FTIR Spec_L3_Rev.pdf

8/10/2019 FTIR Spec_L3_Rev.pdf

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Fourier transform infrared

spectroscopy (FTIR)


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Fourier transform infrared spectroscopy(FTIR) is atechnique which is used to obtain

an infraredspectrumof absorption, emission, photoconductivityor Raman scatteringof a solid, liquidor gas.

An FTIR spectrometer simultaneously collects spectraldata in a wide spectral range. This confers a significant

advantage over a dispersivespectrometer whichmeasures intensity over a narrow range of wavelengthsat a time.

The term Fourier transform infrared

spectroscopyoriginates from the fact that a Fouriertransform(a mathematical process) is required toconvert the raw data into the actual spectrum
http://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Emission_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Raman_scatteringhttp://en.wikipedia.org/wiki/Solidhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Dispersion_(optics)http://en.wikipedia.org/wiki/Fourier_transformhttp://en.wikipedia.org/wiki/Fourier_transformhttp://en.wikipedia.org/wiki/Fourier_transformhttp://en.wikipedia.org/wiki/Fourier_transformhttp://en.wikipedia.org/wiki/Dispersion_(optics)http://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Solidhttp://en.wikipedia.org/wiki/Raman_scatteringhttp://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Emission_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Infrared


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The goal of any absorptionspectroscopy(FTIR, ultraviolet-visible ("UV-Vis")spectroscopy, etc.) is to measure how well a sampleabsorbs light at each wavelength.

The most straightforward way to do this, the

"dispersive spectroscopy" technique, is to shinea monochromaticlight beam at a sample, measurehow much of the light is absorbed, and repeat foreach different wavelength.

This is how UV-Vis spectrometerswork, for example.
http://en.wikipedia.org/wiki/Absorption_spectroscopyhttp://en.wikipedia.org/wiki/Absorption_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Monochromatichttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Monochromatichttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Absorption_spectroscopyhttp://en.wikipedia.org/wiki/Absorption_spectroscopy


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Fourier transform spectroscopy is a less spontaneousway to obtain the same information. Rather thanshining a monochromaticbeam of light at thesample, this technique shines a beam containingmany frequencies of light at once, and measures how

much of that beam is absorbed by the sample. Next, the beam is modified to contain a different

combination of frequencies, giving a second datapoint. This process is repeated many times.

Afterwards, a computer takes all these data andworks backwards to infer what the absorption is ateach wavelength.
http://en.wikipedia.org/wiki/Monochromatichttp://en.wikipedia.org/wiki/Monochromatic


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The beam described above is generated by startingwith a broadbandlight sourceone containing thefull spectrum of wavelengths to be measured.

The light shines into a Michelson interferometeracertain configuration of mirrors, one of which is

moved by a motor. As this mirror moves, eachwavelength of light in the beam is periodicallyblocked, transmitted, blocked, transmitted, by theinterferometer, due to wave interference. Different

wavelengths are modulated at different rates, so thatat each moment, the beam coming out of theinterferometer has a different spectrum.
http://en.wikipedia.org/wiki/Broadbandhttp://en.wikipedia.org/wiki/Michelson_interferometerhttp://en.wikipedia.org/wiki/Wave_interferencehttp://en.wikipedia.org/wiki/Wave_interferencehttp://en.wikipedia.org/wiki/Michelson_interferometerhttp://en.wikipedia.org/wiki/Broadband


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As mentioned, computer processing is

required to turn the raw data (light absorptionfor each mirror position) into the desiredresult (light absorption for each wavelength).

The processing required turns out to be acommon algorithm called the Fouriertransform(hence the name, "Fouriertransform spectroscopy").

The raw data is sometimes called an"interferogram"
http://en.wikipedia.org/wiki/Fourier_transformhttp://en.wikipedia.org/wiki/Fourier_transformhttp://en.wikipedia.org/wiki/Fourier_transformhttp://en.wikipedia.org/wiki/Fourier_transform


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Fourier Transform Infrared (FT-IR) spectrometry wasdeveloped in order to overcome the limitations

encountered with dispersive instruments. The main difficulty was the slow scanning process. A

method for measuring all of the infrared frequenciessimultaneously, rather than individually, was needed.

A solution was developed which employed a verysimple optical device called an interferometer.

The interferometer produces a unique type of signalwhich has all of the infrared frequencies encodedinto it.

The signal can be measured very quickly, usually on theorder of one second or so. Thus, the time element persample is reduced to a matter of a few seconds ratherthan several minutes.


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Most interferometers employ a beam splitter whichtakes the incoming infrared beam and divides it intotwo optical beams.

One beam reflects off of a flat mirror which is fixed inplace. The other beam reflects off of a flat mirrorwhich is on a mechanism which allows this mirror to

move a very short distance (typically a few millimeters)away from the beamsplitter. The two beams reflect off of their respective mirrors

and are recombined when they meet back at thebeamsplitter. Because the path that one beam travels is

a fixed length and the other is constantly changing asits mirror moves, the signal which exits theinterferometer is the result of these two beamsinterfering with each other.


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The resulting signal is called an interferogram

which has the unique property that everydata point (a function of the moving mirrorposition) which makes up the signal hasinformation about every infrared frequency

which comes from the source This means that as the interferogram is

measured, all frequencies are being measured

simultaneously. Thus, the use of theinterferometer results in extremely fastmeasurements


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Because the analyst requires a frequency

spectrum (a plot of the intensity at eachindividual frequency) in order to make anidentification, the measured interferogram signalcan not be interpreted directly.

A means of decoding the individual frequenciesis required. This can be accomplished via a well-known mathematical technique called theFourier transformation.

This transformation is performed by thecomputer which then presents the user with thedesired spectral information for analysis


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The Sample Analysis Process 1. The Source: Infrared energy is emitted from a

glowing black-body source. This beam passes throughan aperture which controls the amount of energypresented to the sample (and, ultimately, to thedetector).

2. The Interferometer: The beam enters theinterferometer where the spectral encoding takesplace. The resulting interferogram signal then exits theinterferometer.

3. The Sample: The beam enters the sample

compartment where it is transmitted through orreflectedoff of the surface of the sample, dependingon the type of analysis being accomplished. This iswhere specific frequencies of energy, which areuniquely characteristic of the sample, are absorbed


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The Sample Analysis Process

4. The Detector: The beam finally passes tothe detector for final measurement. Thedetectors usedare specially designed tomeasure the special interferogram signal.

5. The Computer: The measured signal isdigitized and sent to the computer where theFourier transformation takes place. The finalinfrared spectrum is then presented to theuser for interpretation and any furthermanipulation.


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Why does absorption occur?


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When can absorption occur ?


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As the name suggests,

dispersive spectrometersgenerate spectra byoptically dispersingthe incoming radiationinto its frequency or spectral components.Common dispersive elements include prisms

and gratings An FTIR spectrometer simultaneously collects

spectral data in a wide spectral range. This

confers a significant advantage overa dispersivespectrometer which measuresintensity over a narrow range of wavelengthsat a time
http://spie.org/x32350.xmlhttp://en.wikipedia.org/wiki/Dispersion_(optics)http://en.wikipedia.org/wiki/Dispersion_(optics)http://spie.org/x32350.xml


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Attenuated Total Reflectance (ATR)cell -

ATR products successfully replace constantpath transmission cells and salt plates used foranalysis of liquid and semi-liquid materials.

ATR is well suited for both qualitative andquantitative applications.

Several heating options are available


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mercury cadmium telluride (MCT)

Most mid-infrared (mid-IR) analyses are performed

with the standard deuterated L-alanine dopedtriglycine sulfate (DLaTGS) detector due to its ease ofuse, high sensitivity, and excellent linearity.

When sample measurements must be made at high

speed or when IR throughput is low, the highlysensitive mercury cadmium telluride (MCT) detectorprovides the ability to scan faster than an DLaTGSdetector while maintaining a constant IR response.

The response of the DLaTGS detector is reduced byhalf for every two-fold multiple of the scanningvelocity. Scanning faster with an MCT detector

reduces sampling time without affecting sensitivity.


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Advantages of FT-IR Some of the major advantages of FT-IR over the

dispersive technique include: Speed: Because all of the frequencies are measured

simultaneously, most measurements by FT-IR are madein a matter of seconds rather than several minutes.This is sometimes referred to as the Felgett Advantage.

Sensitivity: Sensitivity is dramatically improved withFT-IR for many reasons. The detectors employed aremuch more sensitive, the optical throughput is muchhigher (referred to as the Jacquinot Advantage) which

results in much lower noise levels, and the fast scansenable thecoaddition of several scans in order toreduce the random measurement noise to any desiredlevel (referred to as signal averaging).


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Advantages of FT-IR Mechanical Simplicity: The moving mirror in

the interferometer is the only continuouslymoving part in the instrument. Thus, there isvery little possibility of mechanical

breakdown. Internally Calibrated: These instruments

employ a HeNe laser as an internalwavelength calibration standard (referred to

as the Connes Advantage). These instrumentsare self-calibrating and never need to becalibrated by the user.


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SUMMARY


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These advantages, along with several others, make measurementsmade by FT-IR extremely accurate and reproducible.

Thus, it is a very reliable technique for positive identification ofvirtually any sample. The sensitivity benefits enable identificationof even the smallest of contaminants.

This makes FT-IR an invaluable tool for quality control or qualityassurance applications whether it be batch-to-batch comparisonsto quality standards or analysis of an unknown contaminant.

In addition, the sensitivity and accuracy of FT-IR detectors, alongwith a wide variety of software algorithms, have dramatically

increased the practical use of infrared for quantitative analysis.

Quantitative methods can be easily developed and calibrated andcan be incorporated into simple procedures for routine analysis


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Thus, the Fourier Transform Infrared (FT-IR)

technique has brought significant practicaladvantages to infrared spectroscopy.

It has made possible the development of

many new sampling techniques which weredesigned to tackle challenging problems whichwere impossible by older technology.

It has made the use of infrared analysisvirtually limitless


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The use of Fourier Transform Infrared spectroscopy (FTIR) todetermine the structure of biological macromolecules has

dramatically expanded. FTIR spectroscopyrequires only small amounts of proteins

(1mM) in a variety of environments.

Therefore, high qualityspectra can be obtained relatively

easy without problems of background fluorescence, lightscattering and problems relatedto the size of the proteins.

The omnipresent water absorption can be subtracted bymathematical approaches.

Methods are nowavailable that can separatesubcomponents that overlap in the spectra ofproteins.

These facts have madepractical biologicalsystemsamenable tostudies by FTIR spectroscopy.


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Basic principles of infrared (IR) absorption

IR spectroscopy is the measurement ofthe wavelength and intensity of theabsorption of infrared light by a sample.

Infrared light is energetic enough toexcite molecular vibrations to higher

energy levels.


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Fourier Transform Infrared (FTIR) spectroscopy

To use the Fourier Transform InfraredSpectroscopy, a continuum source of light(such as a Nernst Globar) is used to producelight over a broad range of infrared

wavelengths. Light coming from this continuum source is

split into two paths using a half-silvered

mirror; this light is then reflected from twomirrors back onto the beam splitter, where itis recombined.


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Because of the "superposition principle"constructive and destructive interference existfor different wavelengths depending on therelative distances of the two mirrors from the

beamsplitter. It can be shown that if the intensity of light is

measured and plotted as a function of the

position of the movable mirror, the resultantgraph is the Fourier Transform of the intensityof light as a function of wavenumber


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In FTIR spectroscopy , the light is directedonto the sample of interest, and the intensityis measured using an infrared detector.

The intensity of light striking the detector is

measured as a function of the mirror position,and this is then Fourier-transformed toproduce a plot of intensity vs. wavenumber.

As radiation

source a MichelsonInterferometer is used (see the drawingbelow).


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It is necessary to increase the sensitivitysomehow, because the absorption due to one

monolayer of molecules typically results in achange in intensity of only about one part in 105. For semiconductors, one way of increasing the

sensitivity is to use multiple internal reflection. In this technique, the edges of the sample are

polished, and the light is sent in at an angle. Thelight bounces around inside the sample, makingabout 30-50 bounces. This increases the

sensitivity by about a factor of 30-50, making itpossible to measure the absorption of less thanone monolayer of molecules on a surface


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Band assignments Amide vibrations

The peptide group, the structural repeat unit of proteins, gives upto 9 characteristic bands named amide A, B, I, II ... VII. The amide A band (about 3500 cm-1) and amide B (about 3100 cm-

1) originate from a Fermi resonance between the first overtone ofamide II and and the N-H stretching vibration.

Amide I and amide II bands are two major bands of the proteininfrared spectrum. The amide I band (between 1600 and 1700 cm-1) is mainly

associated with the C=O stretching vibration (70-85%) and isdirectly related to the backbone conformation.

Amide II results from the N-H bending vibration (40-60%) and from

the C-N stretching vibration (18-40%). This band is conformationallysensitive. Amide III and IV are very complex bands resulting from a mixture of

several coordinate displacements. The out-of-plane motions are found in amide V, VI andVII


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Amide Ais with more than 95% due to the the N-H stretchingvibration. This mode of vibration does not depend on the backboneconformation but is very sensitive to the strength of a hydrogen bond.It has wavenumbers between 3225 and 3280 cm-1for hydrogen bondlengths between 2.69 to 2.85

Amide Iis the most intense absorption band in proteins. It is primilarygoverned by the stretching vibrations of the C=O (70-85%) and C-Ngroups (10-20%). Its frequency is found in the range between 1600 and1700 cm-1. The exact band position is determined by the backboneconformation and the hydrogen bonding pattern.

Amide IIis found in the 1510 and 1580 cm-1region and it is morecomplex than amide I. Amide II derives mainly from in-plane N-Hbending (40-60% of the potential energy). The rest of the potentialenergy arises from the C-N (18-40%) and the C-C (about 10%)stretching vibrations.

Amide III, Vare very complex bands

dependent on the details of theforce field, the nature of side chains and hydrogen bonding. Thereforethese bands are only of limited use for the extraction of structuralinformation.


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CONCLUSION

FTIR is a chemically-specificanalysis technique

It can be used to identifychemical compounds,

and substituent groups

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FTIR Spec_L3_Rev.pdf