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IR and FT-IRPresented By, Chetan M. Jain

First Year M.Pharm(Quality Assurance)

Government College of Pharmacy,Amravati.05/01/23

Infrared Spectroscopy Intoroduction

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Provide useful information about structure of molecule.Absorption of IR radiation by molecule and exhibit absorption spectra.Change in dipole moment.Energy of radiation is directly proportional to wave number and inversely proportional to wavelength.

Region Wavelength (µ) Wave number (cm-1)

Near IR 0.75-2.5 1200-4000

Infrared 2.5-16 4000-625

Far IR 16-200 625-10 05/01/23

Chetan Jain

Origin of IR Spectra UV- Visible spectrum results in changes of electronic as

well as vibrational and rotational level in molecule. IR radiation shows only vibrational and rotational level. Vibrational-Rotational spectrum.

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InstrumentationThe optical components of IR spectrophotometer are

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Light SourceMonochromatorSample HolderDetectorRecorder

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Dispersive instrumentsFT-IR instrumentNon-dispersive instruments

IR Radiation Source Hot bodies continuously emmitting radiation Incandescent lamp

Nernst Glover

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A closed wound nichrome wire is used.A black oxide film formed on the coil gives emissivity by heating.Temp. can be raised to 1100°C.Used in near IR region and failed in IR region.

In IR region – most common source.Consists of hollow rod-2mm in dia and 2-5cm in length.Composed of mixture of oxides like zirconium, yttrium and thorium.Temp: 1000-1800°C.

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Advantages of Nernst glower

Disadvantage

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It emits IR radiation over wide range of wavelength.The intensity of radiation maintains steady and constant over long period of time.Used in air as it is not oxidized.

Frequent mechanical failure.

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Globar Source

Mercury Arc

Tungsten Filament lamp

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It is rod of sintered silicon carbide about 5cm in length and 4mm in diameterTemp- 1300°C. If radiation above 10µ it is used.

Far IR region it is used.It is encolosed in quartz jacket to reduce loss.

Useful for near IR region only.

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Monochromators Select desired frequencies and reject other frequencies. These are of two types

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1) Prism Monochromator2) Grating Monochromator

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Prism Monochromator

Greater range and simplicity. Halogens salts are used. Crystalline NaCl: 5-15µ Crystalline KBr/CBr: 15-40 µ LiF : 0.75-2.5 Two types

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Single beam monochromatorDouble beam Monochromator

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Grating Monochromators Series of parallel straight lines cut into plane

surface. Constructed from glass coated with aluminium. Gives radiation into single order. Advantages

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Not attacked by moisture.Used over considerable wavelength.Sturdy and long lasting.

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Sample holders and samplingof substance

Used for solid, liquid and gas. Material containing sample must be transperent to

IR region. Sample cell constructed from rock salt. Sample must be pure and free from water.

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Sampling of Solids Solid dissolved in solvent

As Solid Film

Mull technique

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Carbon tetrachloride,Chloroform,acetone and cyclohexane.

Sample solution is placed on surface of KBr or Nacl and allowed to evaporate.

Solid sample mixed with nujol to form a paste. This paste is then sandwiched between two salt plates and used for spectral measurement.Nujol has absorption at 2915,1462,1376 and 719cm-1

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Pressed Pellet Technique(Disk Method)

Advatages of pellet tech. over mull tech. is

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The small amount of finally ground solid sample is mixed with 100 times its weight of powdered KBr.This mixture is pressed under high pressure in IR tablet press to form pellet.

Eliminates the problem of bandIf KBr pellet is preserved can be reused Resolution of KBr is superior

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Sampling of liquids For sampling of various cell like sandwitch cell,demountable cell

and cavity cell are used. Made up of Nacl, KBr or thallium bromide. Demountable cell:

Cavity cell:

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It consists of pair of salt plates separeted by gasket made of metal or teflon and together by an clamp.

Matching a parallel-sided hole in a salt block makes it.

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Sampling of Gas The dried gases introduced via a stop cock. The gas sample is introduced into the gas cell made up of

glass or metal cylinder of about 10cm long. The end walls of the gas cell made up of NaCl.

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Detectors Two types of detectors are used

Thermal detectors

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Thermal detectorsPhotodetectors

When IR radiation falls on these detectors they cause heating which give rise to P.D. This P.D. is depends upon amt of radiation. Thermocouple

BolometerThermisterGolay cell 05/01/23Chetan Jain

Thermocouple

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These are dissimilar strips of metal join together at one end.Two fine wires of metals are welded with blackened gold foil, which absorbs radiation.One welded joint(Cold junction) is kept at constant temp.The other welded joint (Hot junction) is exposed to radiation.This exposure of hot junction causes rise in its temp, cause the P.D. proportional to the amt of radiation.

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Bolometers

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These are constructed from metals or semiconductors.When IR radiation falls on bolometer its temp. changes cause change in the resistence of the conductor.This change in the resistence of conductor is depends upon amt of radiation.This made one arm of Wheistone Bridge.If IR radiation falls on the bolometer, the bridge becomes unbalanced due to change in the electrical resistence causes current flow through galvanometer.this amt of current flow is a measure of intensity of radiation.

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Thermister

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These function similar to bolometer.These are the resisters made by fusing several metallic oxides.

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Golay cell

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It consists of metal cylinder.One end of which closed by balanced metal plate and other with metalised diaphragm.Cylinder is filled with gas like xenon.When the radiation falls on blackened metal plate it is heated which causes expansion of gas, which affect the motion of diaphragm.

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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. Photoconductivity cell

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It consists of thin layer of lead sulphide supported on glass and enclosed into an evacuated glass envelope.

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Fourier Transform IR Spectroscopy (FTIR)

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Fourier Transform IR Spectroscopy (FTIR)

Advanced technique. Interferometer(Michelson Interferometer).

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Advantages

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Fellgett AdvantageJacquinot advantage

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Interpretation of IR Spectra

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Applications of IR and FT-IR

Qualitative application Quantitative application.

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References Pharamceutical analysis by A.V. Kasture, S.G. Wadodkar,K.R.

Mahadik, H.N. More, Volume II, Nirali Prakashan, Eleventh edition (2004), p.no. 207-221.

Fundamental of analytical chemistry by D.A.Skoog, D.M.West,F.J.Holler,7th edition p.no.592-597.

Organic Spectroscopy by William Kemp, 3rd edition, Palgrave Publication, p.no19-56.

Instrumental Methods of Analysis by H.A.Willard, L.L.Merritt, J.A.Dean, F.A. Settle, 7th edition, p.no. 288 to 320.

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