The document discusses Fourier transform infrared (FTIR) spectroscopy. It explains that FTIR spectroscopy uses a Michelson interferometer to obtain an infrared spectrum of a sample. The interferometer collects an interferogram that is then Fourier transformed to obtain the spectrum. FTIR spectroscopy provides advantages over dispersive infrared spectroscopy like speed, sensitivity, and mechanical simplicity. It finds applications in identifying organic and inorganic compounds, mixtures, and gases, liquids, and solids.

1. Shivaram
B.PHARMACY
Shiva.pharmacist@gmail.com

2. DEFINITION OF INFRARED
SPECTROSCOPHY
The absorption of light, as it passes through a
medium, varies linearly with the distance the light
travels and with concentration of the absorbing
medium. Where a is the absorbance, the Greek
lower-case letter epsilon is a characteristic constant
for each material at a given wavelength (known as the
extinction coefficient or absorption coefficient), c is
concentration, and l is the length of the light path,
the absorption of light may be expressed by the
simple equation a= epsilon times c times l.

3. INFRARED SPECTROSCOPHY
Infrared spectroscopy is the measurement of the
wavelength and intensity of the absorption of mid-infrared
light by a sample. Mid-infrared is energetic enough to
excite molecular vibrations to higher energy levels.
The wavelength of infrared absorption bands is
characteristic of specific types of chemical bonds, and
infrared spectroscopy finds its greatest utility for
identification of organic and organometallic molecules.
The high selectivity of the method makes the estimation
of an analyte in a complex matrix possible

4. EXAMPLE OF IR

5. THEORY OF INFRARED
ABSORPTION SPECTROSCOPHY
For a molecule to absorb IR, the vibrations or rotations
within a molecule must cause a net change in the dipole
moment of the molecule. The alternating electrical field of
the radiation (remember that electromagnetic radiation
consists of an oscillating electrical field and an oscillating
magnetic field, perpendicular to each other) interacts with
fluctuations in the dipole moment of the molecule.
If the frequency of the radiation matches the vibrational
frequency of the molecule then radiation will be absorbed,
causing a change in the amplitude of molecular vibration

6. MOLECULAR ROTATIONS
Rotational transitions are of little use to the
spectroscopist. Rotational levels are quantized, and
absorption of IR by gases yields line spectra.
However, in liquids or solids, these lines broaden into
a continuum due to molecular collisions and other
interactions

7. INSTRUMENT OF IR

8. INTRODUCTION TO FOURIER TRANSFORM OF
INFRARED SPECTROSCOPHY

9. WHAT IS FTIR SPECTROMETER
A spectrometer is an optical instrument used to measure
properties of light over a specific portion of the
electromagnetic spectrum, 5 microns to 20 microns.
FTIR (Fourier Transform InfraRed) spectrometer is a
obtains an infrared spectra by first collecting an
interferogram of a sample signal using an interferometer,
then performs a Fourier Transform on the interferogram
to obtain the spectrum.
An interferometer is an instrument that uses the
technique of superimposing (interfering) two or more
waves, to detect differences between them. The FTIR
spectrometer uses a Michelson interferometer.

10. FOURIER TRANSFORMS
Fourier transform defines a relationship between a
signal in time domain and its representation in
frequency domain.
Being a transform, no information is created or lost in
the process, so the original signal can be recovered
from the Fourier transform and vice versa.
The Fourier transform of a signal is a continuous
complex valued signal capable of representing real
valued or complex valued continuous time signals

11. SAMPLE ANALYSIS PROCESS
 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” takes place. 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.

13. FTIR THEORY
The spectrometer described here is a modified Bomem MB-100
FTIR.
The heart of the FTIR is a Michelson interferometer .
The mirror moves at a fixed rate. Its position is determined
accurately by counting the interference fringes of a collocated
Helium-Neon laser.
The Michelson interferometer splits a beam of radiation into
two paths having different lengths, and then recombines them.
A detector measures the intensity variations of the exit beam as
a function of path difference.
A monochromatic source would show a simple sine wave of
intensity at the detector due to constructive and destructive
interference as the path length changes.

14. In the general case, a superposition of wavelengths enter
spectrometer, and the detector indicates the sum of the
sine waves added together.
 shows some idealized light sources, and the
interferograms that they would theoretically produce.
The difference in path length for the radiation is known as
the retardation d (OM = OF + d) .
 When the retardation is zero, the detector sees a
maximum because all wavenumbers of radiation add
constructively.
When the retardation is l/2, the detector sees a minimum
for the wavelength l. An interferogram is the sum of all of
the wavenumber intensities

15. FTIR BASICS

16. SCHEMATIC OF MICHELSON
INTERFEROMETER

17. SAMPLE INTERFEROGRAMS AND
THEORETICAL SOURCE INTENSITY

18. FTIR INSTUMENTATION
In a conventional IR spectrophotomer, a sample IR
beam is directed through the sample chamber and
measured against a reference beam at each
wavelength of the spectrum. The entire spectral
region must be scanned slowly to produce good
quality spectrum. In 5.32, we will be using a Nicolet
FTIR Spectrophotometer (Nicolet was heavily
involved in the design of the Hubble telescope!). IR
spectroscopy has been dramatically improved by the
development of the Fourier Transform method in
much the same way as NMR has been revolutionized
by this method.

19.  The heart of an FTIR Spectrophotometer is a Michelson
Interferometer built around the sample chamber. Radiation from an
IR source is directed through the sample cell to a beam splitter. Half
of the radiation is reflected from a fixed mirror while the other half is
reflected from a mirror which moved continuously over a distance of
about 2.5 micrometers. When the two beams are recombined at the
detector, an interference pattern is produced. A single scan of the
entire distance takes about 2 seconds and is stored in the computer.
In order that several scans may be added, they must coincide exactly.
Obviously, this would be impossible
 considering the thermal fluctuations and vibrations in the laboratory.
In order to solve this problem, a helium-neon laser is simultaneously
directed through the Michelson Interferometer and the interference
pattern of the laser is used as a frequency reference. The performance
of an FTIR is dramatically superior to that of conventional
instruments. Generally, only a small amount of sample will produce
an excellent spectrum in a fraction of the time.

20. PREPARATION OF SAMPLE
Due to the sensitivity of the FTIR instrument, the most
convenient and satisfactory method involves simple
evaporation of a solution of the sample (chloroform, ether,
dichloromethane; or even a CDCl3 NMR sample may be
used) onto a KBr salt plate and acquisition of the spectrum
from the thin film remaining. This method provides
excellent spectra with flat baseline unless the thin film is
too powdery in which case
excessive scattering of the light leads to an irregular
baseline. The sample may alternatively be prepared as a
nujol mull (mull accessories: agate mortar and pestle,
nujol and NaCl discs may be obtained from LS).

21. PREPARATION OF INSTRUMENT
If the instrument has just been turned on, then it is necessary to
runa TEST ( F10 ) to be sure that all components are ON. If the
instrument is not turned on or does not check out when the
TEST is performed, then ask the instrument TA for help. In
addition, it is important that N2 is flowing through the
chamber so that most of the CO2 and
H2O are flushed from the chamber and from inside the
instrument. F4 SCAN BACKGROUND is performed with a blank
IR plate in the chamber. F8 then F4 DISPLAY BACKGROUND
will show the spectrum of CO2 and H2O that remain in the
chamber. If the background shows excessive CO2 and H2O,
then be sure the N2 is flowing briskly, wait a minute or two and
try again. Once a good background has been obtained, several
students in succession can use the same background.

22. SCANNING OF SAMPLE
: Place the sample plate in the FTIR and wait for N2 to
purge out the air.
F5 SCAN SAMPLE. Wait until the scan and Fourier
transform are completed.
F8 then F1 DISPLAY SPECTRUM will automatically
subtract the stored background and display the spectrum.
F7 PRINT. Important: Make sure that the printer is on-
line before pressing F7.
Type PEAKPICK S 4000 600 to find the peaks in the
spectrum. This data is printed by pressing F7 . If no one
else is using the instrument next, please turn off the
nitrogen purge

23. FTIR METHODS
• EPA Method 318 - Extractive FTIR Method for Measurement of Emissions from
Mineral Wool and Wool Fiberglass Industries
• EPA Performance Specification 15 for Extractive FTIR CEMS in Stationary
Sources
• EPA Method 320 -Vapor Phase Organic and Inorganic Emissions by FTIR
(extractive)
• EPA Method 321 - Determination of HCl for Portland Cement Industries
• EPA Protocol for Extractive FTIR for Analysis of Gas Emissions
• NIOSH Method 3800 - Organic and Inorganic gases by Extractive FTIR
Spectrometry

24. FTIR BENEFITS
Real-time measurement results.
• Simultaneous analysis of multiple gaseous compounds.
• Measures a wide variety of volatile compounds (Inorganic and
Organic).
• Sensitivity from very low parts per million to high percent levels.
• Provides a precise measurement method which requires no
rigorous external calibration.
• Speed. Measurements take only seconds

25. ADVANTAGES OF FTIR
 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 made in a matter of seconds rather than several minutes. This is sometimes
referred to as the
Felgett Advantage.
• Sensitivity: Sensitivity is dramatically improved with FT-IR for many reasons. The
detectors employed are much more sensitive, the optical throughput is much higher
(referred to as the Jacquinot Advantage) which results in much lower noise levels, and
the fast scans enable the coaddition 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

26. APPLICATIONS OF FTIR
Identification of inorganic compounds and organic
compounds
 Identification of components of an unknown mixture
Analysis of solids, liquids, and gasses
In remote sensing
In measurement and analysis of Atmospheric Spectra
 - Solar irradiance at any point on earth
 - Longwave/terrestrial radiation spectra
Can also be used on satellites to probe the space

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