Spectroscopy
Fourier Transform Infrared
(FT-IR) Spectroscopy
Theory and Applications
THE ELECTROMAGNETIC SPECTRUM
GAMMA RAYS
X RAYS
UV
VISIBLE
INFRARED
Spectroscopy
Introduction to FTInfrared
Spectroscopy
 What is infrared spectroscopy?
 Theory of FT-IR
 FT-IR Advantages?
 New FT/IR4000-6000Series
Spectroscopy
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.
Spectroscopy
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.
Spectroscopy
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
Spectroscopy
Energy levels in Infrared Absorption
Excited
states
hn
Infrared Absorption and
Emission
n
3
n
n2
1
n
h(n1 n0 )
h(n2 - n1)
(overtone)
h(n1 - n0)
Ground
(vibrational)
states
0
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.
Spectroscopy
Infrared Spectroscopy
A molecule can be characterized (identified) by its molecular
vibrations, based on the absorption and intensity of specific
infrared wavelengths.
Spectroscopy
Infrared Spectroscopy
For isopropyl alcohol, CH(CH3)2OH, the infrared absorption
bands identify the various functional groups of the molecule.
Spectroscopy
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.
Spectroscopy
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.
Spectroscopy
Comparison Beetween Dispersion Spectrometer
and FTIR
To separate IR light, a grating is used.
Grating
Detector
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
Light source
Fixed CCM
To select the specified IR light,
A slit is used.
An interferogram is first made
by the interferometer using IR
light.
Detector
B.S.
Sample
Moving CCM
IR Light source
Dispersion
Spectrometer
The interferogram is calculated and transformed
into a spectrum using a Fourier Transform (FT).
FTIR
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.)
Spectroscopy
The Principles of FTIR Method
Interferogram
is made by an interferometer.
Sample
Interferogram
is transformed
into a spectrum using a FT.
BKG
Sample
SB
SB
Sample/BKG
3000
2000
3000
1000
[cm-1]
2000
[cm-1]
%T
IR spectrum
3000
2000
1000
[cm-1]
1000
Spectroscopy
seminar
IR light FTIR
source
IR Light Source
Intensity Distribution and Temperature Dependency versus Wavelength of
Black Body Radiation Energy
105
6000K
104
4000K
103
102
2000K
10
1000K
1
10-1
500K
10-2
300K
10-3
200K
10-4
0.1
0.2
0.5
1
2
5
Wavelength l / mm
10
20
50
100
Spectroscopy
FTIR seminar
FT Optical System Diagram
Light
source
He-Ne gas laser
(ceramic)
Beam splitter
Movable mirror
Sample chamber
(DLATGS)
Fixed mirror
Interferometer
Detector
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
Opposite-phase
interference
wave shape
Movable mirror
Fixed mirror
Ｃ
Movable mirror
0
Signal strength
Spectroscopy
FTIR seminar
Ｉ
(X)
-2l
l
Same-phase interference
wave shape
-l
0
l
2l
Ｄ Interference pattern of light
manifested by the optical-path
difference
Spectroscopy
FTIR seminar
Interference is a superpositioning of waves
Relationship between light source spectrum and the signal output from interferometer
Light source spectrum
(a)
I
Signal output from interference wave
Monochromatic
light
z
A
Wavenumber u
(b)
Time t
Dichroic light
SAz
Wavenumber u
(c)
SI
I(t)
Time t
Continuous
spectrum light
b (u)
Wavenumber u
Time t
All intensities are standardized.
Spectroscopy
FTIR seminar
Sampling of an actual interferogram
Interferometer interferogram
Output of a Laser interferometer
Primary interferometer interferogram
that was sampled
Optical path difference x
Single strength
Spectroscopy
Fourier Transform
Fourier transform
Optical path difference[x]
(Interferogram)
Time axis by FFT
SB
4000
Wavenumber[cm-1]
(Single beam spectrum)
Wavenumber
400
Detector Properties
MCT
Operates at the temperatur
of liquid nitrogen
1010
D* (l, f) (cmHz1/2W-1)
Spectroscopy
FTIR seminar
109
TGS
Operates at room temperature
108
4000
Wavenumber[cm-1]
600
Spectroscopy
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
Spectroscopy
FT-IR Advantages
Fellgett's (multiplex) Advantage
 FT-IR collects all resolution elements with a complete
scan of the interferometer. Successive scans of the FTIR 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.
Spectroscopy
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.
Spectroscopy
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-tonoise for high resolution scans. Accessory use is
limited for a dispersive instrument.
Spectroscopy
FT-IR Application Advantages
 Opaque or cloudy samples
 Energy limiting accessories such as diffuse reflectance or FTIR 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
Spectroscopy
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.
Spectroscopy
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.
Spectroscopy
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
Spectroscopy
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.
Spectroscopy
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.
Spectroscopy
New Features of FTIR4000-6000Series
The highest S/N ratio in the world, 50,000:1 (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
Spectroscopy
FTIR4000 Series
FT/IR-4100
FT/IR-4200
No additional optics for IR microscope interface
Standard apertures for optimum S/N and resolution capability
Easy replacement of light source and detector
Microscope
Polymer shell
Improved instrument design
Compact size
Sample compartment with
same size as a higher class
model
FT/IR-400 Plus
Aperture
Spectroscopy
FTIR4000 Series Purge System
Instrument purge is standard for all models of the FT/IR-4000 Series.
N2gas inlet
Control valve
FT/IR-4000 Series purge design
Spectroscopy
S/N ratio (Oversampling system)
Voice Coil
Conventional method
Accurate mirror drive
And reduce flutter at
low wavenumber range.
FT/IR-4000 & 6000 series
Voice Coil
DAC
DSP
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.
Spectroscopy
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
Spectroscopy
FTIR6000 Series Purge/Vacuum System
N2gas inlet
Instrument purge is standard for all models of the FT/IR-6000 Series.
Purge control valve – front side
FT/IR-6000 Series purge design