CONTENTS

CHAPTER 1 INTRODUCTION PAGES

1.0 Introduction 1-2

1.1 Michelson Interferometer 3

1.2 Fourier transform spectroscopy 3

CHAPTER 2 LITERATURE RIEVIEW

2.0 Conceptual introduction 4

2.1 Measuring an emission spectrum 4-6

2.2 Measuring an absorbtion spectrum 6-7

2.3 History 7

2.4 Infrared spectroscopy 8

CHAPTER 3 METODOLOGY

3.0 Infrared spectroscopy 9

3.1 Theory 9-10

3.2 Number of vibrational modes 10-11

3.3 Special effects 11 3.4

Practical IR spectroscopy 11-12

3.5 Sample preparation 12

3.5.1 Conventional apparatus 12-13

3.6 Application of Infrared spectroscopy 14

3.6.1 Forensic chemistry 14-18

CHAPTER 4 CONCLUSIONS

4.0 Conclusions 19-20

REFERENCES 21

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CHAPTER 1

INTRODUCTION

1.0 Introduction

A.A. Michelson invented in the 1880s a simple interferometer, which basically consists of

a beamsplitter and two plane mirror. Initially, the interferometer was used to study the

speed of light and to fix the standard the meter with the wavelength of a spectra line. The

use of the Michelson interferometer as a spectrometer was started more than half a

century later. In 1949, astrophysicist Peter Fellgett applied the interferometer to measure

the light from celestial bodies, and computed the first Fourier transform spectrum.

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Figure 1.1 : Michelson Interferometer

A Fourier transform spectrometer (abbreviated FTS) is a Michelson interferometer with a

movable mirror. By scanning the movable mirror over some distance, an interference

pattern is produced that encodes the spectrum of the source (in fact, it turns out to be its

Fourier transform). Fourier transform spectrometers have a multiplex advantage over

dispersive spectral detection techniques for signal, but a multiplex disadvantage for noise.

In its simplest form, a Fourier transform spectrometer consists of two mirrors

located at a right angle to each other and oriented perpendicularly, with a beamsplitter

placed at the vertex of the right angle and oriented at a 45° angle relative to the two

mirrors. Radiation incident on the beamsplitter from one of the two "ports" is then divided

into two parts, each of which propagates down one of the two arms and is reflected off

one of the mirrors. The two beams are then recombined and transmitted out the other port.

When the position of one mirror is continuously varied along the axis of the

corresponding arm, an interference pattern is swept out as the two phase-shifted beams

interfere with each other.

1.2 Michelson Interferometer

Michelson Interferometer (Figure 1.1) is an interferometer constructed using a half

silvered mirror inclined at a 45° angle to the incoming beam. Half the light is reflected

perpendicularly and bounces off a beamsplitter; half passes through and is reflected from

a second beamsplitter. The light passing through the mirror must also pass through an

inclined compensator plate to compensate for the fact that the other ray passes through the

mirror glass three times instead of one.

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1.3 Fourier transform spectroscopy

Fourier transform spectroscopy is a measurement technique whereby spectra are collected

based on measurements of the coherence of a radiative source, using time-domain or

space-domain measurements of the electromagnetic radiation or other type of radiation. It

can be applied to a variety of types of spectroscopy including optical spectroscopy,

infrared spectroscopy (FT IR, FT-NIRS), Fourier transform (FT) nuclear magnetic

resonance, mass spectrometry and electron spin resonance spectroscopy. There are

several methods for measuring the temporal coherence of the light, including the

continuous wave Michelson or Fourier transform spectrometer and the pulsed Fourier

transform spectrograph (which is more sensitive and has a much shorter sampling time

than conventional spectroscopic techniques, but is only applicable in a laboratory

environment).

CHAPTER 2

5

LITERATURE REVIEW

2.0 Conceptual introduction

2.1. Measuring an emission spectrum

One of the most basic tasks in spectroscopy is to characterize the spectrum of a light

source: How much light is emitted at each different wavelength. The most straightforward

way to measure a spectrum is to pass the light through a monochromator, an instrument

that blocks all of the light except the light at a certain wavelength (the un-blocked

wavelength is set by a knob on the monochromator). Then the intensity of this remaining

(single-wavelength) light is measured. The measured intensity directly indicates how

much light is emitted at that wavelength. By varying the monochromator's wavelength

setting, the full spectrum can be measured. This simple scheme in fact describes how

some spectrometers work.

Fourier transform spectroscopy is a less intuitive way to get the same information.

Rather than allowing only one wavelength at a time to pass through to the detector, this

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technique lets through a beam containing many different wavelengths of light at once, and

measures the total beam intensity. Next, the beam is modified to contain a different

combination of wavelengths, giving a second data point. This process is repeated many

times. Afterwards, a computer takes all this data and works backwards to infer how much

light there is at each wavelength.

To be more specific, between the light source and the detector, there is a certain

configuration of mirrors that allows some wavelengths to pass through but blocks others

(due to wave interference). The beam is modified for each new data point by moving one

of the mirrors; this changes the set of wavelengths that can pass through.

As mentioned, computer processing is required to turn the raw data (light intensity

for each mirror position) into the desired result (light intensity for each wavelength). The

processing required turns out to be a common algorithm called the Fourier transform

(hence the name, "Fourier transform spectroscopy"). The raw data is sometimes called an

"interferogram".

Figure 2.1: The spectrum of light emitted by the blue flame of a butane torch. The

horizontal axis is the wavelength of light, and the vertical axis represents how much light

is emitted by the torch at that wavelength.

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2.2 Measuring an absorption spectrum

The method of Fourier transform spectroscopy can also be used for absorption

spectroscopy. The primary example is "FTIR Spectroscopy", a common technique in

chemistry. In general, the goal of absorption spectroscopy is to measure how well a

sample absorbs or transmits light at each different wavelength. However, any technique

for emission spectroscopy can also be used for absorption spectroscopy as follows:

Assume you have a working spectrometer that can measure the spectrum of any light

source shining into it (as described above). Shine a broadband light source into the

spectrometer, then shine the same light source through the sample into the same

spectrometer. Comparing the two spectra, it will be obvious which wavelengths get

absorbed by the sample and which wavelengths can pass right through the sample. (More

precisely, the spectrum with the sample, divided by the "background" spectrum without

the sample, equals the fraction of light that the sample can transmit at each wavelength.)

Accordingly, the technique of "Fourier transform spectroscopy" can be used both

for measuring emission spectra (for example, the emission spectrum of a star), and

absorption spectra (for example, the absorption spectrum of a glass of liquid)

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Figure 2.2: An interferogram from a Fourier transform spectrometer. The horizontal axis

is the position of the mirror, and the vertical axis is the amount of light detected. This is

the "raw data" which can be transformed into an actual spectrum

2.3 History

A.A. Michelson invented in the 1880s a simple interferometer, which basically consists of

a beamsplitter and two plane mirror. Initially, the interferometer was used to study the

speed of light and to fix the standard the meter with the wavelength of a spectra line. The

use of the Michelson interferometer as a spectrometer was started more than half a

century later. In 1949, astrophysicist Peter Fellgett applied the interferometer to measure

the light from celestial bodies, and computed the first Fourier transform spectrum.

2.4 Infrared spectroscopy

Infrared spectroscopy (IR spectroscopy) is the subset of spectroscopy that deals with the

infrared region of the electromagnetic spectrum. It covers a range of techniques, the most

common being a form of absorption spectroscopy. As with all spectroscopic techniques, it

can be used to identify compounds and investigate sample composition. A common

laboratory instrument that uses this technique is an infrared spectrophotometer.

The infrared portion of the electromagnetic spectrum is usually divided into three

regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum.

The far-infrared, approximately 400–10 cm−1 (1000–30 μm), lying adjacent to the

microwave region, has low energy and may be used for rotational spectroscopy. The mid-

infrared, approximately 4000–400 cm−1 (30–2.5 μm) may be used to study the

fundamental vibrations and associated rotational-vibrational structure. The higher energy

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near-IR, approximately 14000–4000 cm−1 (2.5–0.8 μm) can excite overtone or harmonic

vibrations. The names and classifications of these subregions are merely conventions.

They are neither strict divisions nor based on exact molecular or electromagnetic

properties.

CHAPTER 3

10

METODOLOGY

3 Infrared spectroscopy

3.1 Theory

Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are

characteristic of their structure. These absorptions are resonant frequencies, i.e. the

frequency of the absorbed radiation matches the frequency of the bond or group that

vibrates. The energies are determined by the shape of the molecular potential energy

surfaces, the masses of the atoms, and the associated vibronic coupling.

In particular, in the Born–Oppenheimer and harmonic approximations, i.e. when

the molecular Hamiltonian corresponding to the electronic ground state can be

approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular

geometry, the resonant frequencies are determined by the normal modes corresponding to

the molecular electronic ground state potential energy surface. Nevertheless, the resonant

frequencies can be in a first approach related to the strength of the bond, and the mass of

the atoms at either end of it. Thus, the frequency of the vibrations can be associated with

a particular bond type.

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3.2 Number of vibrational modes

In order for a vibrational mode in a molecule to be "IR active," it must be associated with

changes in the permanent dipole.

A molecule can vibrate in many ways, and each way is called a vibrational mode.

Linear molecules have 3N-5 degrees of vibrational modes whereas nonlinear molecules

have 3N-6 degrees of vibrational modes (also called vibrational degrees of freedom). As

an example H2O, a non-linear molecule, will have 3*3-6 = 3 degrees of vibrational

freedom, or modes.

Simple diatomic molecules have only one bond and only one vibrational band. If

the molecule is symmetrical, e.g. N2, the band is not observed in the IR spectrum, but

only in the Raman spectrum. Unsymmetrical diatomic molecules, e.g. CO, absorb in the

IR spectrum. More complex molecules have many bonds, and their vibrational spectra are

correspondingly more complex, i.e. big molecules have many peaks in their IR spectra.

The atoms in a CH2 group, commonly found in organic compounds can vibrate in

six different ways: symmetrical and antisymmetrical stretching, scissoring, rocking,

wagging and twisting:

3.3 Special effects

The simplest and most important IR bands arise from the "normal modes," the simplest

distortions of the molecule. In some cases, "overtone bands" are observed. These bands

arise from the absorption of a photon that leads to a doubly excited vibrational state. Such

bands appear at approximately twice the energy of the normal mode. Some vibrations, so-

called 'combination modes," involve more than one normal mode. The phenomenon of

Fermi resonance can arise when two modes are similar in energy, Fermi resonance results

in an unexpected shift in energy and intensity of the bands.

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3.4 Practical IR spectroscopy

The infrared spectrum of a sample is recorded by passing a beam of infrared light through

the sample. Examination of the transmitted light reveals how much energy was absorbed

at each wavelength. This can be done with a monochromatic beam, which changes in

wavelength over time, or by using a Fourier transform instrument to measure all

wavelengths at once. From this, a transmittance or absorbance spectrum can be produced,

showing at which IR wavelengths the sample absorbs. Analysis of these absorption

characteristics reveals details about the molecular structure of the sample. When the

frequency of the IR is the same as the vibrational frequency of a bond, absorption occurs.

This technique works almost exclusively on samples with covalent bonds. Simple

spectra are obtained from samples with few IR active bonds and high levels of purity.

More complex molecular structures lead to more absorption bands and more complex

spectra. The technique has been used for the characterization of very complex mixtures.

3.5 Sample preparation

Gaseous samples require a sample cell with a long pathlength (typically 5–10 cm), to

compensate for the diluteness.

Liquid samples can be sandwiched between two plates of a salt (commonly

sodium chloride, or common salt, although a number of other salts such as potassium

bromide or calcium fluoride are also used). The plates are transparent to the infrared light

and do not introduce any lines onto the spectra.

Solid samples can be prepared in a variety of ways. One common method is to

crush the sample with an oily mulling agent (usually Nujol) in a marble or agate mortar,

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with a pestle. A thin film of the mull is smeared onto salt plates and measured. The

second method is to grind a quantity of the sample with a specially purified salt (usually

potassium bromide) finely (to remove scattering effects from large crystals). This powder

mixture is then pressed in a mechanical die press to form a translucent pellet through

which the beam of the spectrometer can pass. A third technique is the "cast film"

technique, which is used mainly for polymeric materials. The sample is first dissolved in

a suitable, non hygroscopic solvent. A drop of this solution is deposited on surface of KBr

or NaCl cell. The solution is then evaporated to dryness and the film formed on the cell is

analysed directly. Care is important to ensure that the film is not too thick otherwise light

cannot pass through. This technique is suitable for qualitative analysis. The final method

is to use microtomy to cut a thin (20–100 micrometre) film from a solid sample. This is

one of the most important ways of analysing failed plastic products for example because

the integrity of the solid is preserved.

It is important to note that spectra obtained from different sample preparation

methods will look slightly different from each other due to differences in the samples'

physical states.

3.5.1 Conventional apparatus

A beam of infrared light is produced and split into two separate beams. One is passed

through the sample, the other passed through a reference which is often the substance the

sample is dissolved in. The beams are both reflected back towards a detector, however

first they pass through a splitter which quickly alternates which of the two beams enters

the detector. The two signals are then compared and a printout is obtained.

A reference prevents fluctuations in the output of the source affecting the data. The

reference also allows the effects of the solvent to be cancelled out (the reference is usually

a pure form of the solvent the sample is in)

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Figure 3.2: Typical apparatus

3.6 Application of Infrared spectroscopy

Fourier transform spectroscopy is in frequent use in many various practical and scientific

applications. Here, we shall review some of these applications.

3.6.1 Forensic chemistry

Forensic chemistry is the application of chemistry to law enforcement or the failure of

products or processes. Many different analytical methods may be used to reveal what

chemical changes occurred during an incident, and so help reconstruct the sequence of

events. "Forensic chemistry is unique among chemical sciences in that its research,

practice, and presentation must meet the needs of both the scientific and the legal

communities. As such, forensic chemistry research is applied and derivative by nature and

design, and it emphasizes metrology and validation."

3.6.1.1 Methods

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One useful method is the gas chromatograph-mass spectrometer (GCMS), which is

actually two instruments that are attached. The gas chromatograph is essentially a very

hot oven holding a hollow coiled column. A drug sample is diluted in a solvent (e.g.:

chloroform, methanol) and is injected into this column, the solvent will evaporate very

quickly leaving the drug to travel through the column. Different substances are retained in

the column for different amounts of time. The retention time, as compared to a known

standard sample using the same method(same column length/polarity, same flow rate,

same temperature program), can help to provide a positive identification for the presence

of a compound of interest. The column eluent is then fed into a mass spectrometer. A

mass spectrometer bombards the eluant with electrons, causing it to fragment into ions.

These ions are separated by their mass, commonly with the use of a quadrupole mass

analyzer or quadrupole ion trap, and detected by an electron multiplier. This provides a

fragmentation pattern, which functions as a sort of fingerprint for each compound, and is

compared to a reference sample.

3.6.1.2 Spectroscopy

Figure 3.6: IR spectrum showing carbonyl absorption due to oxidative degradation of

polypropylene

Another instrument used to aide in identification of compounds is the Fourier Transform

infrared spectrophotometer (FTIR). The sample is bombarded with infrared radiation.

Polar bonds found in organic compounds have a natural frequency of vibration similar to

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the frequency of infrared radiation. When the frequency of the infrared radiation matches

the natural frequency of the bond, the amplitude of the vibration increases, and the

infrared is absorbed. The output of an infrared spectrophotometer charts the amount of

light absorbed vs. the wavelength, typically with units of percent transmission and

wavenumbers(cm-1). Because both the frequency and the intensity of absorption are

dependent on the type of bond, a skilled chemist can determine the functional groups

present by examining the infrared spectrum.

As with the GCMS the FTIR spectrum can be compared to that of a known

sample, thus providing evidence for the identification of a compound. Spectroscopy can

also help to identify materials used in failed products, especially polymers, additives and

fillers. Samples can be taken by dissolution, or by cutting a thin slice using a microtome

from the specimen under examination. Surfaces can be examined using Attenuated total

reflectance spectroscopy, and the method has also been adapted to the optical microscope

with infra-red microspectroscopy.

Thermoplastics can be analysed using infra-red spectroscopy, UV spectroscopy,

as NMR and ESEM. Failed samples can either be dissolved in a suitable solvent and

examined directly (UV, IR and NMR spectroscopy) or as a thin film cast from solvent or

cut using microtomy from the solid product. Infra-red spectroscopy is especially useful

for assessing oxidation of polymers, such as the polymer degradation caused by faulty

injection moulding. The spectrum shows the characteristic carbonyl group produced by

oxidation of polypropylene, which made the product brittle. It was a critical part of a

crutch, and when it failed, the user fell and injured herself very seriously. The spectrum

was obtained from a thin film cast from a solution of a sample of the plastic taken from

the failed forearm crutch.

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3.6.1.3 Sample integrity

Forensic chemists usually perform their analytical work in a sterile laboratory decreasing

the risk of sample contamination. In order to prevent tampering, forensic chemists must

keep track of a chain of custody for each sample. A chain of custody is a document which

stays with the evidence at all times. Among other information, contains signatures and

identification of all the people involved in transport, storage and analysis of the evidence.

This makes it much more difficult for intentional tampering to occur, it also acts

as a detailed record of the location of the evidence at all times for record keeping

purposes. It increases the reliability of a forensic chemist's work and increases the

strength of the evidence in court.

A distinction is made between destructive and non-destructive analytical methods.

Destructive methods involve taking a sample from the object of interest, and so injures

the object. Most spectroscopic techniques fall into this category. By contrast, a non-

destructive method conserves the integrity of the object, and is generally preferred by

forensic examiners. For example, Optical microscopy cannot injure the sample, so it falls

in this class.

3.6.1.4 Examples

Figure 3.7: Ozone cracking in Natural rubber tubing

Polymers for example, can be attacked by aggressive chemicals, and if under load, then

cracks will grow by the mechanism of stress corrosion cracking. Perhaps the oldest

known example is the ozone cracking of rubbers, where traces of ozone in the atmosphere

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attack double bonds in the chains of the materials. Elastomers with double bonds in their

chains include natural rubber, nitrile rubber and styrene-butadiene rubber. They are all

highly susceptible to ozone attack, and can cause problems like car fires (from rubber fuel

lines) and tire blow-outs. Nowadays, anti-ozonants are widely added to these polymers,

so the incidence of cracking has dropped. However, not all safety-critical rubber products

are protected, and since only ppb of ozone will start attack, failures are still occurring.

Another highly reactive gas is chlorine, which will attack susceptible polymers

such as acetal resin and polybutylene pipework. There have been many examples of such

pipes and acetal fittings failing in properties in the USA as a result of chlorine-induced

cracking. Essentially the gas attacks sensitive parts of the chain molecules (especially

secondary, tertiary or allylic carbon atoms), oxidizing the chains and ultimately causing

chain cleavage. The root cause is traces of chlorine in the water supply, added for its anti-

bacterial action, attack occurring even at parts per million traces of the dissolved gas.

Most step-growth polymers can suffer hydrolysis in the presence of water, often a

reaction catalysed by acid or alkali. Nylon for example, will degrade and crack rapidly if

exposed to strong acids, a phenomenon well known to those who accidentally spill acid

onto their shirts or tights. Polycarbonate is susceptible to alkali hydrolysis, the reaction

simply depolymerising the material. Polyesters are prone to degrade when treated with

strong acids, and in all these cases, care must be taken to dry the raw materials for

processing at high temperatures to prevent the problem occurring.

Many polymers are also attacked by UV radiation at vulnerable points in their

chain structures. Thus polypropylene suffers severe cracking in sunlight unless anti-

oxidants are added. The point of attack occurs at the tertiary carbon atom present in every

repeat unit, causing oxidation and finally chain breakage.

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CHAPTER 4

20

CONCLUSIONS

4.0 Conclusion

The most important practical applications of FT spectrometers is the qualitative and

quantitative analysis of unknown samples. Conventionally, the analysis of a gas phase

sample has taken place so that the user records the IR spectrum of the sample using some

commercial instrument with its maximum resolution. Then, looking at the spectrum with

the naked eye, or using some commercial search program based on the correlation

between the observed and a library spectrum, the user tries to identify the unknown

compound. The multicomponent analysis of gas mixture has usually aken place by the so

called subtraction technique: the library spectrum multiplied by a constant is subtracted

from the observed mixture spectrum. The constant, which gives the desired concentration

of the component, is determined in such a way that the corresponding component just

disappears after subtraction. This analysis method, however, requires high resolution (

) and is slow and laborious to carry out. It requires an experienced

user, and it is very difficult to automatize.

An important scientific application of FTS is molecular spectroscpy. In this

application, infrared FT spectrometers are used to study geometric structures of

molecules, and the behavior of molecules in rotation and vibrations. IR spectra reveal

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quantum mechanical phenomena and interactions in an effective way. In this type of

research, high resolution and accuracy are very important, and FT-IR spectrometry is the

most commonly used method.

REFERENCES

Eric W. Weisstein (2007). Michelson Interferometer. Available online: 14 April 2010.

http://scienceworld.wolfram.com/physics/MichelsonInterferometer.html

Eric W. Weisstein (2007). Fourier transform. Available online: 14 April 2010.

http://scienceworld.wolfram.com/physics/FourierTransform.html

J.Kaunppinen, J.Partanen. (2001). Fourier Transforms In Spectrocopy. Wiley-VCH. Berlin

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