Fourier Transform Infrared FTIR Spectroscopy

1
Fourier Transform Infrared (FT-IR) Spectroscopy

• Theory and Applications

THE ELECTROMAGNETIC SPECTRUM
INFRARED
GAMMA RAYS X RAYS
UV VISIBLE
2
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 shorter than microwaves, and have
  frequencies which are lower than visible 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 is closest to visible light and
  far-infrared refers to the part that is closer 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 molecules in an object. The higher the
  temperature, the more the atoms and molecules
  move and the more infrared radiation they
  produce.
• Any object radiates in the infrared. Even an ice
  cube, emits infrared.

4
What is Infrared? (Cont.)

• Humans, at normal body temperature, radiate most
  strongly in the infrared, at a wavelength of
  about 10 microns (A micron is the term commonly
  used in astronomy for a micrometer or one
  millionth of a meter). In the image to the left,
  the red areas are the warmest, followed by
  yellow, green and blue (coolest).

The image to the right shows a cat in the
infrared. The yellow-white areas are the warmest
and the purple areas are the coldest. This image
gives us a different view of a familiar animal as
well as information that we could not get from a
visible light picture. Notice the cold nose and
the heat from the cat's eyes, 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.

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.
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Infrared Spectroscopy

• A molecule can be characterized (identified)
  by its molecular vibrations, based on the
  absorption and intensity of specific infrared
  wavelengths.

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Infrared Spectroscopy

• For isopropyl alcohol, CH(CH3)2OH, the infrared
  absorption bands 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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Comparison Beetween Dispersion Spectrometer and
FTIR
Dispersion Spectrometer
To separate IR light, a grating is used.
Detector
Grating
Slit
In 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.
Sample
To select the specified IR light, A slit is used.
Light source
FTIR
An interferogram is first made by the
interferometer using IR light.
Fixed CCM
In order to measure an IR spectrum, FTIR takes
only a few seconds. Moreover, the detector
receives up to 50 of the energy of
original light source. (much larger than the
dispersion spectrometer.)
Detector
B.S.
Sample
Moving CCM
The interferogram is calculated and
transformed into a spectrum using a Fourier
Transform (FT).
IR Light source
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The Principles of FTIR Method
Interferogram is made by an interferometer.
Sample
Interferogram is transformed into a spectrum
using a FT.
Sample
BKG
SB
SB
Sample/BKG
1000
3000
3000
2000
2000
1000
cm-1
cm-1
T
IR spectrum
3000
cm-1
2000
1000
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IR light source
FTIR seminar
IR Light Source
Intensity Distribution and Temperature Dependency
versus Wavelength of Black Body Radiation Energy
105
6000K
104
4000K
103
102
2000K
10
Spectral irradiance W l
1000K
1
10-1
500K
10-2
300K
10-3
200K
10-4
2
5
10
20
0.1
0.2
0.5
1
50
100
Wavelength l / mm
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FTIR seminar
FT Optical System Diagram
Light source
He-Ne gas laser
(ceramic)
Beam splitter
Movable mirror
Sample chamber
(DLATGS)
Fixed mirror
Detector
Interferometer
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FTIR seminar
Interference of two beams of light
Movable mirror
Fixed mirror A Movable mirror
Same-phase interference wave shape
-2l
-l
0
l
2l
Continuous phase shift
Fixed mirror B Movable mirror
Opposite-phase interference wave shape
Signal strength
Fixed mirror C Movable mirror
-2l
-l
0
l
2l
Same-phase interference wave shape
0
l
D Interference pattern of light manifested by
the optical-path difference
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FTIR seminar
Interference is a superpositioning of waves
Relationship between light source spectrum and
the signal output from interferometer
Light source spectrum
Signal output from interference wave
I

• Monochromatic
• light
• (b) Dichroic light
• Continuous
• spectrum light

Az
u
Wavenumber

Time t
S
I
SAz
u
Time t
Wavenumber

I(t)
b (u)
Time t
u
Wavenumber
All intensities are standardized.

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FTIR seminar
Sampling of an actual interferogram
Interferometer interferogram
Output of a Laser interferometer
Primary interferometer interferogram that was
sampled
Optical path difference x
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Fourier Transform
Single strength
Fourier transform
SB
400
4000
Optical path differencex
Wavenumbercm-1
(Interferogram)
(Single beam spectrum)
Time axis by FFT Wavenumber
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FTIR seminar
Detector Properties
MCT Operates at the temperatur of liquid nitrogen
1010 109 108
D (l, f) (cmHz1/2W-1)
TGS Operates at room temperature
600
4000
Wavenumbercm-1
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FT-IR Advantages and Disadvantages
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
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FT-IR Advantages

• Fellgett's (multiplex) Advantage
• FT-IR collects all resolution elements with a
  complete scan of the interferometer. Successive
  scans of the FT-IR instrument are coadded and
  averaged to enhance the signal-to-noise of the
  spectrum.
• Theoretically, an infinitely long scan would
  average out all the noise in the baseline.
• The dispersive instrument collects data one
  wavelength at a time and collects only a single
  spectrum. There is no good method for increasing
  the signal-to-noise of the dispersive 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
  and interferometer travel to define resolution.
  To improve signal-to-noise, one simply collects
  more scans.
• More energy is available for the normal infrared
  scan and various accessories can be used to solve
  various sample handling problems.
• The dispersive instrument uses a rectangular slit
  to control resolution and cannot increase the
  signal-to-noise for high resolution scans.
  Accessory use is limited 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 various spectral
  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 at a
  resolution of 1 cm-1 if 4 data points are
  collected within each spectral interval of 1 cm-1
  .
• In order to acquire a spectrum at higher, an
  increased number of data points is needed,
  requiring a longer stroke of the moving mirror.
• For higher resolution instruments an
  aperture is needed in order to improve
  parallelism within interferometer.

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FT-IR Terms and Definitions

• Apodization - a mathematical operation to
  reduce unwanted oscillation and noise
  contributions from the interferogram and to avoid
  aberrations coming from the finite nature of
  real (non theoretical interferograms). Common
  apodization functions include 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 4000 to 400 cm-1.

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New Features of FTIR4000-6000Series
The highest S/N ratio in the world, 50,0001
(FT/IR-6300) (Over sampling with 24-bit
ADC) DSP-driven interferometer and new ADC
(18-bit to 24-bit) Digital control of the moving
mirror drive using an advanced high speed digital
signal processor (DSP) technology The 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 of 7.1, 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
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FTIR4000 Series
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-4200
Microscope
Polymer shell Improved instrument design Compact
size Sample compartment with same size as a
higher class model
Aperture
FT/IR-400 Plus
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FTIR4000 Series Purge System
Instrument purge is standard for all models of
the FT/IR-4000 Series.
Control valve
FT/IR-4000 Series purge design
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S/N ratio (Oversampling system)
Accurate mirror drive And reduce flutter at low
wavenumber range.
FT/IR-4000 6000 series
Conventional method
Voice Coil
Voice Coil
DSP
DAC
Analog circuit
ADC
Pre-amp.
Pre-amp.
Photo coupler
Photo coupler
Clock
24-bit AD
HeNe laser
HeNe laser
Over sampling method
Find the zero crossings, then interpolate a
matching set of IR data points.
Reduction of high frequency noise by over
sampling with a 16 times greater number of
sampling points enables improvement of the S/N
ratio.
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FTIR6000 Series
- Upgradeability - Wide wavenumber range - Full
vacuum capability - Step scan upgrade
FT/IR-6100 / 6200 / 6300
Microscope
FT-Raman
Polymer shell Improved instrument design Compact
size
FT/IR-600Plus
FT/IR-6000 Series Optical design
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FTIR6000 Series Purge/Vacuum System
Instrument purge is standard for all models of
the FT/IR-6000 Series.
Purge control valve front side
FT/IR-6000 Series purge design