Infrared Spectroscopy

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Infrared Spectroscopy
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What is spectroscopy?

• The study of the interaction between radiation
  and matter as a function of wavelength (?).
• Historically, spectroscopy referred to the use of
  visible light dispersed according to its
  wavelength.
• Later, the concept was expanded greatly to
  comprise any measurement of a quantity as
  function of either wavelength or frequency.
• A plot of the response as a function of
  wavelength or more commonly frequency is
  referred to as a spectrum

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Spectrometry Spectrometer

• Spectrometry - the spectroscopic technique used
  to assess the concentration or amount of a given
  species.
• Spectrometer - instrument that performs such
  measurements. (a.k.a spectrograph)
• Spectrometry is often used in physical and
  analytical chemistry for the identification of
  substances through the spectrum emitted from or
  absorbed by them.

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Types of spectroscopy

• Electromagnetic spectroscopy involves
  interactions of matter with electromagnetic
  radiation, such as light.
• Electron spectroscopy involves interactions with
  electron beams.
• Mass spectrometry involves the interaction of
  charged species with magnetic and/or electric
  fields
• Acoustic spectroscopy involves the frequency of
  sound.

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Measurement process

• Most spectroscopic methods are differentiated as
  either atomic or molecular, based on whether or
  not they apply to atoms or molecules.
• They also can be classified on the nature of
  their interaction
• Absorption spectroscopy
• Emission spectroscopy
• Scattering spectroscopy

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Measurement process

• Absorption spectroscopy uses the range of the
  electromagnetic spectra in which a substance
  absorbs.
• (E.g. Atomic absorption spectroscopy,
  Infrared spectroscopy, Nuclear magnetic resonance
  (NMR) spectroscopy)
• Emission spectroscopy uses the range of
  electromagnetic spectra in which a substance
  radiates (emits).
• Scattering spectroscopy measures the amount of
  light that a substance scatters at certain
  wavelengths, incident angles, and polarization
  angles. One of the most useful applications of
  light scattering spectroscopy is Raman
  spectroscopy.

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Infrared Spectroscopy

• Infrared spectroscopy - a technique used to
  identify chemical compounds based on how infrared
  radiation is absorbed by the compounds' chemical
  bonds.
• The most common technique used is absorption
  spectroscopy.
• Infrared spectroscopy exploits the fact that
  molecules have specific frequencies at which they
  rotate or vibrate corresponding to discrete
  energy levels.

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Infrared Absorption
For a molecule to show infrared absorpsions it
must possess a specific feature an electric
dipole moment of the molecule must change during
the vibration. A dipole moment, µ is
defined as the charge value multiplied by the
separation distance between positive and negative
charges. µ qd (C.m)
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Infrared Absorption
The infrared spectrum of a sample is collected by
passing a beam of infrared light through the
sample. Examination of the transmitted light
reveals how much energy was absorbed at each
wavelength. 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.
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Infrared Absorption
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.
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Modes of Vibration
The interactions of infrared radiation with
matter may be understood in terms of changes in
molecular dipoles associated with vibrations.
Vibrations can involve either change in bond
length (stretching) or bond angle (bending).
Some bonds can stretch in-phase (symmetric
stretching) or out-of-phase (asymmetric
stretching) Bending vibrations can be in-plane
(scissoring, rocking) or out-of-plane (wagging,
twisting) bending vibrations.
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Vibrations

• Atoms in the molecule are subjected to number of
  vibrations.

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Modes of Vibration

• The degree of vibrational freedom for polyatomic
  molecules containing (N) atoms is given by 3N 5
  (linear molecules) and 3N 6 (non-linear
  molecules).
• Two other concepts are also used to explain the
  frequency of vibrational modes
• the stiffness of the bond and
• the masses of the atoms at each end of the bond

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Modes of Vibration
The stiffness of the bond can be characterized by
proportionality constant termed the force
constant, k. The reduced mass, µ provides a
useful way of simplifying our calculations by
combining two-bodies problem as one-body. (1/µ)
m1m2/(m1m2) The relation between force
constant, the reduced mass and the frequency of
absorption is ? (1/2p)?(k/µ) or ?
(1/2pc)?(k/µ)
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Theoretical principles

• In infrared spectroscopy wavelength is measured
  in wave numbers which have the units cm-1
• IR radiation does not have enough energy to
  induce electronic transitions as seen with UV.
  Absorption of IR is restricted to compounds with
  small energy differences in the possible
  vibrational and rotational states.
• For a molecule to absorb IR, the radiation must
  interact with the electric field caused by
  changing dipole moment

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Calibration
www.internationalcrystal.net
www.chemistry.oregonstate.edu/courses/ch361-464/ch
362/irinstrs.htm - 9

• This device is precisely calibrated by using
  polystyrene calibration film.
• Size of peaks ? amount of material

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Background Spectrum

• This background spectrum can be used to compare
  with the sample measurement to determine
  transmittance
• Peaks in this region are characteristic of
  specific kinds of bonds, thus can be used to
  identify whether a specific functional group is
  present.

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Example of C-H(functional group) spectra

• Peaks in the region of (3000- 3100) cm-1
  indicates that sp2 hybridized C-H bond are
  present in the sample
• And peaks in range of (2800-3000)cm-1 indicates
  that sp3 hybridized C-H bond are present in a
  sample

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  Acids 1650-1700cm-1 Esters 1740-1750cm-1
Aldehydes 1720-1750cm-1 Ketones
1720-1750 cm-1 Amides1650-1715 cm-1
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FTIR Spectrum of Sample (98 N,N-Dimethylamphetami
ne Hydrochloride)
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What information can FT-IR provide

• It can determine the amount of components in a
  mixture
• It can determine the quality or how consistent a
  sample is
• It can identify unknown materials

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How FTIR works?

• Source Infrared energy is emitted from a glowing
  black-body source. Ends at the Detector
• Interferometer beam enters the interferometer
  where the spectral encoding takes place
• Interferogram signal then exits the
  interferometer
• Beamsplitter takes the incoming beam and divides
  it into two optical beams
• Sample beam enters the sample compartment where
  it is transmitted through or reflected off of the
  surface of the sample
• Detector The beam finally passes to the detector
  for final measurement
• Computer measured signal is digitized and sent
  to the computer where the Fourier transformation
  takes place
• Moving mirror in the interferometer is the only
  moving part of the instrument
• Fixed mirror

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Is FT-IR Qualitative, Quantitative, or
Comparative?

• Qualitative
• -Because each different material is a unique
  combination of atoms, no two compounds produce
  the exact same infrared spectrum
• -the size of the peaks in the spectrum is a
  direct indication of the amount of material
  present

• Quantitative and Comparative
• -Since it is sensitive, accurate, and has
  software algorithms, the quantitative methods can
  be developed and calibrated easily to perform
  various analysis
• -FT-IR is comparative due to the background
  spectrum that is compared to the sample

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Advantages/disadvantages

• Speed
• Sensitivity
• Mechanical simplicity
• Internally calibrated

• Destructive
• Too sensitive that it would detect the smallest
  contaminant

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Forensic Lab use

• A Forensic Scientist would use FT-IR to identify
  chemicals in different types of samples
• Paints
• Polymers
• Coatings
• Rugs
• Contaminants
• Explosive residues

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Sample Preparation - Gaseous
Gaseous samples require little preparation beyond
purification, but a sample cell with a long
pathlength (typically 5-10 cm) is normally
needed. The walls are of glass or brass. Longer
pathlengths are necessary to analyse complex
mixtures and trace impurities. The cell
pathlength can be measured by the method of
counting interference fringes. L n/2(?1 ?2)
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Sample Preparation - Liquids
Liquid samples use solution cells. Two types
of solution cells permanent and demountable.
Permanent cell The pathlength need to be
calibrated regularly if quatitative work is to be
undertaken. Diffucult to clean and can be damaged
by water. Demountable cell Easy to maintain as
it can be readily dismantled and cleaned. The
windows can be repolished, a new spacer supplied
and the cell reassembled.
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Sample Preparation - Solids

• Solid samples can be prepared in four major ways.
  
• Crush the sample with a mulling agent in a
  marble or agate mortar, with a pestle. A thin
  film of the mull is applied onto salt plates and
  measured.
• 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 crushed in
  a mechanical die press to form a translucent
  pellet through which the beam of the spectrometer
  can pass.

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Sample Preparation - Solids

• The 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.
• The final method is to use microtomy to cut a
  thin (20-100 micron) 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.

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Typical Method

• 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 is used for two reasons
• To prevent fluctuations in the output of the
  source affecting the data.
• To allow 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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Typical Method
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Uses and Applications
Infrared spectroscopy is widely used in both
research and industry . It is applied for
detection and identification of different
elements/compounds in solving problems in the
fields of forensics, medicine, oil industry,
atmospheric chemistry, polymer degradation,
pharmacology, etc. Among the more common
spectroscopic methods used for analysis is FTIR
spectroscopy, where chemical bonds can be
detected through their characteristic infra-red
absorption frequencies or wavelengths.
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Uses and Applications
The instruments are now small, and can be
transported, even for use in field trials. With
increasing technology in computer filtering and
manipulation of the results, samples in solution
can now be measured accurately (water produces a
broad absorbance across the range of interest,
and thus renders the spectra unreadable without
this computer treatment). Some machines will
also automatically tell you what substance is
being measured from a store of thousands of
reference spectra held in storage.
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Analysis of Polymers
Many polymer degradation mechanisms such as UV
degradation and oxidation, amongst many other
failure modes, can be investigated using
infra-red spectroscopy.
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UV Radiation
Many polymers are attacked by UV radiation at
vulnerable points in their chain structures.
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.
Polyethylene is also susceptible to UV
degradation, especially those variants which are
branched polymers such as LDPE. The branch
points are tertiary carbon atoms, so polymer
degradation starts there and results in
embrittlement.
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UV Radiation
IR spectrum showing carbonyl absorption due to UV
degradation of polyethylene
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Oxidation
Polymers are susceptible to attack by atmospheric
oxygen, especially at elevated temperatures
encountered during processing to shape. Many
process methods such as extrusion and injection
moulding involve pumping molten polymer into
tools, and the high temperatures needed for
melting may result in oxidation unless
precautions are taken. Oxidation tends to start
at tertiary carbon atoms because free radicals
here at more stable, so last longer and are
attacked by oxygen. The carbonyl group can be
further oxidised to break the chain, so weakening
the material by lowering the molecular weight,
and cracks start to grow in the regions affected.
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Oxidation
IR spectrum showing carbonyl absorption due to
oxidative degradation of polypropylene