Ftir theory and applications

FTIR: THEORY, INSTRUMENTATION AND
APPLICATIONS
Dr. Amol Shinde
Shimadzu Analytical India Pvt Ltd
Mob: 8452912856
Email: amolshinde005@gmail.com

ELECTROMAGNETIC SPECTRUM
UV VIS NIR IR Far IR Microwave
10-5
10-5 10-4 10-3 10-2 10-1
Wavelength (cm)
Energy
0.78 2.5 25 1000Wavelength (mm)
Wavenumber (cm-1) 12,800 4000 400 10

Discovery of Infrared Radiation
William Herschel
1738-1822
German Astronomer
 The infrared radiation was
discovered in 1800.
 Infra = Lower
 Infrared = Lower energy
radiation.
 He observed an invisible radiation
in solar spectrum having lower
energy than red light, by means
of its effect on a thermometer.

 Electromagnetic radiation is an oscillating
electric force field transmitted through
space in the form of a transverse wave
 Responsible for phenomena such as
transmission, absorption, refraction and
reflection
Wave Properties

PROPERTIES OF LIGHT
n = frequency (cycle / second)
0 2 4 6 8 10
-1.0
-0.5
0.0
0.5
1.0
lOne cycle
nl = cA
A = amplitude
c = speed of light (3.0 * 1010 cm / sec)
l = wavelength (nm , μm )

E = Energy (ergs)
h = Planck’s Constant (erg sec)
c = Speed of light (μm/sec)
l = Wavelength (μm)
n = Frequency (cycles/sec)
E = hc/l = hn
ENERGY OF LIGHT

ELECTRONIC TRANSITION
Excited state E1
DE = E1 - E0 = hv
Energy level
Radiatione
Energy difference
DE
Grand state E0
e

 High energy radiation causes
 Electronic transition
 Low energy radiation causes
 Vibrational transition
 Rotational transition
ABSORPTION OF RADIATION

ABSORPTION OF RADIATION
Energy
Vibrational transition
Rotational
transition R
v1
v2
v0
5
4
3
2
1
0
Electronic
transition
E1
E0
v0
v1
v2
E = hn in which h is Planck’s constant (6.626 * 10-34 Joules
and n is the frequency of the radiation

Vibrational motion
•A polyatomic molecule containing N atoms
will have 3N possibilities for motion
•Of these 3N possibilities or degree of
freedom, 3 must be attributed for the
translation of the entire molecule
•The remain 3N – 3 possibilities are for
rotation and internal vibration in the
molecule
•Therefore
3N – 3-2 is for linear molecules
3N – 3-3 is for non-linear molecules

Types of Vibrations
1. Stretching Vibrations
Bond length changes
2. Bending Vibrations
Bond angle changes

 When the molecule is irradiated with
electromagnetic radiation, the vibrating bond
will absorb energy if the frequencies of the
light and the vibration are identical
 Each of the different vibration modes give rise
to different absorption band
Absorption of Infrared Radiation

Absorption of Infrared Radiation
 A molecule must undergo a net change in
dipole moment as a consequence of
vibrational or rotational motions in order to
absorb infrared radiation

Can all vibrations absorb IR?
 Frequencies fall outside the normal infrared regions
 Vibration does not produce a fluctuating dipole therefore
cannot interact with fluctuating electric fields of the
infrared light
Cl H
N N
Example of Dipole and Dipole movement
• When HCl vibrates, the
dipole (charge separation)
increases
• N2 has no dipole and is
infrared inactive
23

v1
v2
v3
Linear molecule CO2
v1
v2
v3
Non-linear molecule H2O
Only the vibration,which Changes Dipole moment
of molecule, has IR absorption
Can all vibrations absorb IR?

GROUP WAVENUMBERS
 Associated with the movements of a few atoms
 Largely independent of the rest of the
molecule
 Examples: C = O, C - Cl, NH, CH and OH

Fingerprint Region
• Complex region of the infrared spectrum
below 1500 cm-1.
• Functional group region
• Vibrational modes involve many atoms
• Characteristic of the molecule as a whole

Regions of Infrared Spectrum
4000 2500 2000 1500 625
2.5 4 5 7 16
Wavenumber (cm-1)
Wavelength (10-6m)

OVERTONE
 Relatively weak intensity that may not be observed
 Found in high wavenumber regions
 Appear at integer multiples of fundamental vibrations
 Occur at frequency about 2 or 3 times that of the
fundamental lines
 Not very common in mid-infrared spectra of molecules

Infrared Window Materials
 Used for transmission infrared radiation
 Used as a media for powdered samples
 Used as a cell for liquid samples

Consideration of Window Materials
 wavenumber transmission range
 chemical properties of sample and crystal
 physical properties of materials
 cost of materials

Material R.I Transmission
Range (cm-1)
% T
Sodium Chloride 1.49 600-50000 90
Potassium Bromide 1.52 340-40000 90
Calcium Fluoride 1.38 1100-50000 95
KRS-5 2.37 250-16600 70
Germanium 4.0 660-5500 50
Silica/Quartz 1.42 2500-50000 85
Diamond 2.38 125-40000 70
Characteristics of Window Materials

Sodium Chloride NaCl
 Widely used material for infrared cell windows
 Relatively the lowest cost
 Wide transmission range
 Low reflection losses
 Soluble in water and glycerine
 Slightly soluble in alcohol
 Hygroscopic, thus fog easily

Potassium Bromide KBr
 Most widely used window material
 Good resistance to thermal and mechanical shock.
 Wider transmission range
 Dissolve readily in water, glycerine and
 Alcohol
 More hygroscopic than sodium chloride
 Scratches and deforms much readily than
 sodium chloride

Calcium Fluoride CaF2
 Insoluble in water
 Useful for aqueous and deuterium oxide
solutions analysis
 Good resistance to most acids and alkalis but is
attacked by ammonium salts
 Hard , and hence high mechanical strength
 Limited transmission range

 KRS-5 (Thallium Bromide/ Iodide)
 Difficult to dissolve in water
 Attacked by aqueous solutions of high pH and
solutions of phosphates
 Toxic, usually only harmful if absorbed through a
cut or ingested
 Widely used as an internal reflection plate in ATR
 Difficult to polish

Germanium Ge
 Inert to chemicals
 Insoluble in water but attacked by hot
 Sulfuric acid
 Hard and very brittle
 Mostly used for ATR prism
 High reflection losses which lead to low
transmission levels

Quartz/ Silica
 Commonly used for near infrared region
 Very resistant material
 Soluble in hydrofluoric acid and slightly soluble in
alkalis
 Three types namely, natural, ultraviolet and
infrared grade

Diamond
 Hardest window material used in infrared
spectroscopy
 Superior physical and chemical properties
 Mostly used for ATR reflector plate for
intractable samples
 Extremely expensive
 Having IR absorption 3000~1500cm-1

Thumb rules for IR interpretation
Rule 1:
Bending is easier than stretching-- happens at lower energy (lower wavenumbe
C-H Stretch Alkane 2800-3000 cm-1
C-H Bend Alkane 1300-1400 cm-1
Rule 3:
Heavier atoms move slower than lighter atoms
C-Cl 550-850 O-H more than 3300
C-H more than 2800
Rule 2:
Triple bond ˃ Double bond ˃ Single bond
Alkyne 2100-2260 cm-1
Alkene 1600-1700

Types of FTIR
 Dispersive Infrared Spectroscopy
 Utilizes diffraction grating for wavenumber
selection by using a monochromator
 Fourier Transform Infrared Spectroscopy
 Uses an interferometer to replace the dispersive
device and generate an interference beam that is
then exposed to the sample

Interference Phenomenon
Bright zone
Dark zone
X=0 X=2x1.25=2.5µ
A)Fixed mirror
Light reflected
by
B)Movable mirror
X=2(l1-l2)=0
C)Movable mirror
X=2 (l1-l2)=1.25µm
2.5μm
1.25µm
2.5μm of monochromatic light
5μm of monochromatic light

Interference Phenomenon
Interferogram
Each wavelength
Addition

Background spectrum
CO2
H2O
H2O

Interferogram
Fourier
transformed
Air
Polystyrene film
S/B
Infrared spectrum
Power
spectrum
(1768-1830)
HOW FTIR WORKS

BASIC COMPONENTS
Light Source
Interferometer
Sample
Compartment
Detector
Processing Unit
Output
FTIR Basic
Components
Laser
Reference

INFRARED LIGHT SOURCE
 Infrared source consist of inert solid which is
heated electrically to temperature 1500 ~ 2000oK
 The continuous radiation which is emitted
resembles that of black body radiation

MICHELSON INTERFEROMETER
 Consist of 2 plane mirrors at right angle to each
other
 Beam splitter reflects 1/2 of the source intensity
to the movable mirror and transmits the other 1/2
to the fixed mirror
Spectrum
Fourier
Transform
Interferogram
Detector

TYPES OF INFRARED DETECTOR
 Thermocouples detector
 Pyro-electric detector (DLATGS, TGS, etc)
 MCT detector (Mercury Cadmium Tellurium)

Temperature controlled DLATGS detector
 High sensitivity deuterated L-alanine triglycine
sulfate detector (DLATGS)
 Maintain instrument temperature below
detector curie point (61oC)
 Detector is doped with L-alanine so that
electrical field is automatically re-poled when
temperature exceeding curie point

Advantages of Pyroelectric detector
 Excellent S/N ratio
 Stable
 Fast response
 Wide range

MCT DETECTOR
 High sensitivity
 Faster response
 Necessary to cool by LN2
 Narrow band

Advantages of FTIR Vs Dispersive
 Multiplex advantage (Fellgett)
Scans all wavenumbers at a time
 Aperture advantage (Jacquinot)
No dispersive elements, so high throughput
 Laser reference advantage (Connes)
Internal wavelength calibration
 Application advantage
High speed and sensitive measurement
possible

Hinges using film
(4 positions)
Linkage part
Top plate (fixed)
Dynamic Alignment System
Processing circuit
He-Ne laser
Beam
splitter
Piezo actuator
Fixed mirror
Movable mirror
Sampling
circuit for
interference
condition
determination
Sensor B
Sensor A
Light source
Sample compartment
FJS system
Processing circuit
Flexible Joint
Support

Dynamic Alignment System
 Laser light passing through the interferometer is
detected by photodiodes
 Signals are generated
 Difference between the result and optimum
interference condition is calculated
 Result is fed back to the interferometer
 The difference is eliminated by the use of a piezo
actuator

ADVANTAGES
 Stable and prompt measurement is achieved
through dynamic alignment mechanism
 To ensure that alignment of the optics is
accurate
 Energy is stabilized in short time

PIEZO ACTUATOR
Power amplifier
Piezo actuator
Fixed mirror
Contracts or releases to
tilt the angle of the
fixed mirror for optimal
interference

Data Manipulations
 Peak table generation
 Baseline correction
 Smoothing
 Normalization
 Deconvolution
 Derivative calculations
 JCAMP & ASCII file
conversions
 Film Thickness
 Purity check algorithm
 Peak ratio calculations
 Spectrum subtraction
 Data conversion
calculations:
 Kubelka-Munk
 Kramers-Kronig
 ATR Correction

Deciding Factors
 Physical nature of the sample
 Type of information of the sample
 Limitation of a technique

• What is the objective of the analysis
• Background information of the sample
• Solubility of the sample
• Hardness of the sample
• Degradation of polymers
• Reactivity
What information one should have?

Transmission techniques
1. Potassium Bromide (KBr) pellet method
1. Liquid film method
2. Film method
3. Nujol method (very thick suspension)

1. Attenuated total reflectance (ATR)
2. Diffuse reflectance (DRS)
3. Specular reflectance
4. Reflection Absorption Spectroscopy
5. Micro sampling techniques
Reflection techniques

KBR pellet method
 Popular method for measuring powdered samples
 Suitable for hard and brittle polymers
 Polymer in the form of small particles dispersed
in a disc of potassium bromide

Hydraulic Press and Evacuable Die
 KBr discs are prepared by grinding the polymer
sample (2mg) with KBr (100-200 mg) and
compressing the whole into a transparent disc
 A good spectrum will depend on whether the
sample is finely grind
 Compression to a cohesive disc is done by using
an evacuable die
 13mm in diameter

Hydraulic Press and Evacuable Die

Mini-Hand Press
 3mm diameter KBr pellet with briquetting frame
 Hand-driven press
 Small amount of sample
 Simple and quick

Hydraulic Press Mini-hand Press
13mm in diameter KBr
pellet
3mm in diameter KBr
pellet with briquetting
frame
With hydraulic press and
vacuum pump
Hand-driven press
Takes longer time Simple and quick
High through put Low through put but
enough for FTIR
To be able to keep in
desiccator
Make each measurement
Hydraulic Press vs. Mini-Hand Press

Caffeine powder
Mini-hand Press, KBr pellet method
4cm-1, 45scans

LIQUID FILM METHOD
1. Demountable cell
2. Sealed liquid cell
3. Fixed thickness cell

Demountable cell
 Qualitative analysis only
 Non-volatile liquid and paste
 Use spacer of 0.025 mm ~ 0.1 mm (25 µm to 100 µm )
thickness for low absorption liquid
 No spacer is needed for high absorption liquid

Sealed liquid cell
 Qualitative and quantitative analysis
 Volatile and Non-volatile liquid
 Use spacer of 0.025 mm ~ 0.5 mm (25 µm to 500
µm ) thickness for low absorption liquid

 THREE CRITERIAN FOR SOLVENT
1. Sample must be soluble or miscible with the solvent
2. Solvent must not have absorption bands in the region of
interest
3. Diluent must not react with the sample
Sealed liquid cell

Fixed Thickness Cell
 For quantitative measurement of liquid or volatile
samples
 3 types of cell window
NaCl, KBr, KRS-5
 Thickness
0.025mm ~ 5.0mm

Gas cells
 For quantitative measurement of gas samples
 5 cm and 10 cm pathlength.

Long Pathlength Gas cells
 Measurement of low concentration of gas samples
 1 meter and 2 meter is the pathlength
 MCT detector is used

Microsample measurement
1. Reflection type beam condenser
2. Single reflection ATR
3. IR Microscope

Reflection Beam Condenser
Can be used to measure
liquid of 2-3 µL

IR Spectra of polyester fibre with RBS 8000
Reflection Beam Condenser

ATR measurement
 The sample is held in contact with a prism made of highly refractive index
material which transmits infrared rays.
 The incident angle of IR beam is larger than the critical angle hence the IR
beam is totally reflected by the interface between sample and the prism.
Principle of ATR Measurement
Long wavelength penetrate deeper
Lower critical angle , more penetration
Penetration depth calculation

Characteristic of Prisms
Transparency,
hardest
Insoluble2.3840000~650Diamond
Yellow, hardInsoluble2.420000~650ZnSe
Silver, fragileInsoluble4.05000~700Ge
Orange, soft,
poisonous
Practically
insoluble
2.3720000~290KRS-5
CharacteristicsSolubility in
water
Refractive
index
Wavenumber
range (cm-1)
Material

Effect of sample contact
ATR spectra of polystyrene at different clamping power

Effect of Wavelength
1.27μm1.66μm2.78μm5.03μm25μm(400cm-1
)
0.51μm0.66μm1.11μm2.01μm10μm(1000cm-1
)
0.25μm0.33μm0.56μm0.66μm5.0μm(2000cm-1
)
0.13μm0.17μm0.28μm0.50μm2.5μm(4000cm-1
)
60º45º60º45°Wavelength
(Wavenumber)
Incident angleIncident angle
Ge (n=4.0)KRS-5, ZnSe &
Diamond (n=2.4)

Acrylonitrile-butadiene rubber containing carbon black as
a reinforcement
3000 1500 400
KRS-5
Ge
Incident angle = 45o
Effect of R.I of prism
Anomalous
dispersion of
refractive index

Vertical Variable Angle ATR
By varying the incident angle of IR rays, the sample
profile can be studied
ATR 8000

Why Diamond ATR?
Diamond ATR makes up for all of the limitations of ATR
while maintaining all of the advantages
 100X reduction in ATR element size allows intimate
contact with samples and greatly reduces samples by
requirement
 Mechanical strength of the diamond ATR element
allows for compression of the sample for intimate
contact
 Resistant to all corrosive and abrasive solvents and
samples
 Cleans up easily because of the low friction
coefficient of diamond

DIAMOND ATR
DIAMOND ATR
1reflection Diamond ATR
DuraSamplIR
1 reflection Diamond prism1 reflection Diamond prism
contact size : about 1.5mm in diameter
contact size : about 1.5mm in diameter
Sample pressure device

Diffuse Reflectance
DRS method is applicable to almost all samples that can
be pulverized.
Sample preparation is easy
Specular
reflectance
lightDiffuse
reflectance light
Incident light

Diffuse Reflectance
To minimize the specular reflectance
 Grind the sample and KBr to fine particle
 Dilute the sample to 1~5% of KBr powder
 Do not press the sample surface

Diffuse Reflectance Measurement
DRS 8000

Diffuse Reflectance Measurement
SiC sandpaper disk
Sample
 SiC sampler is effective in sampling sample from large
forming and plastic products.
 Holder can be directly attached to DRS
 Emery paper is used as blank

mirror
sample
MCT
CCD
TRANSMISSION REFLECTION

ATR Objective mirror
1. Cone type prism
2. 45 degree angle
3. 15x magnification
4. Slide on for visible measurement and ATR
Measurement

Microvise holder
Used to hold film samples

Diamond Compression Cell
Trans.

Spectra of single yarn before and after compression
20um
Before
After
Diamond Compression Cell

Microscopy modes:
 Reflectance mode (surface contaminants)
1. Reference material
2. Gold mirror
3. Aluminum coated mirror
4. Aluminum foil
 Transmission mode (film, fibers)
1. Diamond cell can be used to compress the sample
for transmittance measurement
2. Put small sample on a BaF2 window

Ftir theory and applications

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