Fourier transform Infrared Spectrometer

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Fourier transform Infrared Spectrometer Maria Rose Kuriakose 1 st Year M. Pharm ACPS, Pariyaram.

OVERVIEW:

OVERVIEW INTRODUCTION FTIR COMPARISION ADVANTAGES & DISADVANTAGES REFERENCES

IR SPECTROSCOPY:

IR SPECTROSCOPY INTRODUCTION Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used to determine the functional groups. The infrared region of the spectrum composes of radiation with wave numbers ranging from about 12,500 to 50cm -1 / wave lengths from 0.8 to 200µ. Infrared region lies between visible and microwave region.

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The infrared region constitutes 3 parts a) The near IR (0.8 -2.5µm) (12,500-4000cm -1 ) b) The middle IR (2.5 -15µm) (4000-667cm -1 ) c) The far IR (15-200µm) (667-50cm -1 ) most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region. It gives sufficient information about the structure of a compound.

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The absorption of infra red radiation » Vibrational transitions which are accompanied by a change in dipole moment of the molecule are called infrared active transitions . Because they absorbs the IR radiation. Applied frequency = natural frequency of vibration.

FOURIER TRANSFORM IR SPECTROMETER:

FOURIER TRANSFORM IR SPECTROMETER INTERFEROMETER… In the FT-IR instrument, the sample is placed between the output of the interferometer and the detector. The sample absorbs radiation of particular wavelengths. An interferogram of a reference is needed to obtain the spectrum of the sample. After an interferogram has been collected, a computer performs a Fast Fourier Transform , which results in a frequency domain trace (i.e. intensity vs. wave number).

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The detector used in an FT-IR instrument must respond quickly because intensity changes are rapid . Pyroelectric detectors or liquid nitrogen cooled photon detectors must be used. Thermal detectors are too slow. To achieve a good signal to noise ratio, many interferograms are obtained and then averaged. This can be done in less time than it would take a dispersive instrument to record one scan.

Interferometer consists of……….:

Interferometer consists of………. Drive mechanism Beam splitter Source and transducers.

Drive mechanism..:

Drive mechanism.. Speed and planarity of the moving mirror must be constant. Displacement of the mirror can be measured by a motor driven micrometer screw.

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Additional features of mirror system …. Sampling the interferogram at precisely spaced retardation intervals. Determining exactly the zero retardation point in order to permit signal averaging.

Beam splitter:

Beam splitter These are constructed of transparent materials having refractive indices 50% radiation is reflected and 50% is transmitted. Mylar sandwiched between two plate of a low refractive index solid. Thin film of germanium or silicon deposited on cesium iodide or bromide , Nacl, KBr are satisfactory for mid IR regions. Iron(III) oxide is deposited on calcium fluoride for near IR region.

Sources and Transducers:

Sources and Transducers Generally thermal transducers are not adapted to FTIR because of their slow responses time. Pyroelectric transducers: Exhibit fast response time , and allow them to track the changes in time domain signal from interferometer. Triglycine sulfate pyroelectric transducers are widely used for mid IR . Photoconduction transducers: Where better sensitivity or faster response times are required by liquid-nitrogen cooled , mercury/cadmium or indium antimonite, lead sulfide photoconductive transducers are employed.

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WORKING

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To improve the accuracy and precision can be accomplished by using three interferometers rather than one . The component and radiation path for each of the three interferometer system are indicated by subscript 1,2,3. System 1 is an IR system which provides an interferogram System 2 is known as Laser – fringe reference system which provides sampling – interval information. System 3 is White-light system

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S 1 includes mirror, beam splitter, transducer. S 2 includes Helium /neon laser. S 3 includes Tungsten source, transducers, a mirror system is fixed to give a zero retardation.

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Triple interferometer leads to remarkable precision and high reproducibility. This system is capable of providing spectra with a resolution lies in between 0.1-1 cm -1. To obtain 0.001 cm-1 resolution use more sophisticated system for maintaining the alignment of moving mirror.

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COMPARISON OF DISPERSIVE IR AND FT-IR

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Dispersive IR FT-IR There are many moving parts so mechanical slippage and wear. Calibration against reference spectrum is measure of frequency . Stray light with the instrument causes spurious reading. To improve resolution , IR is passed through small slit. Only radiation of narrow frequency will falls on the detector at one time. Only mirror can move . Use of laser provide high frequency. No effect, all signals are modulated. Larger beams were used . Falls simultaneously.

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Sample is subjected to thermal effect. Any emission of IR radiation by the sample falls on the detector. Slow scan speed permit monitoring sample under rapid change. No effect. Will not fall on the detector. Rapid scan speed permit monitoring sample under rapid change.

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ADVANTAGES AND DISADVANTAGES

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much faster, seconds vs. minutes use signal averaging to increase signal-to-noise (S/N) higher inherent S/N – no slits, less optical equipment, higher light intensity high resolution (<0.1 cm -1 ) High sensitive Study of sample with high absorbance Investigation of kinetic studies / detection of chromatographic elutes. other main adv. Includes…. Flagett advantage: Improved resolution

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Jaquinot advantages : In FT-IR there is no wastage of energy because there is no dispersive unit. Slits used in monochromator will reduce the energy . But in FT-IR there is no slits , only moving part is mirror. Cones advantages: In dispersive IR , we can’t use internal reference, but here we use internal reference (Neon-laser lamp). Disadv. single-beam, requires collecting blank can’t use thermal detectors – too slow

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REFERENCE Principles of Instrumental Analysis- Douglas A Skoog p.no: 392-399 Instrumental Methods of Analysis- H. Willard p.no: 302-304

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THANK YOU………

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