FOURIER -TRANSFORM INFRARED SPECTROMETER [FTIR]

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Content

Introduction of Infrared Spectroscopy Difference between Infrared and Fourier Transform Infrared

Spectroscopy Introduction of FTIR Sample preparation Instrumentation Michelson interferometer Interferogram Advantages Disadvantage References

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Why Infrared Spectroscopy ?

An Infrared spectrum represents a fingerprint of a sample with absorption peaks which correspond to the frequencies of vibrations between the bonds of the atoms making up the material-Because each different material is a unique combination of atoms, no two compounds produce the exact same spectrum, therefore IR can result in a unique identification of every different kind of material!

FingerPrint

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What is Infrared?

Infrared waves have wavelengths longer than visible but shorter than

microwaves.

The Infrared region is divided into 3 regions : Near IR : 0.8 to 2.5 u

Infrared region : 2.5 to 15 u Far IR : 15u to 200 u

Infrared radiation stimulates molecular vibrations.

Infrared spectra are traditionally displayed as %T versus wave number.

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Stretching: Change in inter-atomic distance along bond axis

Bending: Change in angle between two bonds.

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To separate IR light, a grating is used.

Grating

Light source

Detector

Sample

Slit

To select the specified IR light, A slit is used.

Dispersion Spectrometer

In order to measure an IR spectrum,the dispersion Spectrometer takesseveral minutes.Also the detector receives onlya few % of the energy oforiginal light source.

Fixed mirror

B.S.

Moving mirror

IR Light source

Sample

detector

An interferogram is first made by the interferometer using IR light.

The interferogram is calculated and transformedinto a spectrum using a Fourier Transform (FT).

FTIR

In order to measure an IR spectrum,FTIR takes only a few seconds.Moreover, the detector receivesup to 50% of the energy of original light source.(much larger than the dispersion spectrometer) .

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• Difference between FTIR and dispersive IR:

Sr.no.

Dispersive IR Fourier Transform IR

1 There are many moving parts resulting in mechanical slippage and wear. Only the mirror moves during an experiment

2 Calibration against reference spectra is required to measure frequency .

Use of a laser provides high frequency (to 0.01 cm-1).

3 Slow scanning speed. Rapid scan speeds permit monitoring sample undergoing rapid change.

4 In order to improve resolution only a small amount of the IR beam may be allowed to pass through the slits.

A much larger beam may be used at all times.Data collection is easier .

5 Only radiation of a narrow frequency range falls on the detector at any one time .

All frequencies of radiation fall on the detector simultaniously .

6 The samples is subject to thermal effect from the focused beam .

The sample is not subject to thermal effects.

7 Less sensitive, time consuming. High sensitivity ,precision & faster .

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FTIR

FTIR is a powerful tool for identifying the type of chemical bond present in a molecule.

FTIR have greatly extended the capabilities of IR and applied to many areas that are difficult to analyze by dispersive instrument.

In dispersive IR , IR light is separated into it’s individual frequency by dispersion using a grating monochromatic

All frequencies and intensities can be simultaneously determined.FTIR gives a plot of intensity v/s frequency.

The instrument used for FTIR is Scanning Michelson interferometer. FTIR frequencies are allowed to intersect to produce an interference pattern and

this pattern is analyzed mathematically using “FOURIER TRANSFORM”, to determine the individual frequencies and their intensities.

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FTIR Instrumentation

The Michelson interferometer consist of following parts:

1) A moving mirror.

2) A fixed mirror.

3) A beam splitter.

4) A radiation source.

5) FTIR Detector.

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FTIR Instrumentation

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Sample Preparation

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•To obtain an spectrum, the sample must be

placed in a “container” or cell that is transparent

in the IR region of the spectrum.

•Sodium chloride or salt plates are a common

means of placing the sample in the light beam of

the instrument.

Sample Cell

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Sources

Nernst Glower

Hollow rod about 2mm in diameter & 30 mm in length,composed

of rare earth oxide. Externally heated (1000-1800 C). Radiation over wide wavelength range.Globar source

Rod of sintered silicon carbide,50mm in length & 4-8mm in diameter,

Self starting.

Heated to temp.between 1300-1700 C.

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Tungsten Incandescent Lamp Black body source Used in near IR instrument.

Nichrome/Rhodium wire Coiled , heated Emits in Mid-IR Black oxide layer formation.

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FTIR DETECTOR

A)Thermal Detector

1) Thermocouple- Two junctions of dissimilar metals connected together. Electricity directly propotional to energy difference between 2

connections.

2)Bolometer- One arm of Wheastone bridge & similar strip of metal is used as balancing

arm of bridge which is not expose to radiation. Amount of current flowing is directly prop. to amount of radiation falling

on the detector.

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B)Photon detector

Interaction between incident photon and semiconductor.

Energy from photon stricking electron in detector and raises them from non-conducting to conductance state.

E.g. MCT detector.

C) Pyroelectric Detector

DTGS Detector

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Interferogram

The Interferogram consist of two modulated waves :

1) Consructive Interference.

2) Destructive Interference.λλλλλλλλλλλλλλλλλλλλλλλλλ=++++λλλ

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= n Constructive Interference will take place for any value of when the two beams are in phase n=0,1,2,3,…..

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Destructive InterferenceFixed Mirror

Moving Mirror

= ( n + 1/2 ) Totally destructive interference takes place when optical path difference is 1/2 or some multiple of it-beams are completely out of phase

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Advantages

• Fellgette advantage- High speed.

• Jacquinot advantage-High sensitivity.

• Slitless system.

• All of the energy of the source is utilized in FTIR.

• Rapid scan time with high sensitivity.

• Connes advantage.

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Disadvantages

FTIR instruments have a single beam, whereas dispersive

instruments usually have a double beam.

Interferograms are difficult to interpret without first

performing a Fouriertransform to produce a spectrum.

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References William Kemp, Organic Spectroscopy, Published by Palgrave,

Third edition.Gurdeep R.Chatwal, Sham K.Anand,Instrumental method of

Chemical analysis, Himalaya Publishing house,fifth edition.Silverstein, A Spectroscopic Identification of Organic

compound,sixth edition.Skoog wele Hollers, Analytical chemistry Introduction ,

Saunderls college publishing house ,eight edition.B.K Sharma, Instrumental method of chemical analysis, Goel

publication, second edition.Y.R Sharma, Elementary Organic Spectroscopy,Principles and

Chemical applications, S.Chand Publication, sixth edition.

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HabartH .Willard,Lynne L. Merritt,Johan A. Dean,

Frank A.Settle,Instrumental method of Analysis, CBS

Publisher and Distributors, New Delhi.

Dyer R John, Application of Absorption Spectroscopy

of unknown Organic compound, PHT, pvt. Ltd. New

Delhi.

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