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Fourier Transform Infrared(FT-IR) Spectroscopy
Theory and Applications
THE ELECTROMAGNETIC SPECTRUM
INFRAREDGAMMA RAYS X RAYS UV VISIBLE
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Introduction to FTInfrared
Spectroscopy
What is infrared spectroscopy?
Theory of FT-IR
FT-IR Advantages?
New FT/IR4000-6000Series
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What is Infrared? Infrared radiation lies between the visible and microwave portions
of the electromagnetic spectrum.
Infrared waves have wavelengths longer than visible and shorterthan microwaves, and have frequencies which are lower thanvisible and higher than microwaves.
The Infrared region is divided into: near, mid and far-infrared.
Near-infrared refers to the part of the infrared spectrum that isclosest to visible light and far-infrared refers to the part that iscloser to the microwave region.
Mid-infrared is the region between these two.
The primary source of infrared radiation is thermal radiation. (heat)
It is the radiation produced by the motion of atoms and moleculesin an object. The higher the temperature, the more the atoms andmolecules move and the more infrared radiation they produce.
Any object radiates in the infrared. Even an ice cube, emitsinfrared.
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What is Infrared? (Cont.)
Humans, at normal body temperature, radiatemost strongly in the infrared, at a
wavelength of about 10 microns (A micronis the term commonly used in astronomy fora micrometer or one millionth of a meter). In
the image to the left, the red areas are thewarmest, followed by yellow, green and
blue (coolest).
The image to the right shows a cat in theinfrared. The yellow-white areas are thewarmest and the purple areas are the coldest.
This image gives us a different view of afamiliar animal as well as information that wecould not get from a visible light picture.Notice the cold nose and the heat from the cat'seyes, mouth and ears.
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Infrared Spectroscopy
The bonds between atoms in the molecule stretch and bend,
absorbing infrared energy and creating the infrared
spectrum.
Symmetric Stretch Antisymmetric Stretch Bend
A molecule such as H2O will absorb infrared light when the vibration
(stretch or bend) results in a molecular dipole moment change
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Energy levels in Infrared Absorption
Infrared absorption occurs among the ground vibrational states, the
energy differences, and corresponding spectrum, determined by the
specific molecular vibration(s). The infrared absorption is a net
energy gain for the molecule and recorded as an energy loss for the
analysis beam.
hExcited
states
Ground
(vibrational)
states
h(1 -0 )h(1 - 0)
h(2 - 1)(overtone)
Infrared Absorption and
Emission
1
2
0
3
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Infrared Spectroscopy
A molecule can be characterized (identified) by its molecularvibrations, based on the absorption and intensity of specific
infrared wavelengths.
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Infrared Spectroscopy
For isopropyl alcohol, CH(CH3)2OH, the infrared absorptionbands identify the various functional groups of the molecule.
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Capabilities of Infrared Analysis
Identification and quantitation of organic solid,liquid or gas samples.
Analysis of powders, solids, gels, emulsions,
pastes, pure liquids and solutions, polymers, pure
and mixed gases.
Infrared used for research, methods development,
quality control and quality assurance applications.
Samples range in size from single fibers only 20
microns in length to atmospheric pollution studies
involving large areas.
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Applications of Infrared Analysis
Pharmaceutical research Forensic investigations
Polymer analysis
Lubricant formulation and fuel additives
Foods research
Quality assurance and control
Environmental and water quality analysis
methods
Biochemical and biomedical research
Coatings and surfactants
Etc.
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To separate IR light, a grating is used.
Grating
Light source
Detector
Sample
Slit
To select the specified IR light,
A slit is used.
Dispersion
SpectrometerIn order to measure an IR spectrum,
the dispersion Spectrometer takes
several minutes.
Also the detector receives only
a few % of the energy of
original light source.
Fixed CCM
B.S.
Moving CCM
IR Light source
Sample
Detector
An interferogram is first made by
the interferometer using IR light.
The interferogram is calculated and transformed
into a spectrum using a Fourier Transform (FT).
FTIRIn order to measure an IR spectrum,
FTIR takes only a few seconds.
Moreover, the detector receives
up to 50% of the energy of originallight source.
(much larger than the dispersion
spectrometer.)
Comparison Beetween Dispersion Spectrometer and
FTIR
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Interferogram
is made by an interferometer.
Interferogram
is transformed
into a spectrum using a FT.
BKG
SB
3000 2000 1000
[cm-1]
Sample
SB
Sample
3000 2000 1000
[cm-1]
Sample/BKG
IR spectrum
%T
3000 2000 1000 [cm-1]
The Principles of FTIR Method
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FTIR seminar
Intensity Distribution and Temperature Dependency versus Wavelength of
Black Body Radiation Energy
2 5 2010
105
104
103
102
10
1
10-1
10-2
10-3
10-4
0.1 0.2 0.5 1 50 100
Wavelength / m
6000K
4000K
2000K
1000K
500K
300K
200K
Spe
ctralirradianceW
IR light source
IR Light Source
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FTIR seminar
Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light
source
(ceramic)
Detector
(DLATGS)
Beam splitter
FT Optical System Diagram
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Fixed mirror
Movable mirror
Fixed mirror
Movable mirror
Fixed mirror
Movable mirror
Same-phase interference
wave shape
Opposite-phase
interference
wave shape
Same-phase interference
wave shape0
Movable mirror
Interference pattern of lightmanifested by the optical-path
difference
Continuous phase shift
Signalstrength
(X)
-2 - 0 2
-2 - 0 2
FTIR seminar
Interference of two beams of light
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Relationship between light source spectrum and the signal output from interferometer
(a) Monochromatic
light
(b) Dichroic light
(c) Continuous
spectrum light
All intensities are standardized.
Light source spectrum Signal output from interference wave
Time t
Time t
Time tI(t)
I
( )
Wavenumber
Wavenumber
Wavenumber
IAz
Az
FTIR seminar
Interference is a superpositioning of waves
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FTIR seminar
Interferometer interferogram
Output of a Laser interferometer
Primary interferometer interferogram
that was sampled
Optical path difference x
Sampling of an actual interferogram
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4000 400
SBFourier transform
Optical path difference[x]
(Interferogram) (Single beam spectrum)
Wavenumber[cm-1]
Single
st
rength
Time axis by FFT Wavenumber
Fourier Transform
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FTIR seminar
TGS
Operates at room temperature
MCT
Operates at the temperatur
of liquid nitrogen
D*(,
f)(cmHz
1/2W-1)
1010
109
108
Wavenumber[cm-1]4000 600
Detector Properties
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1.Better sensitivity and brightness
- Allows simultaneous measurement over the entire wavenumber range- Requires no slit device, making good use of the available beam
2.High wavenumber accuracy
- Technique allows high speed sampling with the aid of laser light interference fringes
- Requires no wavenumber correction
- Provides wavenumber to an accuracy of 0.01 cm-1
3. Resolution
- Provides spectra of high resolution
4. Stray light
- Fourier Transform allows only interference signals to contribute to spectrum.
Background light effects greatly lowers.
- Allows selective handling of signals limiting intreference
5. Wavenumber range flexibility
- Simple to alter the instrument wavenumber range
CO2 and H2O sensitive
FT-IR Advantages and Disadvantages
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FT-IR Advantages
Fellgett's (multiplex) Advantage
FT-IR collects all resolution elements with a completescan of the interferometer. Successive scans of the FT-IR instrument are coadded and averaged to enhance thesignal-to-noise of the spectrum.
Theoretically, an infinitely long scan would average outall the noise in the baseline.
The dispersive instrument collects data one wavelengthat a time and collects only a single spectrum. There isno good method for increasing the signal-to-noise of thedispersive spectrum.
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FT-IR Advantages
Connes Advantage
an FT-IR uses a HeNe laser as an internal wavelength
standard. The infrared wavelengths are calculated
using the laser wavelength, itself a very precise and
repeatable 'standard'.Wavelength assignment for the FT-IR spectrum is very
repeatable and reproducible and data can be compared
to digital libraries for identification purposes.
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FT-IR Advantages
Jacquinot Advantage
FT-IR uses a combination of circular apertures andinterferometer travel to define resolution. To improvesignal-to-noise, one simply collects more scans.
More energy is available for the normal infrared scanand various accessories can be used to solve varioussample handling problems.
The dispersive instrument uses a rectangular slit tocontrol resolution and cannot increase the signal-to-noise for high resolution scans. Accessory use islimited for a dispersive instrument.
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FT-IR Application Advantages
Opaque or cloudy samples
Energy limiting accessories such as diffuse reflectance or FT-
IR microscopes
High resolution experiments (as high as 0.001 cm-1 resolution)
Trace analysis of raw materials or finished products
Depth profiling and microscopic mapping of samples
Kinetics reactions on the microsecond time-scale
Analysis of chromatographic and thermogravimetric sample
fractions
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FT-IR Terms and Definitions
Resolution (common definition) The separation of the variousspectral wavelengths, usually
defined in wavenumbers (cm-1).
A setting of 4 to 8 cm-1 is sufficient
for most solid and liquid samples.Gas analysis experiments may need
a resolution of 2 cm-1 or higher.
Higher resolution experiments will
have lower signal-to-noise.
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FT-IR Terms and Definitions
Resolution FT/IR Case
A spectrum is said to be collected ata resolution of 1 cm-1 if 4 datapoints are collected within eachspectral interval of 1 cm-1 .
In order to acquire a spectrum athigher, an increased number of data
points is needed, requiring a longerstroke of the moving mirror.
For higher resolution instrumentsan aperture is needed in order toimprove parallelism withininterferometer.
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FT-IR Terms and Definitions
Apodization - amathematical operation toreduce unwantedoscillation and noisecontributions from the
interferogram and to avoidaberrations coming fromthe finite nature of real(non theoreticalinterferograms). Commonapodization functionsinclude Beer-Norton,Cosine and Happ-Genzel.
Apodization
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FT-IR Terms and Definitions
Scan mode - Either single
beam or ratio. Single
beam can be a scan of the
background (no sample)
or the sample. Ratio
mode always implies the
sample spectrum divided
by, or ratioed against, the
single beam background.
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FT-IR Terms and Definitions
Scan(s) - a complete cycle of movement of the
interferometer mirror. The number of scans collected
affects the signal-to-noise ratio (SNR) of the final
spectrum. The SNR doubles as the square of the
number of scans collected; i.e. 1, 4, 16, 64, 256, .
Scan speed or optical path velocity - the rate at which
the interferometer mirror moves. For a DTGS detector,
the SNR decreases as the scan speed increases.
Scan range - spectral range selected for the analysis.
The most useful spectral range for mid-infrared is 4000to 400 cm-1.
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The highest S/N ratio in the world, 50,000:1 (FT/IR-6300) (Over sampling with 24-bit ADC)
DSP-driven interferometer andnew ADC (18-bit to 24-bit)
Digital control of the moving mirror drive using an advanced high speed digital signal processor (DSP) technologyThe outstanding performance of the ADC (Analog-to digital converter) and DSP (Digital signal processor) allows very rapid and accurate correction
for the effects of velocity and position errors.
Autoalignment for all models (The interferometer optics can always be aligned by the PC)
In addition to proven technology for Rapid scanning and vacuum capabilities;
a Step scan capability enables time-resolved studies similar to research models by Nicolet, Bruker and Bio-Rad.
IR imaging with IMV-4000 multi-channel microscope for all models (Rapid scanning with a linear array MCT detector )
PC communication and control using USB
Aperture of7.1, 5.0, 3.5, 2.5, 1.8, 1.2, 0.9, 0.5 mm diameter for FT/IR-4100/4200
Spectra Manager II (cross-platform software suite for JASCO spectroscopy systems) (Spectra Manager CFR: 21 CFR Part
11 compliance)
Research model capability (Upgradeable wavelength extension, high resolution, step scan)
Improved Water Vapor and CO2 Compensation
New Features of FTIR4000-6000Series
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Polymer shell
Improved instrument design
Compact sizeSample compartment with
same size as a higher class
model
FT/IR-400 Plus
Aperture
No additional optics for IR microscope interface
Standard apertures for optimum S/N and resolution capability
Easy replacement of light source and detector
FT/IR-4100
FT/IR-4200Microscope
FTIR4000 Series
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FT/IR-4000 Series purge design
N2gas inlet
Control valve
Instrument purge is standard for all models of the FT/IR-4000 Series.
FTIR4000 Series Purge System
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Conventional method
Find the zero crossings, then interpolate
a matching set of IR data points.
Over sampling method
Reduction of high frequency noise by over sampling with a 16 times greater
number of sampling points enables improvement of the S/N ratio.
Pre-amp.
Analog circuit
Photo coupler
Voice Coil
HeNe laser
Photo coupler
Pre-amp.
ADC
DSPDAC
Clock
24-bit AD
Voice Coil
HeNe laser
Accurate mirror drive
And reduce flutter at
low wavenumber range.
FT/IR-4000 & 6000 series
S/N ratio (Oversampling system)
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FT/IR-6000 Series purge design
N2gas inlet
Purge control valve front side
Instrument purge is standard for all models of the FT/IR-6000
Series.
FTIR6000 Series Purge/Vacuum System
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