Principles of FTIR

Principles of FTIRPerkinElmer

Infrared SpectroscopyNear infrared780 nm 2.5 mMid Infrared2.5 m 25 mFar Infrared25 m 2000 m12500 4000 cm-14000 400 cm-1400 10 cm-1

Peak absorptions are shown as the region facing downwardsInfrared radiation sourcesample100% transmission

FTIR

FT-IR4000-400

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Dispersive Infrared SpectrophotometerFourier Transform Infra-Red Spectrophotometer(Fourier Transform Infra-Red Spectroscopy, FTIR)Current technologyFTIRPerkinElmer Spectrum 100Dispersive IRPerkinElmer Model 297Infrared SpectrophotometerTypes of infrared spectrophotometer

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Dispersive Infrared Spectrophotometerlight sourcereferencesampleChopper mirrorThermocouple detectorgratingRadiation from the source isdispersedby the grating into single wavelength components.The sample beam ratioed to the reference beam produces a plot of the sample spectrum

spectrumsample compartment

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Fourier Transform Infra-Red Spectrophotometer (FTIR)

spectrum

light sourcesampleHe-Ne Lasermoving mirrorinterferogrambeamsplitterfixed mirror

detectorinterferometerSample compartment

Fourier transform

Radiation emitted from the source is split into two with a beamsplitter in the interferometer. The fixed and moving mirrors reflect each of the beam back to the beamsplitter, where the two beams recombine into one and falls on the detectorThe two beams combine constructively or destructively, varying as the optical path difference, when the moving mirror is moved. When the combined beam is transmitted through the sample, it is detected as an interferogram and contains all infrared information on the sample. The infrared spectrum is obtained from the interferogram by the mathematical process of Fourier transformation

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What is a beamsplitterinterferometer=produces optical path difference in radiationchanges phase difference of radiationrequires precise movement!!

radiation sourcefixed mirrormoving mirror

beamsplitter

0/4/2I0R0I0R02R0T0I0I0T0I0T0I0I0(R02+T02)

opd= 0

opd= /2Optical path difference opdopd= As the moving mirror moves continuously to a further distance away, the intensity of the combined beam at the center of the beamsplitter changes from I0radiation from fixed mirrorradiation from moving mirrorcombined radiationthink about the interference of monochromatic radiation

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intensity is strongest at 0 optical path difference Light interference of multiple wavelengths(interferogram)0/2IOPDmonochromatic radiation of wavelength two wavelengths radiation120OPDmultiple wavelengths1234560OPDIntensityIntensityIntensity

Role of Laser0OPDIntensityinterferogram obtainedsampled interferogramdetermine the sampling intervalusing the periodicity of the laser interferogramThe interferogram is a function of distanceaccuracy of sampling interval is required

What is Fourier transformFouriers theoryThe period of the trigonometric function is the wavelength. The magnitude is the intensityRevert to their respective wavelength interferograms

!!All functions can be represented as the sum of trigonometric functions

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interferogramFourier transform

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Convert to spectrumWavelengthIntensitywavelengthintensity

Energy in infrared region and interferogram

continuous wavelength radiationinterferogramFTsum of trigonometric (cos) functions

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General formula for Fourier seriesfunction of distancewavelength function must be integrated from to

The interferometer must be moved an infinite distance to obtain an interferogram which will meet the requirements of the Fourier transform.

An apodization function which is simply a weighting is applied to the interferogram, convert this interferogram to an interferogram similarto that obtained with the moving mirror moving to an infinite distance, before performing the Fourier transform.function of wavelength

spectruminterferogramAs it is impossible

Problems encountered in actual FTIR measurementsWhat is apodization function

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Apodization functionFT with limited rangeFT with apodization function applied to the limited rangeA decay function with increasing |x|The interferogram is made to converges to zero at the edge.There are several apodization functions, such as Norton-Beer and Triangular, which affects the peak width and peak height differently.

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Phase CorrectionThe centerburst is not at optical path difference 0due to delay in acquisition of electronic signals, optical beam distortions etc)

phase correctionstrongest interferogram intensity position (centerburst) is set to optical path difference "0" position

deviation from the optical path difference 0positiondeviation from the optical path difference0position In general, several points centered around the centerburst are subjected to the correction

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background measurementObtaining FTIR spectruminterferogramsample measurement

apodizationphase correctionFourier transformtransmittance calculationintensity ratio4000.020001000400.01.12040608096.2cm-1EGY 4000.020001000400.00.020406083.2cm-1EGY 4000.020001000400.00.02040608091.3cm-1%T infrared spectrumintensity spectrumIntensity spectrumFourier transform

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Short measurement time

All frequencies are measured simultaneously in an interferometer with Fourier transform.The measurement time is reduced in a multi-wavelength measurementReaction process measurementRapid measurement time allows a chemical reaction or kinetics to be monitoredHigh signal to noise ratio

Throughput advantage (Jacquinot Advantage)Throughput advantage of FTIR is 100 times better than a dispersive IR.Fellgett advantage (Multiplex Advantage)FTIR can measure the entire wavelength range simultaneouslyMeasuring very small samplesAn infrared microscope system allows these small samples to be measured easilyMeasuring dark samplesSamples with high carbon content (such as black rubber) can be measuredImprovement in wavelength accuracy (Connes Advantage)

FTIR is calibrated with a He-Ne laserHe-Ne laser has a very stable frequency. Therefore, FTIR will have excellent long term stability and repeatability in the spectrumAdvantages of FTIRAdvantages of FT-IRcompared to dispersive IR

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FTIR

FT-IR4000-400

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