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Introduction to
Fourier Transform
Infrared Spectrometry

© 2001 Thermo Nicolet Corporation
All rights reserved, worldwide
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In
infrared spectroscopy, IR radiation is passed through a sample
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The resulting
spectrum represents the molecular
absorption and transmission,
creating a molecular fingerprint
of the sample
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This makes
infrared spectroscopy useful for
several types of analysis
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It covers both the
basic theory of FT-IR and how it works as well as discussing some the practical aspects of FT-IR use
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T H E O R Y

O F

F T - I R

Why Infrared Spectroscopy?
Infrared spectroscopy has been a workhorse technique for materials analysis in the laboratory for over
seventy years
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Because each different material is a unique combination of atoms, no two compounds produce the
exact same infrared spectrum
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In addition, the size of the peaks in the
spectrum is a direct indication of the amount of material present
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Older Technology
The original infrared instruments were of the dispersive type
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This was accomplished by the use
of a prism or grating
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A grating is a more modern dispersive element which better
separates the frequencies of infrared energy
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This results in a spectrum which is a plot of
intensity vs
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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 – one second scans can be co-added together to ratio out random noise
• It has greater optical throughput
• It is mechanically simple with only one moving part

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Why FT-IR?
Fourier Transform Infrared (FT-IR) spectrometry was developed in order to overcome the limitations
encountered with dispersive instruments
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A method
for measuring all of the infrared frequencies simultaneously, rather than individually, was needed
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The interferometer produces a unique type of signal which has all of the infrared frequencies “encoded”
into it
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Thus,
the time element per sample is reduced to a matter of a few seconds rather than several minutes
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One beam reflects off of a flat mirror which is fixed in place
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The two beams reflect off of their
respective mirrors and are recombined when they meet
back at the beamsplitter
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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
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Thus, the use of the interferometer results in extremely fast measurements
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A means of “decoding” the individual frequencies is required
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This transformation is
performed by the computer which then presents the user with the desired spectral information for analysis
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The Source: Infrared energy is emitted from a glowing black-body source
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The resulting interferogram signal then exits the interferometer
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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
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The detectors used
are specially designed to measure the special interferogram signal
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The Computer: The measured signal is digitized and sent to the computer where the Fourier
transformation takes place
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Spectrometer
1 Source

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Interferometer

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Polystyrene run as film

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% T 40
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Computer

Spectrum

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This is normally a measurement with no sample in the beam
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This technique results in a spectrum which has all of the instrumental characteristics removed
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A single background
measurement can be used for many sample measurements because this spectrum is characteristic of
the instrument itself
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This is sometimes referred to as the
Felgett Advantage
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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)
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Thus, there is very little possibility of mechanical breakdown
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These instruments are self-calibrating
and never need to be calibrated by the user
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Thus, it a very reliable technique for positive identification of virtually
any sample
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This
makes FT-IR an invaluable tool for quality control or quality assurance applications whether it be
batch-to-batch comparisons to quality standards or analysis of an unknown contaminant
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Quantitative methods
can be easily developed and calibrated and can be incorporated into simple procedures for routine analysis
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It has made possible the development of many new sampling
techniques which were designed to tackle challenging problems which were impossible by older
technology
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