Fourier Transform Infrared Spectrometry (FTIR) and Textile

Fourier Transform Infrared
Spectrometry (FTIR) and Textile
Azmir Latif
M.Sc in Textile Engineering

What is Infrared Region?
 Infrared radiation lies between the visible and
microwave portions of the electromagnetic spectrum.
 Infrared waves have wavelengths longer than visible
and shorter than microwaves, and have frequencies
which are lower than visible and higher than
microwaves.
 The Infrared region is divided into: near, mid and far-
infrared.
 Near-infrared refers to the part of the infrared
spectrum that is closest to visible light and far-
infrared refers to the part that is closer to the
microwave region.
 Mid-infrared is the region between these two.
Monday, November 06,
2017

Monday, November 06, 2017
Terahertz gap
1011 – 1013 Hz

 The primary source of infrared radiation is thermal
radiation. (heat)
 It is the radiation produced by the motion of atoms and
molecules in an object. The higher the temperature, the
more the atoms and molecules move and the more
infrared radiation they produce.
 Any object radiates in the infrared. Even an ice cube,
emits infrared.

According to Principle of IR,
Molecular Vibration takes place as a result
of Absorption of IR radiation when –
Applied Infrared Frequency = Natural frequency of vibration
•Every bond or functional groups requires different frequency
for Absorption. Hence characteristics Peak is observed for
every functional group or part of the molecule.

What is FT-IR?

 FT-IR stands for Fourier Transform Infrared, the
preferred method of infrared spectroscopy. In infrared
spectroscopy, IR radiation is passed through a sample.
Some of the infrared radiation is absorbed by the sample
and some of it is passed through (transmitted).
 The resulting spectrum represents the molecular absorption
and transmission, creating a molecular fingerprint of the
sample.
 Like a fingerprint no two unique molecular structures
produce the same infrared spectrum. This makes infrared
spectroscopy useful for several types of analysis.

 So, what information can FT-IR provide?
 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

Dispersive Spectrometer FTIR
In order to measure an IR
spectrum,
The dispersion
Spectrometer takes several
minutes.
Also the detector receives
only a few % of the energy
of original light source.
In order to measure an IR
spectrum,
FTIR takes only a few
seconds.
Moreover, the detector
receives up to 50% of the
energy of original light
source. (much larger than
the dispersion
spectrometer.)

To separate IR light, a grating is used.
Grating
Light source
Detector
Sample
Slit
To select the specified IR light,
A slit is used.
Fixed CCM
B.S.
Moving CCM
IR Light source
Sample
Detector
An interferogram is first
made by the interferometer
using IR light.
The interferogram is calculated and transformed
into a spectrum using a Fourier Transform (FT).

THEORY OF FT-IR:
 Fourier Transform Infrared (FT-IR) spectrometry was
developed 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 frequencies
simultaneously, rather than individually, was needed.
 A solution was developed which employed a very simple
optical device called an interferometer. The interferometer
produces a unique type of signal which has all of the infrared
frequencies “encoded” into it.
 The signal can be measured very quickly, usually on the
order of one second or so. Thus, the time element per sample is
reduced to a matter of a few seconds rather than several
minutes.

 Most interferometers employ a beamsplitter which
takes the incoming infrared beam and divides it into two
optical beams.
 One beam reflects off of a flat mirror which is fixed in
place. The other beam reflects off of a flat mirror which 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 the beamsplitter.
Because the path that one beam travels is a fixed length and
the other is constantly changing as its mirror moves, the
signal which exits the interferometer is the result of these two
beams “interfering” with each other.
 The resulting signal is called an interferogram which
has the unique property that every data point (a function of
the moving mirror position) which makes up the signal has
information 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 the interferometer results in extremely fast
measurements.
 Because the analyst requires a frequency spectrum (a
plot of the intensity at each individual frequency) in order
to make an identification, the measured interferogram
signal can not be interpreted directly.
 A means of “decoding” the individual frequencies is
required. This can be accomplished via a well-known
mathematical technique called the Fourier transformation.
This transformation is performed by the computer which
then presents the user with the desired spectral
information for analysis.

Superiority of FT-IR
Dispersive infrared spectrometers suffer from several
disadvantages in sensitivity, speed and wavelength
accuracy. Most of the light from the source does not in fact
pass through the sample to the detector, but is lost in the
narrowness of the focusing slits; only poor sensitivity
results. Since the spectrum takes minutes to record, the
method cannot be applied to fast processes.
Dispersive infrared spectrometers scan over the
wavelength range and disperse the light by use of a
grating, these spectrometers suffer from wavelength
inaccuracies associated with the backlash in the mechanical
movements, such as in the rotation of mirrors and
gratings.
An entirely different principle is involved in Fourier
Transform infrared spectroscopy, which centres on a
Michelson interferometer, so that the method can also be
called interferometric infrared spectroscopy.

Fourier transform infrared spectroscopy is preferred
over dispersive or filter methods of infrared spectral
analysis for several reasons:
• It is a non-destructive technique.
• It provides a precise measurement method which
requires no external calibration.
• It can increase speed, collecting a scan every second.
• It can increase sensitivity.
• It has greater optical throughput.
• It is mechanically simple with only one moving part.

SAMPLING TECHNIQUES
1.Liquid Samples:
Neat sample
Diluted solution
Liquid cell
2.Solid Samples:
Neat sample
Cast films
Pressed films
KBr pellets
Mull
3.Gas Samples:
Short path cell
Long path cell

The Sample Analysis Process:
The normal instrumental process is as follows:
1.The Source: Infrared energy is emitted from a glowing black-
body source. This beam passes through an aperture which
controls the amount of energy presented to the sample (and,
ultimately, to the detector).
2.The Interferometer: The beam enters the interferometer where
the “spectral encoding” takesplace. The resulting interferogram
signal then exits the interferometer.
3.The Sample: The beam enters the sample compartment where
it is transmitted through or reflected off of the surface of the
sample, depending on the type of analysis being accomplished.
This is where specific frequencies of energy, which are
uniquely characteristic of the sample, are absorbed.
4.The Detector: The beam finally passes to the detector for final
measurement. The detectors used are specially designed to
measure the special interferogram signal.

5.The Computer: The measured signal is digitized and sent to
the computer where the Fourier transformation takes place.
The final infrared spectrum is then presented to the user for
interpretation and any further manipulation.
Because there needs to be a relative scale for the
absorption intensity, a background spectrum must also be
measured. This is normally a measurement with no sample in
the beam. This can be compared to the measurement with the
sample in the beam to determine the “percent transmittance.”
This technique results in a spectrum which has all of the
instrumental characteristics removed.
Thus, all spectral features which are present are
strictly due to the sample. A single background measurement
can be used for many sample measurements because this
spectrum is characteristic of the instrument itself.

sample light

goal – spectrum
of absorption

beam source sample detector
absorption
It shines a monochromatic light beam…

absorption
…and measures how much is absorbed

absorption
beam number
A beam contains many frequencies of light at
once and absorption of that beam is measured

absorption
beam number
Next, beam is modified to give a second data
point

absorption
beam number
Process is repeated many times

Afterwards, computer processing is required

Infrared Spectroscopy
For isopropyl alcohol, CH(CH3)2OH, the infrared
absorption bands identify the various functional groups
of the molecule.

cancer
cavity
Visible image THz image

Visible image THz image
ceramic
knife

Advantages of FT-IR
•Speed: Because all of the frequencies are measured
simultaneously, most measurements by FT-IR are made in a
matter of seconds rather than several minutes.
•Sensitivity: Sensitivity is dramatically improved with FT-IR
for many reasons. The detectors employed are much more
sensitive, the optical throughput is much higher which results
in much lower noise levels, and the fast scans enable the co
addition of several scans in order to reduce the random
measurement noise to any desired level (referred to as signal
averaging).
•Mechanical Simplicity: The moving mirror in the
interferometer is the only continuously moving part in the
instrument. Thus, there is very little possibility of mechanical
breakdown.

• Internally Calibrated: These instruments employ a HeNe
laser as an internal wavelength calibration standard.
These instruments are self-calibrating and never need to
be calibrated by the user.

Tuesday, November 07, 2017
Qualitative analysis of textile fiber by Fourier Transform Infrared (FTIR)
By FTIR we only know the name of fiber is identified. By this technique
we can identify the exact composition of fiber like 80 % polyester 20 %
cotton. Fiber polymer identification is made by comparison of the fiber
spectrum with reference spectra. Fiber samples may be prepared and
mounted for microscopical infrared analysis by a variety of techniques. It
is used to determine the chemical formula for the textile fiber. We can
identify textile fiber qualitatively to see the FTIR spectra. Infrared
spectra of fibers are obtained using an IR spectrophotometer coupled
with an IR microscope. When infar- red radiation is composed of
electromagnetic waves with wavelengths between 1 and 15 µm, it is
passed through a material. It is most highly absorbed at certain
characteristic frequencies. By using an infar-red spectrometer, the
variation in absorption can be found and plotter against wavelength, or,
more commonly, its reciprocal, the wave-number.

Infrared absorptions occur in vibrating molecules where there is a changing dipole moment between
the atoms. In this sense, Infrared spectroscopy is not sensitive to symmetric vibrations such as –C-C-.
Therefore for polymeric materials, the main chain or backbone structure is often not detected using
this technique and it is usually the side chain motions to atoms that give rise to absorptions.
Infrared spectroscopic absorbance bands of textile fiber by FTIR
Fiber Characteristic absorbance bands (cm‾1)
Cotton 3450 3250 2900 1630 1430 1370 1100 970 550
Wool 3400 3250 2900 1720 1600 1500 1220
Silk (fibroin) 3300 2950 1710 1630 1530 1500 1440 1220 610
540
Rayon 3450 3250 2900 1650 1430 1370 1060 970 890
Triacetate 3500 2950 1750 1430 1370 1230 1040 900
PET (Poly ethylene
terephthalate)
1730 1410 1340 1250 1120 1100 1020 870 730
Nylon 6 3300 3050 2950 2850 1630 1530 1450 1250 680
570
Nylon 66 3300 2950 2850 1630 1530 1470 1270 930 680 570
Polyacrylonitrile 2950 2250 1730 1450 1360 1220 1160 1060 540
Polyethylene 2900 2850 1470 1460 1370 740 720
Polypropylene 2970 2940 2850 1450 1370 1160 990 970 840
Polyvinyl alcohol 3400 2950 1430 1400 1090 1050 1020 850 790

Polymeric analysis of cotton fiber
IR bands corresponding to vibrations in the hydrogen bond networks of
Iβ cellulose (like cotton).
Wave number (cm‾1) Assignment
665, 705 O-H out of plane bending in COH alcoholic groups
1035 C-O mainly of C6H2-O6H primary alcohols (secondary
conformation)
1060 C-O mainly of C3-O3H secondary alcohol
1115 C-O mainly of C2-O2H secondary alcohols
1160 Mainly antisymmetric C-O-C glycoside
1315, 1335, 1430 O-H(…) (in plane) bending of C-O-H alcohol groups
3275 O-H(…) mainly of C2O2-H secondary alcohols
3305 O-H(…) mainly of C6H2O6-H primary alcohols (minor
conformation)
3340 O-H(…) mainly of C3O3-H secondary alcohols
3410 O-H(…) mainly of C6H2O6-H primary alcohols (dominant
conformation)

REFERENCES :-
•Introduction to Fourier Transform Infrared Spectrometry By
Thermo Nicolet Corporation
•An article on FTIR SAMPLING TECHNIQUES by Hue Phan.
TN.101
•Instrumental Methods Of Chemical Analysis by Gurdeep R.
Chatwal & Sham K. Anand
•P. R. Griffiths and J. A. de Haseth, Fourier-Transform Infrared
Spectroscopy, Wiley- Interscience, New York,
Chichester,Brisbane, Toronto, Singapore,1986
•FTIR Spectroscopy By Jorge. E. Perez and Richard T. Meyer
CIC
• Photonics, Inc. 3825 Osuna Rd. NE Ste. 6 & 7. Albuquerque,
NM 87105.

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