources looses their effectiveness. ACHARYA & BM REDDY COLLEGE OF PHARMACY 11

12. Mercury arc lamp: For the far IR region, special high-pressure mercury arc lamps are used.( wave no <200cm -1 Tungsten filament lamp: The ordinary tungsten filament lamp is a convenient source for the near-IR region of 4000 to 12800 cm -1 . ACHARYA & BM REDDY COLLEGE OF PHARMACY 12

13. 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. Temperature can be raised to 1100°c. Gives less energy then Nernst and globar Incandescent lamp ACHARYA & BM REDDY COLLEGE OF PHARMACY 13

14. Incandescent lamp An incandescent light bulb or lamp is an electric light which produces light with a wire filament heated to a high temperature by an electric current passing through it, until it glows The hot filament is protected from oxidation with a glass or quartz bulb that is filled with inert gas. ACHARYA & BM REDDY COLLEGE OF PHARMACY 14

15. Rhodium wire: Rhodium wire heater sealed in a cylinder has been used. Nichrome wire: It is used when the required wavelength range and intensity are not too great. ACHARYA & BM REDDY COLLEGE OF PHARMACY 15

16. Sampling solid liquid gas Solid films NaCl / KBr cell NaCl/ KBr cell Solid run in solution Mull technique Pressed pellet technique ACHARYA & BM REDDY COLLEGE OF PHARMACY 16

17. monochromator The sample in the IR spectroscopy absorbs only certain frequencies. Hence monochromator is used to select desired frequencies and reject the others. The prism or grating type monochromators are employed in infrared work. However prism instruments are favored because of greater range and simplicity. ACHARYA & BM REDDY COLLEGE OF PHARMACY 17

18. Neither quartz nor glass is sufficient transparent to infrared, radiations above 3.5 microns. These Can not be used as they absorb satisfactory in IR region. Hence crystals of certain halogen salts , which transmit infrared freely have been utilized for optical parts. Quartz is employed as material for prism construction and is used for near IR region. It strongly absorbs beyond 4 microns. ACHARYA & BM REDDY COLLEGE OF PHARMACY 18

19. The great bulk of analytical work in IR is done crystalline NaCl as the Prism material. Beyond 20 microns NaCl absorbs strongly. So cant be used. Crystalline KBr and Cerium bromide are satisfactory for the far IR region (15-40 microns). Lithium fluoride provides Prism material in near IR region(1-5 micron). ACHARYA & BM REDDY COLLEGE OF PHARMACY 19

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21. Note : All the common IR transmitting material except quartz are, however, water soluble and are easily stretched. protection from condensation of moisture with desiccants or with heat is thus necessary. The protection of hygroscopic materials against moisture is accomplished by enclosing the entire Monochromator in a sealed house which is either evacuated or desiccated. ACHARYA & BM REDDY COLLEGE OF PHARMACY 21

22. iR DETECTORS The measurement of infrared radiation is difficult as a result of the low intensity of available sources and the low energy of the infrared photon. As a consequence of these properties, the electrical signal from an infrared detector is small, and its measurement requires large amplification. ACHARYA & BM REDDY COLLEGE OF PHARMACY 22

23. The convenient phototubes are generally not applicable in the infrared because the photons in this region lack the energy to cause photoemission of electrons. Thermal detectors, whose responses depend upon the heating effect of the radiation, can be employed for detection of infrared wavelengths. With these devices, the radiation is absorbed by a small black body and the resultant temperature increase is measured. ACHARYA & BM REDDY COLLEGE OF PHARMACY 23

24. Two types of detectors are used Thermal detectors Photodetectors ACHARYA & BM REDDY COLLEGE OF PHARMACY 24

25. Thermal detectors When IR radiation falls on these detectors they cause heating which give rise to potential difference(P.D). This P.D is depends upon amount of radiation Thermocouple Bolometer Thermister Golay cell Pyroelectric detectors ACHARYA & BM REDDY COLLEGE OF PHARMACY 25

26. thermocouples It is the most commonly used detector in infrared spectrophotometry. Thermocouples are the dissimilar strips of metals joined together at one end. Here two fine wires of metals which have different thermoelectrical properties are welded with blackened gold foil, and which absorbs the radiations. ACHARYA & BM REDDY COLLEGE OF PHARMACY 26

27. One welded joint (cold junction) is kept at constant temperature and the other welded joint (hot junction) is exposed to radiations. This exposure of hot junction causes a rise in its temperature. Thus, as the two junctions are at different temperatures, it causes a potential difference which is proportional to degree of heating of hot junction (or amount of radiations falling on the hot junction). ACHARYA & BM REDDY COLLEGE OF PHARMACY 27

28. A thermocouple is closed in a evacuated steel casing with a IR transmitting window, KBr to avoid losses of energy by convection. ACHARYA & BM REDDY COLLEGE OF PHARMACY 28

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30. bolometer A bolometer is based upon the fact that the electrical resistance of a metal changes with increase of temperature. The constructed metal or semiconductor that exhibits a large change in electrical resistance as a function of temperature. When radiation falls on the bolometer, its temperature changes and hence the resistance of conductor also changes. ACHARYA & BM REDDY COLLEGE OF PHARMACY 30

31. The degree of changes in resistance is a measure of amount of radiation falls on the bolometer. Bolometer is made in one arm of the Wheatstone bridge and a similar strip of metal is used as balancing arm of the bridge, which is not exposed to Infrared radiations. When no infrared radiations fall on the bolometer, the bridge remains balanced. As the radiations fall on the bolometer, the bridge becomes unbalanced due to change in electrical resistance and thus the electrical current flows through galvanometer G. ACHARYA & BM REDDY COLLEGE OF PHARMACY 31

32. ACHARYA & BM REDDY COLLEGE OF PHARMACY 32 The amount of current flowing through galvanometer is a measure of the intensity of the radiations falling on the detector.

33. thermistor Functions similar to bolometer but the resistor made by fusing together several metallic oxides which have negative thermal coefficient of electrical resistance. In bolometer as the temperature increases resistance also increases but in thermistors the resistance decreases. Decrease in resistance leads to the flow of current across the resistor. Degree of current flow is a measure of amount of radiation fallen on thermistor. ACHARYA & BM REDDY COLLEGE OF PHARMACY 33

34. Golay detectors In Golay detector xenon is contained in a small cylindrical chamber, which contains a blackened membrane. One end of the cylinder is sealed with an infrared window; the other consists of a flexible diaphragm which is silvered on the outside. A light beam is reflected from this silvered surface to the cathode of a vacuum phototube. ACHARYA & BM REDDY COLLEGE OF PHARMACY 34

35. When infrared radiation enters the cell, the blackened membrane is warmed which in turn heats the xenon by conduction. The resulting increase in pressure causes distortion of the silvered diaphragm. As a consequence, the fraction of the reflected light beam that strikes the active surface of the phototube is changed; a change in photocurrent results, which can be related to the power of the infrared beam. ACHARYA & BM REDDY COLLEGE OF PHARMACY 35

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37. Pyroelectric detector Pyroelectric detector consists of a thin plate of pyroelectric crystal of 0.25 to 12 mm 2 in size. The crystal is placed between two electrodes which are connected to an external electric circuit. One of the electrodes should be transparent to IR radiations. When the crystal is exposed to IR radiation, it absorbs the heat leading to change in its dipole moment. ACHARYA & BM REDDY COLLEGE OF PHARMACY 37

38. This change causes generation of electrical charges which are attracted towards the electrodes. Accumulation of charge at the electrodes creates a potential hence, producing electric current in the external circuit. The most common pyroelectric is Triglycine sulphate (TGS) & also deuterated Triglycine sulphate. ACHARYA & BM REDDY COLLEGE OF PHARMACY 38

39. Photon detectors Used in near IR region Consists of semiconductors like lead sulphide, lead telluride When radiation are fall on these they goes to higher level and produce signal ACHARYA & BM REDDY COLLEGE OF PHARMACY 39

40. Photoconductivity cell Type of photon detector It consists of thin layer of lead sulphide or lead telluride supported on glass. Enclosed into an evacuated glass envelope. When IR falls, conductance of the cell increases. As the conductance increases more current flows. Amount of current flows proportional to the IR radiation. ACHARYA & BM REDDY COLLEGE OF PHARMACY 40

41. Semiconductor detectors Type of photon detecors Semiconductors: insulators when no radiation falls and conductors when radiation falls Exposure to radiation causes very rapid change in their electrical resistance Rapid response on exposure to IR ACHARYA & BM REDDY COLLEGE OF PHARMACY 41

42. IR photon displaces electron in the detector and changes conductivity Eg. Lead sulphide , Lead telluride, indium antimonide, germanium doped with Cu or Hg Response time is less than 1 micro second Sensitive, rapid, wide acceptance ACHARYA & BM REDDY COLLEGE OF PHARMACY 42

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44. References Introduction to Instrumental analysis by Robert D. braun . Pg.no:353-364 Instrumental method of analysis by H.A.Willard , L.LMerritt , J.A.Dean , F.A Settle, 7 TH edition Instrumental method of chemical analysis by Gurdeep R Chatwal, sham k Anand Pg.no: 2.41-2.50 ACHARYA & BM REDDY COLLEGE OF PHARMACY 44

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