General

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General

• This is the subtitle

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Introduction to Fourier Transform Infrared
Spectroscopy
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What is FTIR?

• FTIR stands for Fourier Transform Infra Red, the
  preferred method of infrared spectroscopy.
• A method for measuring all of the infrared
  frequencies simultaneously, rather than
  individually as with dispersive instruments

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Why Infrared Spectroscopy ?

• An Infrared spectrum represents a fingerprint of
  a sample with absorption peaks which correspond
  to the frequencies of vibrations between the
  bonds of the atoms making up the material-Because
  each different material is a unique combination
  of atoms, no two compounds produce the exact same
  spectrum, therefore IR can result in a unique
  identification of every different kind of
  material!

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Infrared Spectroscopy is simply the study of the
interaction of Infrared light with matter!!

• Most powerful aspect is the ability to identify
  complete unknowns
• direct correlation between the wavenumbers at
  which a molecule absorbs infrared radiation and
  its chemical structure

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Infrared Spectroscopy
UV
VIS
NIR
IR
Far IR
Microwave
10-5
10-5
10-4
10-3
10-2
10-1
Wavelength (cm)
Energy (Frequency)
Light, or more properly, electromagnetic
radiation, can be described in terms of
frequency ? or wavelength ? . Energy of
radiation increases with increasing frequency
and decreases with increasing wavelength
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Wavenumbers (cm-1) - standard


IR spectral units

• Modern infrared instruments record the absorption
  of energy as a function of decreasing frequency
  (decreasing energy) from left to right
• W ( cm-1) 1/?
• A frequency-related unit known as a wavenumber
  is expressed as reciprocal centimeters- number
  of vibrations occurring over 1 cm

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WAVENUMBER SCALE
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Origins of Infrared Absorptions

• All atoms within molecules are in constant
  vibration
• all bonds with a permanent dipole (slightly
  electronegative charge) will absorb infrared
  radiation at the appropriate vibrational
  frequency (??/ ?X ? 0 )
• the vibrational frequency will depend on the
  atoms associated with the bond as well as the
  bond strength

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Symmetric stretching
In-plane bending
Antisymmetric stretching
Out-of-plane bending
When the molecule is irradiated with IR
electromagnetic radiation, the vibrating bond
will absorb energy if the frequencies of the
light and the vibration are identical
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Nature of Molecular Vibrations Normal Modes
STRETCHING
VIBRATING BONDS
CONTRACTING
When a molecule absorbs IR radiation, its
chemical bonds vibrate. The bonds can
stretch, contract, and bend. Vibrational motion
can be broken down into a number of constituent
vibrations called normal modes (Rough Guide-
of atoms in a molecule helps

determine of vibrations a molecule
possesses)
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Factors affecting frequency of
molecular vibrations

• Mass of the atoms - heavier molecules will result
  in slower movement and result in lower
  frequencies
• Elasticity of the spring (force constant) - a
  stronger spring (bond) will cause the vibration
  to be more rapid than a weaker one- HIGHER
  FREQUENCY
• Bond Strength - increasing of bonds increases
  frequency of vibration

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Dispersive IR Instruments

• Infrared light from the source passes through the
  sample, then goes through a slit into a grating
  to disperse the light into a spectrum of its
  component wavenumbers.
• The grating is rotated so that different
  wavenumber slices pass through the slit and are
  sampled by the detector

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A Dispersive Infrared Spectrophotometer
Shimadzu IR-470
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Older Technology- limitations

• Slow Measurements Because dispersive
  instruments measure each frequency individually,
  a single measurement may take as long as 15
  minutes
• Relatively Insensitive Older instruments
  employed detectors which required a large amount
  of sample. Today we can detect as little as 0.01
  of a compound in a particular matrix with FTIR
• Mechanically Complex- Because dispersive
  instruments have lots of moving parts, they are
  susceptible to mechanical failure increasing
  possible instrument down time and the slits throw
  most of the sample beam away at any one time
  anyhow

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Advantages of FTIR (technique)

• Universal technique
• sensitivity 10-6 grams
• fast and easy
• relatively inexpensive
• rich information
• sensitive to molecules-anything that contains
  chemical bonds
• vast majority of molecules in the universe absorb
  mid-infrared light, making it a highly useful
  tool

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Disadvantages of FTIR

• Cannot detect atoms or monoatomic ions - single
  atomic entities contain no chemical bonds
• Cannot detect molecules comprised of two
  identical atomS symmetric-such as N2 or O2.
• Aqueous solutions are very difficult to analyze-
  water is a strong IR absorber
• Complex mixtures - samples give rise to complex
  spectra

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Michelson Interferometer
IR radiation from the source is separated by the
beamsplitter into two beams that travel different
pathlengths.When these two separated beams are
recombined, an interference pattern is generated
and passed through the sample. The FTIR relies on
a mathematical (Fourier Transform) reconstruction
of the spectrum from this interference pattern
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Constructive Interference
? n? Constructive Interference will take
place for any value of ?
when the two beams are in phase
n0,1,2,3,..
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Destructive Interference
Fixed Mirror
Moving Mirror
? ( n 1/2 ) ? Totally destructive
interference takes place when optical path
difference is 1/2 ? or some multiple of it-beams
are completely out of phase
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At all path differences other than ZPD (zero path
difference), a variation of constructive and
destructiveinterference takes place - this
modulated radiation is denoted by how many
times a wave may switch between being light and
dark known as the frequency
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where Fv Modulation
frequency Fv 2VW V moving
mirror velocity in cm/sec
W wavenumber passing
through interferometer -Both of these
wavenumbers are distinguished by the detector as
a cosine wave, but their frequencies
are different as expected from the equation above
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Interferogram of a Broadband Source
A plot of light intensity versus optical path
difference is an Interferogram These are the
fundamental measurements obtained by an FTIR
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Relationship between Interferogram and Power
Spectrum
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FTIR HhOWHHrinciples
How an Interferogram becomes a Spectrum!
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Fourier Transform of an Interferogram

• The wavelength and intensity data at each point
  in the scanned spectrum are encoded so that all
  of the spectral information is acquired at once
  it is mathematically decoded through Fourier
  Transform.
• Essentially, wavelength and intensity are related
  mathematically to mirror position and velocity.

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What is Apodization?????? (comes from
Greek word a podi or no feet)

• Calculating A Fourier Transform involves
  performing a mathematical integral on the IFG.
  Ideally the limits of this integral should be /-
  infinity (infinity for OPD and of data points
  collected to Fourier Transform properly) - This
  is impossible, so the IFG and the integral must
  be truncated at some point, the limits being 0
  OPD and the maximum OPD
• An unfortunate outcome of truncating the signal
  is the lineshapes become distorted causing
  sidelobes which are sinusoidal undulations in
  the baseline. These lobes, or feet are
  suppressed by multiplying the IFG by an
  Apodization Function

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Main Side-Effect of Apodization!!

• - Spectral Resolution Will be Reduced - AFs
  vary in how well they suppress sidelobes, and how
  much they degrade resolution
• AF that provides the highest resolution, and does
  the worst job of suppressing sidelobes, is the
  boxcar apodization (only use with gases or where
  utmost resolution is required)
• AF recommended for Condensed Phase samples is
  Happ-Genzel or Medium Beer-Norton

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Relationship between the Interferogram and the
Spectrum
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Multiplex (Fellgett) Advantage
All wavelengths measured all the time-because all
data is measured simultaneously, measurements are
made in seconds rather than several minutes

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Jacquinot ( Throughput) Advantage
'
All infrared radiation passes through the sample
and strikes the detector at once-there are no
slits to restrict the energy increased
sensitivity and reduced noise automatically
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Connes (Frequency Calibration) Advantage
Because the He-Ne laser emission occurs at
exactly 632.8nm, it is used to set the exact
wavenumber(cm-1) scale in the IR spectrum
Spectrum of high wavenumber accuracy is obtained
and you never have to calibrate
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The Signal-Averaging Advantage

• Fast scans with an FTIR enable the coaddition of
  many scans in order to reduce the random
  measurement noise-use the computer to do this
  quite easily
• The positive and negative fluctuations in the
  random noise level cancel themselves out as more
  scans are added together

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FTIR 8300/8400/8700/8900 SOURCE

• Purpose provide radiant energy in the infrared
  region of the electromagnetic spectrum
• Ceramic Source/Globar- bright and very intense -
  inert solid is heated electrically to 1500-2000 K
  -guaranteed for 5000 hours

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BEAMSPLITTER

• KBr is almost universally used as a substrate
  material in FTIR beamsplitters-this material does
  not split the beam since it transmits in the IR
• Instead, a thin coating of Germanium is
  sandwiched between two pieces (disks) of KBr, and
  it is this Ge coating that splits the beam. So
  they are referred to as Ge on KBr beamsplitters
• Disadvantage KBrs natural tendency to absorb
  moisture-fogging

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FTIR 83/84/87/8900 Interferometer
Mirror is the only
moving part

• FJS -Flexible Joint Support-
  unique-patented
• Shimadzu system
• Swing system with smooth film support
  system.
• Base plate swings while keeping parallel against
  the
• top plate.
• Mirror is mounted in front of the base.
• Mirror runs parallel with theTop plate
• Advantage
• Long endurance and precision because of
  noncontact
• free from wear
  
  
  
• High linearity and precision almost equal to an
  air
• bearing system

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Friction-less Mirror Drive Requires No
Maintenance

• Unique, patented Shimadzu design employs a
  friction-less electro-magnetic moving mirror
  drive supported by two tough, durable polyimide
  sheets (a flexible joint support system).
• Stability and linearity of response is equivalent
  to the Air-bearing Drive without the need for
  pure, dry air

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FTIR83/84/87/8900 DETECTORS

• DLATGS (deuterated L-Alanine doped tri-glycine
  Sulfate) - Shimadzu patented design
• pyroelectric bolometer - changes in the amount of
  infrared radiation striking the detector cause
  the temperature of the DTGS element to change,
  measured as a voltage change
• -Temperature controlled high sensitivity
  detector-better sensitivity than traditional DTGS
  detector
• MCT Detector (optional) - AIM Microscope required
• ----mercury cadmium telluride (HgCdTe)- consists
  of an alloy of 3 elements, and it is a
  semi-conductor. Energy is detected by use of a
  bandgap (photons exceeding the energy bandgap
  will be detected)

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He-Ne Laser

• Laser acts as an internal wavenumber standard. -
  He-Ne gives off light at precisely 15,798.637
  cm-1. All infrared frequencies are measured
  relative to it.
• Laser is used to track the position of the moving
  mirror. - so the optical path difference can be
  measured precisely and any adjustments will be
  made instantaneously to the fixed mirror
  positioning

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Dynamic Alignment System
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Dynamic Auto-Alignment for Maximum Reproducibility

• Dynamic feedback loop - maintains the optimum
  alignment conditions during measurement, a
  consistently stable and reproducible spectrum is
  obtained.
• Continuous laser referencing - operates
  continuously at 5,000 to 10,000 Hz
• Auto-Adjust Function - provides quick and easy
  fine tuning of the interferometer. It eliminates
  the need for tedious mechanical adjustments

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Dynamic Auto-Alignment (contd)

• Recalls last position at power-down
• Minimizes start-up time
• 5 min or less for Qualitative Analysis
• 30 min for Quantitative Analysis
• Eliminates the need to keep the FTIR on
  continuously

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Triple Protection of the Interferometer
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Interferometer Optical Component Protection
features

• A high-tech proprietary anti-moisture coating is
  applied directly to the KBr beamsplitter surface
• Optical compartment is sealed and desiccated with
  Calcium Oxide - will not re-release moisture back
  into the optical compartment
• KRS-5 Exit Window does not react with water, it
  is impervious to moisture and will never fog-
  Disadvantage only 70 transmissivity

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Common Causes of Instrument Problems?

• Beam blocked by something
• Low throughput accessory
• Poor (vibration,humidity) operational environment
• Failing Source
• Fogged Beamsplitter
• Poorly Aligned (tuned) moving mirror
• Failing detector
• Electronic Failures

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Initial Suggestions to Follow

• Follow infra red beam (Are laser spots in the
  sample compartment visible on your hand?) - Make
  sure no obstructionsin light path/ Does problem
  exist with Accessories removed?
• Turn power on/off - Look for laser lights. If not
  present, replace laser tube/laser power supply

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More Suggestions !

• Inspect beamsplitter for fogging, and replace if
  necessary
• Check the Moving Mirror in Hayato 105 (Service
  Mode) and see if it moves freely-it should since
  we are forcing it to move- If it does not then
  inspect with different scanning speeds to observe
  movement- if none- replace the CPU boards- FTIR
  Main/Digital II

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