Fourier transform infrared spectroscopy

Fourier Transform Infrared Spectroscopy (FTIR)
By: Asaye Dessie
Getaneh Alamir
Fourier Transform Infrared Spectroscopy (FTIR)
By: Asaye Dessie
Getaneh Alamir

Content
 Introduction
o Brief history of development of FTIR
o Theoretical background
 Mathematical expressions of Fourier transforms
 Components of FTIR spectroscopy
 The working principles of Michelson interferometer
 Generating the spectrum
 Principle of Absorption
 FT-IR Analysis
 Characterization of textile fibers by FTIR spectra
 Advantages of FT-IR
 Limitation of FTIR
 Application of FT-IR in Textiles
 Reference
Content
 Introduction
o Brief history of development of FTIR
o Theoretical background
 Mathematical expressions of Fourier transforms
 Components of FTIR spectroscopy
 The working principles of Michelson interferometer
 Generating the spectrum
 Principle of Absorption
 FT-IR Analysis
 Characterization of textile fibers by FTIR spectra
 Advantages of FT-IR
 Limitation of FTIR
 Application of FT-IR in Textiles
 Reference

BRIEF HISTORY OF DEVELOPMENT OF FTIR
 FT-IR spectrometry was developed in order to overcome the
limitations encountered with dispersive instruments.
 The limitation of Dispersive was the slow scanning process and
measuring individual infrared frequencies.
 A method for measuring all of the infrared frequencies
simultaneously and very simple optical was needed.
 Fourier had developed mathematical transform (FT) method in 1700.
 Albert Michelson had perfected FT-IR instrument in 1887 and design
the spectra of organic and his interferometer in 1891.
 FT-IR was combined with personal computers to make widely used,
versatile, and cost-effective method of analysis in 1980s
 FT-IR spectrometry was developed in order to overcome the
limitations encountered with dispersive instruments.
 The limitation of Dispersive was the slow scanning process and
measuring individual infrared frequencies.
 A method for measuring all of the infrared frequencies
simultaneously and very simple optical was needed.
 Fourier had developed mathematical transform (FT) method in 1700.
 Albert Michelson had perfected FT-IR instrument in 1887 and design
the spectra of organic and his interferometer in 1891.
 FT-IR was combined with personal computers to make widely used,
versatile, and cost-effective method of analysis in 1980s

THEORETICAL BACKGROUND
 Spectroscopy is the study of matter and its properties by
investigating light, sound, or particles that are emitted, absorbed or
scattered by the matter under investigation.
 It is the study of the interaction between light and matter.
 Infrared frequencies of light are used to study fundamental vibrations
and associated rotational-vibrational structure via vibrational
resonance and selective absorption.
Infrared SpectroscopyInfrared Spectroscopy
 Spectroscopy is the study of matter and its properties by
investigating light, sound, or particles that are emitted, absorbed or
scattered by the matter under investigation.
 It is the study of the interaction between light and matter.
 Infrared frequencies of light are used to study fundamental vibrations
and associated rotational-vibrational structure via vibrational
resonance and selective absorption.

IR region is subdivided into three regions, near IR, mid IR and far IR.

Cont.…
 Generally there are two types of infrared spectroscopy
Dispersive infrared spectroscopy
Fourier transform infrared spectroscopy
 Dispersive spectrophotometers, which use a monochromatic to
produce an infrared spectrum one resolution element at a time.
 Michelson interferometers, which use a moving mirror
adjustment to create an interferogram, from which all resolution
elements are determined simultaneously.
 Generally there are two types of infrared spectroscopy
Dispersive infrared spectroscopy
 Dispersive spectrophotometers, which use a monochromatic to
produce an infrared spectrum one resolution element at a time.
 Michelson interferometers, which use a moving mirror
adjustment to create an interferogram, from which all resolution
elements are determined simultaneously.

FOURIER TRANSFORM INFRARED SPECTROSCOPY
 FT-IR stands for Fourier Transform InfraRed, the preferred method
of infrared spectroscopy.
 Fourier infrared spectroscopy is the study of interactions between
matter and electromagnetic fields in the IR region.
 In this spectral region, the EM waves mainly couple with the
molecular vibrations.
 A molecule can be excited to a higher vibrational state by absorbing
IR radiation.
 It covers a range of techniques, mostly based on absorption spectroscopy
 This makes infrared spectroscopy useful for several types of analysis.
 FT-IR stands for Fourier Transform InfraRed, the preferred method
of infrared spectroscopy.
 Fourier infrared spectroscopy is the study of interactions between
matter and electromagnetic fields in the IR region.
 In this spectral region, the EM waves mainly couple with the
molecular vibrations.
 A molecule can be excited to a higher vibrational state by absorbing
IR radiation.
 It covers a range of techniques, mostly based on absorption spectroscopy
 This makes infrared spectroscopy useful for several types of analysis.

Cont.…
 FT-IR can provide the following information.
It can identify unknown materials
It can determine the quality or consistency of a sample
It can determine the amount of components in a mixture
 Infrared spectroscopy can result in a positive identification
(qualitative analysis) of every different kind of material.
 With modern software algorithms, infrared is an excellent tool for
quantitative analysis.
 FT-IR can provide the following information.
It can identify unknown materials
It can determine the quality or consistency of a sample
It can determine the amount of components in a mixture
 Infrared spectroscopy can result in a positive identification
(qualitative analysis) of every different kind of material.
 With modern software algorithms, infrared is an excellent tool for
quantitative analysis.

MATHEMATICAL EXPRESSIONS OF FOURIER TRANSFORM
tTRANSFORMS FTIR spectrometer operates on a different principle called Fourier
transform.
 The mathematical expression of Fourier transform can be expressed
as:
 And the reverse Fourier transform is;
Where: ω is angular frequency and
x is the optical path difference.
F(ω) is the spectrum and
f(x) is called the interferogram.
i is called square root of -1
 FTIR spectrometer operates on a different principle called Fourier
transform.
 The mathematical expression of Fourier transform can be expressed
as:
 And the reverse Fourier transform is;
Where: ω is angular frequency and
x is the optical path difference.
F(ω) is the spectrum and
f(x) is called the interferogram.
i is called square root of -1

Components of FTIR Spectroscopy
 Source: Infrared energy is emitted from a glowing black-body source.
 Interferometer: The beam enters the interferometer where the
“spectral encoding” takes place.
 Sample: The beam enters the sample compartment where it is
transmitted through or reflected off of the surface of the sample.
 The Detector: Detectors transform the input energy into an output then
converted to a signal.
 The Computer: The measured signal is digitized and sent to the
computer where the Fourier transformation takes place.
 Moving mirror: It is the only moving part of the instrument.
 Fixed mirror: It is a stationary mirror
 Source: Infrared energy is emitted from a glowing black-body source.
 Interferometer: The beam enters the interferometer where the
“spectral encoding” takes place.
 Sample: The beam enters the sample compartment where it is
transmitted through or reflected off of the surface of the sample.
 The Detector: Detectors transform the input energy into an output then
converted to a signal.
 The Computer: The measured signal is digitized and sent to the
computer where the Fourier transformation takes place.
 Moving mirror: It is the only moving part of the instrument.
 Fixed mirror: It is a stationary mirror

Working principles of Michelson Interferometer
 Light from the light source is directed to the beam splitter.
 Half of the light is reflected and half is transmitted.
 The reflected light goes to the fixed mirror where it is reflected back
to the beam splitter.
 The transmitted light is sent to the moving mirror and is also
reflected back towards the mirror.
 At the beam splitter, each of the two beams (from the fixed and
moving mirrors) are split into two:
One goes back to the source and
The other goes towards the detector.
 Light from the light source is directed to the beam splitter.
 Half of the light is reflected and half is transmitted.
 The reflected light goes to the fixed mirror where it is reflected back
to the beam splitter.
 The transmitted light is sent to the moving mirror and is also
reflected back towards the mirror.
 At the beam splitter, each of the two beams (from the fixed and
moving mirrors) are split into two:
One goes back to the source and
The other goes towards the detector.

Cont.
 The two beams reaching the detector come from the same source and have
an optical path difference determined by the positions of the two mirrors,
 That means they have a fixed phase difference and the two beams interfere.
 The two beams interfere constructively or destructively for a particular
frequency by positioning the moving mirror.
 If the moving mirror is scanned over a range, a sinusoidal signal will be
detected for that frequency with its
maximum corresponding to constructive interference and
minimum corresponding to destructive interference.
 This sinusoidal signal is called interferogram – detector signal (intensity)
against optical path difference.
 The two beams reaching the detector come from the same source and have
an optical path difference determined by the positions of the two mirrors,
 That means they have a fixed phase difference and the two beams interfere.
 The two beams interfere constructively or destructively for a particular
frequency by positioning the moving mirror.
 If the moving mirror is scanned over a range, a sinusoidal signal will be
detected for that frequency with its
maximum corresponding to constructive interference and
minimum corresponding to destructive interference.
 This sinusoidal signal is called interferogram – detector signal (intensity)
against optical path difference.

Generating the Spectrum
 Interferogram is determined experimentally in FTIR spectroscopy,
and the corresponding spectrum – frequency against intensity plot,
is computed using Fourier transform.
 This transformation is carried out automatically and the spectrum is
displayed.
 The detector sees all the frequencies simultaneously.
 It is imperative to record a relevant background spectrum for each
sample examined.
 Interferogram is determined experimentally in FTIR spectroscopy,
and the corresponding spectrum – frequency against intensity plot,
is computed using Fourier transform.
 This transformation is carried out automatically and the spectrum is
displayed.
 The detector sees all the frequencies simultaneously.
 It is imperative to record a relevant background spectrum for each
sample examined.

Cont.
Background spectrum:
 The empty beam background (no sample in the light path) is
recorded first.
 This spectrum shows the instrument energy profile.
Sample spectrum:
 The sample is placed in the combined beam.
 The sample spectrum is the ratio of the spectrum containing sample
against that of the background.
 In recording the background spectrum, the light path should be made
as close to that of the sample spectrum as possible.
Background spectrum:
 The empty beam background (no sample in the light path) is
recorded first.
 This spectrum shows the instrument energy profile.
Sample spectrum:
 The sample is placed in the combined beam.
 The sample spectrum is the ratio of the spectrum containing sample
against that of the background.
 In recording the background spectrum, the light path should be made
as close to that of the sample spectrum as possible.

Principle of Absorption
 At temperatures above absolute zero, all the atoms in molecules are in
continuous vibration with respect to each other.
 As a molecule vibrates , a regular fluctuation in the dipole moment
occurs.
A Dipole Moment = Charge Imbalance in the molecule
 When the frequency of a specific vibration is equal to the frequency of
the IR radiation directed on the molecule, the molecule absorbs the
radiation and amplitude of the vibration increases.
 At temperatures above absolute zero, all the atoms in molecules are in
continuous vibration with respect to each other.
 As a molecule vibrates , a regular fluctuation in the dipole moment
occurs.
A Dipole Moment = Charge Imbalance in the molecule
 When the frequency of a specific vibration is equal to the frequency of
the IR radiation directed on the molecule, the molecule absorbs the
radiation and amplitude of the vibration increases.

Cont.
 The major types of molecular vibrations are Stretching and Bending
Stretching -along the line of the chemical bond
Bending - out of the line with the chemical bond.
 The absorbed Infrared radiation and the associated energy is
converted into these type of motions.
 Stretching > Bending
 The major types of molecular vibrations are Stretching and Bending
Stretching -along the line of the chemical bond
Bending - out of the line with the chemical bond.
 The absorbed Infrared radiation and the associated energy is
converted into these type of motions.
 Stretching > Bending

Table of Characteristic IR Absorptions

FT-IR Analysis
In fiber characterization by FTIR, analysis is done in two ways:
1. Qualitative Analysis
 For qualitative identification purposes, the spectrum is commonly presented as
transmittance vs wave number.
 It is possible to identify a functional group of a molecule by comparing its
vibrational frequency on an IR spectrum to an IR stored data bank.
 Functional groups have their characteristic fundamental vibrations which give rise
to absorption at certain frequency range in the spectrum.
 However, several functional groups may absorb at the same frequency range, and
a functional group may have multiple-characteristic absorption peaks, especially
for 1500 – 650 cm-1, which is called the fingerprint region.
 In addition, the size of the peaks in the spectrum is a direct indication of the
amount of material present.
In fiber characterization by FTIR, analysis is done in two ways:
1. Qualitative Analysis
 For qualitative identification purposes, the spectrum is commonly presented as
transmittance vs wave number.
 It is possible to identify a functional group of a molecule by comparing its
vibrational frequency on an IR spectrum to an IR stored data bank.
 Functional groups have their characteristic fundamental vibrations which give rise
to absorption at certain frequency range in the spectrum.
 However, several functional groups may absorb at the same frequency range, and
a functional group may have multiple-characteristic absorption peaks, especially
for 1500 – 650 cm-1, which is called the fingerprint region.
 In addition, the size of the peaks in the spectrum is a direct indication of the
amount of material present.

Fingerprint RegionFingerprint Region
 More complex and more difficult to interpret.
 Small structural differences results in significant in spectral
differences
 Complete interpretation impossible
 Complete identification requires 100% match between sample’s and
standard’s spectra in the finger print region

Cont.
Functional groups vibration and its relationships with regions of infrared absorption

Cont.
2. Quantitative Analysis
o Absorbance (A) is used for quantitative analysis due to its linear
dependence on concentration.
o It is given by Beer-Lambert law; absorbance is directly proportional
to the concentration and path length of sample:
Where:
A- is absorbance,
ε -the molar extinction coefficient or molar absorptivity,
c -the concentration and
l- the path length (or the thickness) of sample.
o Thus the intensity of the peaks in the FT-IR spectrum is proportional
to the amount of substance present, for identical ε and c.
A=ϵcl
2. Quantitative Analysis
o Absorbance (A) is used for quantitative analysis due to its linear
dependence on concentration.
o It is given by Beer-Lambert law; absorbance is directly proportional
to the concentration and path length of sample:
Where:
A- is absorbance,
ε -the molar extinction coefficient or molar absorptivity,
c -the concentration and
l- the path length (or the thickness) of sample.
o Thus the intensity of the peaks in the FT-IR spectrum is proportional
to the amount of substance present, for identical ε and c.
A=ϵcl

Characterization of Cotton by FTIR spectraCharacterization of Cotton by FTIR spectra
O-H- 3335cm-1 broad,
medium
2850 cm-1 CH2 stretch
1478 cm-1 ( H-C-H and H-O-
C bend),
1379cm-1, 1334 cm-1 (H-C-
C, H-C-O, and H-O-C bend),
1108cm-1 (C-C and C-O
stretch),
910 cm-1 ( C-O-C in plane,
symmetric), and
516-379 cm-1 (skeletal C-O-
C, C-C-C,O-C-C and O-C-O
bend)
medium
1478 cm-1 ( H-C-H and H-O-
C bend),
1379cm-1, 1334 cm-1 (H-C-
stretch),
symmetric), and
bend)
medium
1478 cm-1 ( H-C-H and H-O-
C bend),
1379cm-1, 1334 cm-1 (H-C-
stretch),
symmetric), and
bend)
medium
1478 cm-1 ( H-C-H and H-O-
C bend),
1379cm-1, 1334 cm-1 (H-C-
stretch),
symmetric), and
bend)Spectra of a single cotton fiber

Characterization of Polyester by FTIR spectraCharacterization of Polyester by FTIR spectra
Spectra of a single polyester fiber

Characterization of Nylon by FTIR spectraCharacterization of Nylon by FTIR spectra

ADVANTAGES OF FTIR
 Fourier transform infrared spectroscopy is preferred over dispersive
or filter methods of infrared spectral analysis for several reasons:
It is a non-destructive technique
Speed
Sensitivity
Mechanical simplicity
Internally calibrated (self-calibrating)
 Fourier transform infrared spectroscopy is preferred over dispersive
or filter methods of infrared spectral analysis for several reasons:
It is a non-destructive technique
Speed
Sensitivity
Mechanical simplicity
Internally calibrated (self-calibrating)

LIMITATIONS OF FTIR
It cannot be used to detect all the vibration modes in a molecule.
It is not possible to know molecular weight of substance
It is not possible to know whether it is pure compound or a mixture
of compound.
Interferogram are difficult to interpret without first performing a
Fourier transform to produce a spectrum.
Accuracy of FT-IR remains true if there is no change in atmospheric
conditions throughout the experiment.
It cannot be used to detect all the vibration modes in a molecule.
It is not possible to know molecular weight of substance
It is not possible to know whether it is pure compound or a mixture
of compound.
Interferogram are difficult to interpret without first performing a
Fourier transform to produce a spectrum.
Accuracy of FT-IR remains true if there is no change in atmospheric
conditions throughout the experiment.

Application of FT-IR in Textile
• Identification of compounds by matching spectrum of unknown
compound with reference spectrum (fingerprinting)
• Identification of functional groups in unknown substances
• Identification of reaction components and kinetic studies of reactions
• Identification of molecular orientation in polymer films.
• Detection of molecular impurities or additives present
• The same way it determines Percentage of trash particles or foreign
matter present in fiber, yarn or fabric.
• Identification of polymers, plastics, and resins.
• Identification of compounds by matching spectrum of unknown
compound with reference spectrum (fingerprinting)
• Identification of functional groups in unknown substances
• Identification of reaction components and kinetic studies of reactions
• Identification of molecular orientation in polymer films.
• Detection of molecular impurities or additives present
• The same way it determines Percentage of trash particles or foreign
matter present in fiber, yarn or fabric.
• Identification of polymers, plastics, and resins.

Fourier transform infrared spectroscopy

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