Double-Beam Infrared Spectrophotometer

DoubleBeam Infrared Spectrophotometer

The commonly used instrument for the study of absorption in the infrared region is called double beam infrared spectropholometer. The main components of this spectrophotometer are:
(i) Source of light
(ii) Sample compartment
(iii) Optical chopper
(iv) Monochromator
(v) Detector
(vi) Amplifier and
(vii) Recorder
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The source of infrared radiation is either a hot filament called Nernst glower or a hot carborundum rod called Globar. The Nernst filament is made up of a hollow rod about 2 mm in diameter. It is composed of a mixture of oxides of zirconium, thorium and yttrium. The Globar is made up of a cylindrical rod of silicon carbide about 7 mm in diameter. When heated electrically to a temperature of about 1300-1800 °C  these rods emit infrared radiations.
The infrared radiations emitted from the source are divided into two beams of equal intensity. These beams are focused into the sample compartment. One beam passes through the ‘sample and is called the sample beam. The other beam serves as the reference beam. absorption by the sample is measured directly from the difference in intensity between the two beams. This intensity difference is usually measured by the optical null method.
The two beams are reflected to a light chopping device which is a mirror rotating at a rate of about 10 cycles per second. As the chopper rotates, it causes the sample beam and the reference beam to be reflected alternately to the monochromator.
A monochromator consists of the entrance and the exit slits, a prism or grating and minors to deflect or focus the radiation. The prism or grating separates the polychromatic radiation into monochromatic radiation. The prism is usually made from glass or fused quartz. The front face of the prism is coated with aluminium which can reflect upto96 % infrared radiations. Grating is usually made from a soft metal like aluminium. The grating monochromator has the following two advantages over the prism monochromator. (i) It is not attacked by moisture. (ii) It can be used over a wide range of wave length.
The detector works on the principle of a thermocouple, which converts the infrared (thermal) energy to electrical energy. The current will be proportional to the intensity of radiation falling on the thermocouple. When the sample has absorbed radiation of a particular frequency, the detector will be receiving alternately an intense transmitted reference beam and a weak sample beam. Thus an alternating current is produced which is then passed on to the amplifier.
The amplifier is so designed that it can only amplify the alternating current. The amplifier is coupled to a small servo-motor which derives the optical wedge into the reference beam until the detector receives radiation of equal intensity from both the sample and the reference beams.
The movement of the optical wedge is coupled to a recorder pen so that the movement of the wedge in and out of the reference beam is accompanied by a simultaneous and proportional movement of the pen on the printed paper which records the absorption bands.
Infrared spectra may be obtained for gases, liquids or solids.
(I) A gas sample is examined in an evacuated as cell ranging in length from a few centimeters to some meters. Gaseous samples are of two types:
(i)    Strongly absorbing, such as fluorocarbons which are filled in the gas cell under a pressure of about 5 mm. Hg.
(ii)Weakly absorbing such as HCl which is filled at a pressure of about 0.5 mm Hg.
(2)    A liquid sample may be examined neat (pure) or in solution. A small drop of the liquid sample is pressed between two optically polished circular flat plates of rock salt (NaCl). As a result film of about 0.1 mm thickness is produced. The plates which are held together by capillary action are placed in the path of the sample beam. For liquid samples that dissolve sodium chloride, silver chloride plates may be used. Solutions are examined in cells from 0.1 mm to 1 mm in thickness. The amount of solute required in the range of 1 mg to 15 mg and the concentration varies from 0.05 to 10 %. Solvents used must be dry and transparent. Carbon tetrachloride, Carbon disulphide and Chloroform are most commonly used solvent.
(3)    A solid sample is examined either as a mull or as a KBr disc. Mulls are prepared by grinding about 2 to 5 mg of the solid in a mortar with pestle by adding one or two drops of a mulling agent. The paste of the mull is then applied between two optically polished NaC1 plates. The commonly used mulling agent is “Nujol” which is a mixture of high molecular weight paraffin hydrocarbons. Nujol is commonly consists of high boiling petroleum oil. When Nujol is used as a mulling agent, the peaks at 2850, 1460 and 1380 cm-1 are observed due to stretching and bending of C-H bonds present in Nujol. These regions can be studied by using hexachlorobutadiene and chlorofluorocarbon oils as complementary mulling agents. KBr discs are prepared by grinding and mixing about 1 mg of the solid sample with about 100 mg of dry powdered KBr in a mortor. This mixture is then pressed in a special die under a pressure of about 20,000 psi to form a transparent disc.

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