This document discusses the working principle of Fourier transform infrared spectroscopy (FTIR). It begins with an introduction to FTIR, describing how it was developed and its advantages over dispersive infrared spectroscopy. It then provides details on the description of FTIR, including its components and how it uses an interferometer to measure infrared absorption over a range of wavelengths simultaneously. The principle of FTIR is explained, noting how it uses a Michelson interferometer to convert the radiation into a slower oscillating signal that is transformed using Fourier transform into a conventional frequency domain spectrum. In conclusion, it states that FTIR has enabled real-time monitoring of chemical processes by providing information about chemical structure from infrared absorption measurements.

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ABDUL LATIF UNI ERSITY
KHAI PUR
ROLL NO. BC0113-01
NAME: ABDUL-RAHMAN SHAIKH
CLASS: BS (P-IV)
SEMESTER: 2nd
DEPARTMENT: BIOCHEMISTRY
SUBJECT: ANTIMICROBIAL
TEACHER: RESAPECTABLE SIR GHULAM FAREED NAREJO
ASSIGNMENT TOPIC: WORKING PRINCIPLE OF FTIR

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Contents
Introduction to FOURIER TRANSFORM INFRA-RED (FTIR)................................................................... 3
Description ........................................................................................................................................ 3
Principle of FTIR................................................................................................................................... 5
Conclusion ............................................................................................................................................ 7
References............................................................................................................................................. 8

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Working principle of
FTIR
Introduction to FOURIER TRANSFORM INFRA-RED (FTIR)
In 1887, Albert Michelson (German born American physician) perfected this instrument and
used it for several measurements in his study of light and relativity. [1] The past few years have seen
rapid growth in the use of infrared spectroscopy for at-line, on-line, and even in-line analysis. This
progress has been made possible by developments in the design of both FTIR instruments and
equipment to interface these instruments to chemical processes. It has been driven by the need for
real-time monitoring of the chemistry underlying various processes and by infrared's ability to
provide a wealth of information about chemical structure. The present paper reviews some of the
more important developments, with emphasis on the optical and mechanical hardware available for
interfacing the FTIR to the process. Finally, it reviews a number of representative applications areas
in which process FTIR is currently being used. [2]
Description
In many ways, mid-infrared spectroscopy would appear to be the ideal technology for on line
chemical analysis. After all, IR spectroscopy is the only analytical method which provides both
ambient temperature operation and the ability to directly monitor the vibrations of the functional
groups which characterize molecular structure and govern the course of chemical reactions. In
principle, IR also offers the advantages of continuous (near real-time) operation and low
maintenance compared to gas chromatography and low cost and structural specificity compared to
mass spectroscopy. The term "infrared" generally refers to any electro-magnetic radiation falling in
the region from 0.7 /xm to 1000 /xm. However, the region between 2.5 /xm and 25 /xm (4000 to 400
cm"1) is the most attractive for chemical analysis. This "mid-IR" region includes the frequencies
corresponding to the fundamental vibrations of virtually all of the functional groups of organic
molecules. These spectral lines are typically narrow and distinct, making it possible to identify and
monitor a band corresponding to the specific structural feature that is to be modified by a reaction.
As a result, quantitative calibrations performed in the mid-IR are usually straightforward and robust,
being largely immune to the effects of spurious artifacts.

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Despite its obvious attractiveness, mid-IR did not find widespread use in process analysis until quite
recently. Instead, for the past several years, much more attention has been directed toward the use
of near-IR for on-line spectral analysis. This may seem somewhat surprising in view of the fact that
in the near-IR one is often dealing with combination frequencies and harmonics of mid-IR functional
group frequencies. These near IR bands tend to be weak and broadly overlapping making it
impossible to single out distinct bands for analysis. This necessitates the use of fairly sophisticated
statistical methods to correlate observed spectra with the process variables of interest.
These methods are very powerful but are also quite capable of producing spurious results,
particularly when they encounter a condition that was not anticipated during calibration. This is the
infamous "false sample" problem endemic to near-IR, In contrast, the mid-IR region is a
spectroscopist's dream, with meaningful, well understood absorption bands often adjacent to weakly
absorbing regions, making calibrations largely independent of effects such as source variations,
changes in overall sample transmission, or scattering. Despite these advantages, the widespread
application of mid-IR on the process line had to await- technological advances in three general areas:
• FTIR spectrometers capable of reliable operation in the process environment.
• Methods for transmitting the IR radiation to and from the measurement location.
• Robust sample interfacing equipment capable of providing consistent results in the process
environment and of dealing with the very strong absorptions generally encountered in the mid-IR.
The particular need for these advances has to do with some specific fundamental differences between
mid- and near-IR. For example, the radiation source power available in the mid-IR is much lower
due to the nature of the black body radiation curve. At the same time, mid-IR detectors capable of
operating at room temperature are less sensitive than their near-IR counterparts. These two factors
together necessitate the use of the Fourier transform infrared (FTIR) approach rather than the far less
sensitive dispersive approach commonly used in the near-IR.
The transmission of radiation to and from the measurement site is more problematic in the
mid-IR due to the need for high throughput combined with the limited selection of optical materials
which transmit in this region. Ironically, this latter problem arises from the very fact that most
molecular vibrations fall in the mid-IR region.
Sample interfacing in the mid-IR is often complicated by the fact that the absorptions
corresponding to the fundamental molecular vibrations are orders of magnitude stronger than their
near-IR overtones. As a result, the simple transmission cells which can be used for near-IR liquid
analysis are usually not suitable for use in the mid-IR.
Despite the considerable challenges. Mid-IR does offer attractive benefits in the form of
distinct and meaningful bands, robust and straightforward calibrations, proven diagnostic methods,
and insensitivity to spectral artifacts. Fortunately, the final obstacles to the widespread

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implementation of process mid-IR have been now been surmounted, and as a result, the field
is starting to experience accelerated growth. The following sections will outline some of the
advances that have made this possible as well as some of the previously available technology
now being applied in process FTIR. The final sections will give some specific examples of
process FTIR hardware and the types of applications to which it is being applied. [2]
Principle of FTIR
Conventional spectroscopy is frequency domain spectroscopy in which radiant power data are
recorded as function of frequency. In time domain spectroscopy, which is achieved by Fourier
Transform (FT), radiant power data is recorded as a function of time. In previous case, radiant power
(ν) is plotted against frequency (ν1) (Hz) while in later, against the time. [3]
Michelson interferometer (MI) changes the frequency of electromagnetic radiation (EMR)
from source to proportionately slower oscillating signal. The sum of slower oscillating signal is
carried to the computer which mathematically separates the signal into individual oscillations and
calculate the oscillations of corresponding frequencies of observed radiation. This data is
continuously recorded. The amplitude of each resolved oscillations is a function of intensity of
radiation. A mathematical method called Fourier Transform (FT) is used to convert time domain
spectrum to conventional frequency domain spectrum. [4]
Fig: 01. Basic components of FTIR
Fig: 02 Parts of FTIR

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The unique part of an FTIR spectrometer is the interferometer. A Michelson type
plane mirror interferometer is displayed in fig: no. 3. Infrared radiation from the source
is collected and collimated (made parallel) before it strikes the beam splitter. The beam
splitter ideally transmits one half of the radiation, and reflects the other half. Both
transmitted and reflected beams strike mirrors, which reflect the two beams back to the
beam splitter. Thus, one half of the infrared radiation that finally goes to the sample gas
has first been reflected from the beam splitter to the moving mirror, and then back to the
beam splitter. The other half of the infrared radiation going to the sample has first gone
through the beam splitter and then reflected from the fixed mirror back to the beam
splitter. When these two optical paths are reunited, interference occurs at the beam splitter
because of the optical path difference caused by the scanning of the moving mirror. [5]
Fig: 03.
Fig: 04

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Conclusion
FTIR has been driven by the need for real-time monitoring of the chemistry underlying various
processes and by infrared's ability to provide a wealth of information about chemical structure.

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References
1. William Kemp,
Organic Spectroscopy,
Infrared Spectroscopy,
3rd Edn, PALGRAVE, New York: 43, (199).
2. W.M. Doyle
Axiom Analytical, Inc., 18103-C Sky Park South, Irvine, CA 92714 (USA)
3. Skoog, Holler & Nieman,
Principles of instrumental analysis,
5th Edn, Sounders College Publishing,
USA: 184, (1998).
4. Robert D. Braun,
Introduction to Instrumental Analysis,
Infrared Spectroscopy,
Pharma Book Syndicate,
Hyderabad: 371-73, (2006).
5. Principles of Fourier Transfer Infrared Spectroscopy
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