IR Spectroscopy.ppt IR Spectroscopy is a method in which molecules vibrate due

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School of Health and Allied Sciences
Instrumentation of IR SPECTROSCOPY
Instrumentation of IR SPECTROSCOPY
Phr. Sharvendra Nath Maurya
Phr. Binod Bhandari
Pokhara University

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Dispersive Infrared spectrophotometers
 The IR spectrometer consists of three basic components, i.e. IR
radiation source, monochromator, and detector.
 The instrument produces a beam of infrared radiation from a hot
filament wire and uses mirrors to divide it into two parallel beams of
equal intensity radiation.
 The sample is placed in one beam, and a reference in the other. The
beam then passes into the monochromator which disperses it into a
continuous spectrum of frequencies of IR light.
August 18, 2025 INS 591: Presentation 2022 2

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 The monochromator consists of a beam of chopper (a rapidly rotating
sector) that passes the two beams alternatively to a diffraction grating.
 The slowly rotating diffraction grating varies the frequency of radiation
reaching the thermocouple detector, which senses the ratio between
the intensities of the reference and sample beams.
 In this way, the detector determines which frequencies have been
absorbed by the sample. Once the signal from the detector is
amplified, the recorder draws the resulting spectrum.

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August 18, 2025 INS Presentation 2024 5

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Sample Handling
a. Solids
• Pressed Pellet Technique: In this technique, a small amount of finely ground
sample is mixed with 0.1-2% of dried powdered KBr/NaCl and compressed
(other alkali metal halides also can be used) under very high pressure to obtain
a transparent disk about 1cm in diameter and 1-2 mm thick.

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• Nujol Mull Technique: It involves grinding the compound with mineral oil to
create a suspension of finely ground sample dispersed in the mineral oil. The
thick suspension is placed between the salt plates. Nujol band appears at
2924, 1462, and 1377cm-1
.

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• Solid run in solution
Solid is dissolved in non-aqueous inert solution
A drop of this solution is placed on alkali metal disc
Allow to evaporate leaving a thin film of solute
Allow thin film in spectrometer
Some solvents used are chloroform, carbon tetrachloride, acetone

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Solid films:
If a solid is amorphous in nature
Dissolve the sample in a suitable volatile solvent
Sample is deposited on the surface of KBr or NaCl cell
Evaporate by gentle heat
•This method is useful for rapid qualitative analysis and useless for
quantitative analysis.

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b. Liquids
A drop of liquid organic compound is placed between a pair of a polished
sodium chloride or potassium bromide plates known as salt plates. When the
plates are squeezed gently then a thin liquid forms between them. Organic
compounds analyzed by this technique must be free of water.
c. Gases
Gas samples are introduced into a glass cell made up of Nacl. The IR radiation
passes through the sample and chemical vapours in the sample will absorb IR
energy at different wavelength.

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Monochromators
•They are used to select radiation of any desired frequency from source and
eliminate other frequencies.
Prisms:
•By rotation of prism, different wavelengths of the spectrum can be made to
pass through an exit slit and through the sample. Glass prisms cannot be used
as it absorbs IR.
•Metal halide prisms
•Nacl Prism (2-15µm)

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Diffraction Grating:
Gratings are commonly employed in the design of the instruments and offer
better resolution at higher frequency than the prisms. They offer much better
resolution at low frequency.

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Detectors
Thermocouples
•The underlying principle of thermocouple is that two dissimilar wires are
joined head to tail, then a difference between head and tail causes a current to
flow in the wires. This current shall be directly proportional to the intensity of
radiation falling on the thermocouple

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Bolometers
•They are based on the principle that make use of the increase in resistance of
a metal with increase in temperature. When two platinum foils are
appropriately incorporated into a wheatstone bridge, and a radiation is allowed
to fall on the foil, a change in resistance is observed ultimately. This causes an
out-of-balance current that is directly proportional to the incident radiation.
Photo conducting Detectors
•It consists of a thin film of semiconductor (ex. PbS) on a non-conducting glass
surface and sealed in a vacuum. The absorption of light by semiconductor
moves from non-conducting to conducting state. The decrease in resistance
leads to increase in current. Its range is: 10,000 -333 cm-1 at room
temperature.

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Golay detector
•In this specific instance, the absorption of infrared radiation affords expansion
of an inert gas in a cell chamber. One wall of the cell chamber is provided with
a flexible mirror and the resulting distortion alters the intensity of illumination
falling on a photocell from the reflected beam of light.

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• Pyroelectric Detectors
In this detector, pyroelectric (ceramic, lithium tantalate) material get polarized
(separation of (+) and (-) charges) in presence of electric field. Polarization is
dependent on temperature. It measures degree of polarization related to
temperature of crystal. Fast response is obtained and is good for FTIR.

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Fourier Transform Infrared Spectrometer
• Infrared spectra are obtained by detecting changes
in the absorbance (or transmittance) intensity as a
function of frequency.
• Fourier Transform Infrared Spectrometers have
replaced the Dispersive instrument for most of the
application because of their superior speed and
sensitivity.

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Principle of FTIR
• The Michelson interferometer consists of a beam splitter, a moving mirror,
and a stationary mirror. The beam splitter divides the light beam into two
halves, which are reflected by the moving and fixed mirrors before being
recombined by the beam splitter.
• As the moving mirror makes reciprocating movements, the optical path
difference to the fixed mirror changes, causing the phase difference to shift
over time. Interference light is created in the Michelson interferometer by
recombining the light beams. An interferogram records the intensity of the
interference light, with the optical path difference recorded along the
horizontal axis.

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Different parts of FTIR instrumentation include:
1. The Source: A broadband emitter, such as a mid-IR ceramic source, a far-
infrared mercury lamp, or a near-infrared halogen lamp, is used as the light
source.
2. The Interferometer
 The interferometer, which consists of a beam splitter, a stationary mirror,
and a moving mirror, is the heart of an FTIR spectrometer. The beam splitter
is a semi-transparent mirror that divides a collimated light beam into two
optical channels. Half of the light is transferred to the moving mirror and half
is reflected to the stationary mirror. The moving and stationary mirrors
reflect the two light beams, which are recombined at the beam splitter
before going through the sample chamber and onto the detector.

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3. The sample
Depending on the type of analysis being performed, the beam enters the sample
compartment and is either transmitted through or reflected off the surface of the
sample. This is where certain frequencies of energy that are unique to the sample are
absorbed.
4. Detector
FTIR detectors are used to measure and convert the transmitted or reflected light
from a sample into an electrical signal. The sensitivity and wavelength range of the
data that can be captured is determined by the type and material of the detector.
The detector converts the beam into photons, which are then translated into
measurable electric signals that the computer can read.

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 The following are some examples of common detectors:
• Room temperature DLATGS is a regular analysis tool.
(DLATGS = Deuterated L-Alanine-doped TriGlycine Sulfate- crystal
material used as a pyroelectric detector).
• Cooled liquid nitrogen is employed in sensitive applications.
• Si-photodiodes are employed in near-IR and visible infrared
applications.
• Silicon far-infrared bolometers

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Fourier transforms infrared spectroscopy is favored over
dispersive or filter methods of infrared spectral analysis:
 is a non-destructive procedure.
 provides a precise measurement method that does not require external calibration.
 increase speed by collecting a scan every second.
 increase sensitivity by combining scans taken at intervals of one second to cancel
out random noise.
 offers higher optical throughput.
 has a simplified mechanical design with only one moving part.

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Advantages of FTIR
• It has a higher speed. Because all of the frequencies are detected simultaneously,
most FT-IR measurements are completed in seconds rather than minutes. This is
also known as Felgett Advantage.
• It has high sensitivity. For a variety of reasons, FT-IR improves sensitivity
considerably. The detectors used are much more sensitive, the optical throughput is
much higher, resulting in much lower noise levels, and the fast scans allow for the
coaddition of several scans to reduce random measurement noise to any desired
level.
• It is a very accurate and reproducible method, making it suitable for background
subtraction.

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• is an extremely reliable approach for positively identifying almost any material.
• use a HeNe laser as an internal wavelength calibration standard (the Connes
Advantage). These instruments are self-calibrating and never require user
calibration.
• used to capture IR data from very small samples.
• It has a high throughput, often known as the Jaquinot advantage. The radiant power
reaching the detector is significantly higher than in a dispersive device.
• It has high precision. The laser in an FT-IR spectrometer serves as the instrument’s
reference signal and time keeper. It also moves at the same rate as other
components inside its own system. This can provide reliable measurements without
being influenced by outside sources such as sunlight or temperature fluctuations.

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Applications of FTIR
• used to examine industrially manufactured materials in various quality
control processes.
• is a common first step in material analysis. A change in the
spectrometer’s absorption band characteristic patterns suggests a
change in the material composition or possible contamination.
• used to dry polymers, photoresist materials, and polyimides.
• investigates the interactions between matter and electromagnetic
radiation, which appear as a spectrum.

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• enabled the diagnosis of various organ diseases as well as the
quantification of various biomolecules such as proteins, nucleic
acids, and lipids.
• unique approach for characterizing the variation in fuel
stability of several biodiesel /antioxidant mixtures.
• In most failure analysis investigations, it is used to determine
breakdown, oxidation, and uncured monomers.
 FTIR is also used for :
• high-resolution experiments

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• trace analysis in raw materials or final products.
• Reactions on the microsecond time scale.
• Chromatographic and thermo-gravimetric sample fraction analysis.
• used to identify reaction components and conduct kinetic studies on
reactions.
• used for compound identification by matching the spectra of an
unknown substance with a reference spectrum (fingerprinting).
• utilized for functional group identification in unknown compounds. For
example, Ketones, Aldehydes, Carboxylic Acids, and so on.

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Limitations of FTIR
• The molecule must be active in the infrared range. (When exposed to IR
radiation, a minimum of one vibrational motion must change the
molecule’s net dipole moment for absorption to be noticed.)
• For the majority of samples, minimal elemental information is provided.
• The material being tested must be transparent in the spectral region of
interest.

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Example: FTIR of H. rosa-
sinensis flower extract
750
1500
2250
3000
3750
1/cm
60
80
100
%T
3649.83
3404.89
2922.72
2853.29
2363.40
1993.09
1919.80
1869.66
1740.43
1632.43
1543.71
1458.85
1402.92
1254.41
1161.83
1076.97
1040.32
835.88
722.09
619.87
Extract 1
• 1632 and 1740 [Aldehyde (C=O) group Str.]
• 2363 [Nitrile (CN) group]
• 619 [alkanes]
• 722 [benzene ring=CH]
• 835 [C-H bond]
• 1161 [C-O and C-OH bonds]
• 1254 [ C-O-C bond of aromatic acid ester
and C-OH groups of phenolic compounds]
• 1402-1431 [Alcohol (C-OH bond)]
• 1543 [Aromatic C=C bond]
• 2863-2922 [=C-H]
• 3404 [Alcohol group]
Thapa, S., Maurya, S. N., Manjunath, K., Mahmood, A. A., Devi, K., Varghese,
S. A., Lamsal, A., Tamang, B., & Biradar, M. S. (2025). LC-MS profiling and
multi-target mechanistic insights of Hibiscus rosa-sinensis in diabetes:
Network pharmacology, molecular docking, MD simulation, PCA, and in-
vitro α-amylase inhibition. Pharmacological Research - Modern Chinese
Medicine, 16(April), 100636. https://doi.org/10.1016/j.prmcm.2025.100636

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Refrences
• Siddiqui, A. A., & Siddiqui, S. (n.d.). Infrared spectroscopy. In
Pharmaceutical Analysis (4th ed., pp. 189–221). New Delhi: CBS
Publishers & Distributors Pvt. Ltd.
• https://chem.washington.edu/sites/chem/files/styles/large/public/
images/pelletpress_0.gif?itok=nIKRHnKn
• https://encryptedtbn0.gstatic.com/images?
q=tbn:ANd9GcQqyzsYGNbsx5LDYA65QKEyhKePzYcd_Rrp1w&s
• https://www.orgchemboulder.com/Technique/Procedures/IR/Images/
Solidjustright.jpg

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Thank You!

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