1. Infrared Spectroscopy
Syllabus:
Infrared absorption spectroscopy:
instrumentation, FTIR, advantages of
FTIR, applications of IR, qualitative and
quantitative analysis, advantages
and limitations of quantitative IR methods.
2. Infrared Spectroscopy
• IR Spectroscopy is the interaction between matter and IR
radiation.
• All molecular species except “homonuclear diatomics” (e.g.,
O2, H2, N2, etc.) are detectable
• IR light absorption due to changes in rotational and
vibrational energy in molecule
• Absorbed energy causes molecular motions which create a
net change in the dipole moment.
3. Infrared radiation
Region cm-1 μm
Near IR 12000 – 4000 0.8 – 2.5
Mid IR 4000 – 400 2.5 – 25
Far IR 400 – 10 25 – 1000
Vibrations of Molecules
Symmetrical
stretching
Antisymmetrical
stretching
Scissoring
Rocking Wagging Twisting
4. Theory of Infrared Absorption Spectroscopy
• IR photons have low energy. The only transitions that have
comparable energy differences are molecular vibrations and rotations.
5. Theory of Infrared Absorption Spectroscopy
• In order for IR absorbance to occur two conditions must be met:
1. There must be a change in the dipole moment of the molecule as a
result of a molecular vibration (or rotation). The change (or
oscillation) in the dipole moment allows interaction with the
alternating electrical component of the IR radiation wave.
Symmetric molecules (or bonds) do not absorb IR radiation since
there is no dipole moment.
2. If the frequency of the radiation matches the natural frequency of
the vibration (or rotation), the IR photon is absorbed and the
amplitude of the vibration increases.
6. DE = hn
• There are three types of molecular transitions that occur in IR
a) Rotational transitions
• When an asymmetric molecule rotates about its center of mass, the
dipole moment seems to fluctuate.
• DE for these transitions correspond to n < 100 cm-1
• Quite low energy, show up as sharp lines that subdivide vibrational
peaks in gas phase spectra.
b) Vibrational-rotational transitions
• complex transitions that arise from changes in the molecular dipole
moment due to the combination of a bond vibration and molecular
rotation.
c) Vibrational transitions
• The most important transitions observed in qualitative mid-IR
spectroscopy.
• n = 13,000 – 675 cm-1 (0.78 – 15 mM)
7. IR Spectroscopy
I. Introduction
C. The IR Spectroscopic Process
1. The quantum mechanical energy levels observed in IR spectroscopy are those of
molecular vibration
2. We perceive this vibration as heat
3. When we say a covalent bond between two atoms is of a certain length, we are
citing an average because the bond behaves as if it were a vibrating spring
connecting the two atoms
4. For a simple diatomic molecule, this model is easy to visualize:
8. IR Spectroscopy
I. Introduction
C. The IR Spectroscopic Process
5. There are two types of bond vibration:
• Stretch – Vibration or oscillation along the line of the bond
• Bend – Vibration or oscillation not along the line of the bond
H
H
C
H
H
C
scissor
asymmetric
H
H
CC
H
H
CC
H
H
CC
H
H
CC
symmetric
rock twist wag
in plane out of plane
9. C. The IR Spectroscopic Process
6.As a covalent bond oscillates – due to the oscillation of the dipole
of the molecule – a varying electromagnetic field is produced
7.The greater the dipole moment change through the vibration, the
more intense the EM field that is generated
Infrared Spectroscopy
10. C. The IR Spectroscopic Process
8. When a wave of infrared light encounters this oscillating EM field
generated by the oscillating dipole of the same frequency, the two
waves couple, and IR light is absorbed
9. The coupled wave now vibrates with twice the amplitude
Infrared Spectroscopy
IR beam from spectrometer
EM oscillating wave
from bond vibration
“coupled” wave
11. D. The IR Spectrum
1. Each stretching and bending vibration occurs with a characteristic frequency as
the atoms and charges involved are different for different bonds
The y-axis on an IR
spectrum is in units of %
transmittance
In regions where the EM
field of an osc. bond
interacts with IR light of
the same n –
transmittance is low (light
is absorbed)
In regions where
no osc. bond is
interacting with IR
light, transmittance
nears 100%
Infrared Spectroscopy
12. IR Spectroscopy
D. The IR Spectrum
2. The x-axis of the IR spectrum is in units of wavenumbers, n, which is the number
of waves per centimeter in units of cm-1 (Remember E = hn or E = hc/l)
13. IR Spectroscopy
D. The IR Spectrum
3. This unit is used rather than wavelength (microns) because wavenumbers
are directly proportional to the energy of transition being observed –
chemists like this, physicists hate it
High frequencies and high wavenumbers equate higher energy
is quicker to understand than
Short wavelengths equate higher energy
4. This unit is used rather than frequency as the numbers are more “real” than
the exponential units of frequency
5. IR spectra are observed for the mid-infrared: 600-4000 cm-1
6. The peaks are Gaussian distributions of the average energy of a transition
14. IR Spectroscopy
D. The IR Spectrum
7. In general:
Lighter atoms will allow the oscillation to be faster – higher energy
This is especially true of bonds to hydrogen – C-H, N-H and O-H
Stronger bonds will have higher energy oscillations
Triple bonds > double bonds > single bonds in energy
Energy/n of oscillation
17. Mechanical Model of Stretching Vibrations
1. Simple harmonic oscillator.
• Hooke’s Law (restoring force of a spring is proportional to the
displacement)
F = -ky
Where: F = Force
k = Force Constant
(stiffness of spring)
y = Displacement
• Natural oscillation frequency of a mechanical oscillator depends on:
a) mass of the object
b) force constant of the spring (bond)
• The oscillation frequency is independent of the amount of energy
imparted to the spring.
18. • Frequency of absorption of radiation can be predicted with a modified
Hooke’s Law.
Where: n = wavenumber of the abs. peak (cm-1)
c = speed of light (3 x 1010 cm/s)
k = force constant
m = reduced mass of the atoms
2
1
2
1
m
n
k
c
yx
yx
MM
MM
m
Where: Mx = mass of atom x in kg
My = mass of atom y in kg
• Force constants are expressed in N/m (N = kg•m/s2)
- Range from 3 x 102 to 8 x 102 N/m for single bonds
- 500 N/m is a good average force constant for single bonds when
predicting k.
- k = n(500 N/m) for multiple bonds where n is the bond order
19. IR Sources and Detectors
Sources - inert solids that heat electrically to 1500 – 2200 K.
• Emit blackbody radiation produced by atomic and molecular oscillations
excited in the solid by thermal energy.
• The inert solid “glows” when heated.
• Common sources:
1. Nernst glower - constructed of a rod of a rare earth oxide (lanthanide)
with platinum leads.
2. Globar - Silicon carbide rod with water cooled contacts to prevent
arcing.
3. Incandescent wire - tightly wound wire heated electrically. Longer life
but lower intensity.
20. Detectors – measure minute changes in temperature.
1. Thermal transducer
• Constructed of a bimetal junction, which has a temperature dependant
potential (V). (similar to a thermocouple)
• Have a slow response time, so they are not well suited to FT-IR.
2. Pyroelectric transducer
• Constructed of crystalline wafers of triglycine sulfate (TGS) that have a
strong temperature dependent polarization.
• Have a fast response time and are well suited for FT-IR.
3. Photoconducting transducer
• Constructed of a semiconducting material (lead sulfide,
mercury/cadmium telluride, or indium antimonide) deposited on a glass
surface and sealed in an evacuated envelope to protect the
semiconducting material from the environment.
• Absorption of radiation promotes nonconducting valence electrons to a
conducting state, thus decreasing the resistance (W) of the semiconductor.
• Fast response time, but require cooling by liquid N2.
21. FTIR Background
• FTIR is a modern spectroscopic method which
operates in the IR region (molecular vibrations and
rotations)
• The “FT” in FTIR gives the wavelength selection
method (Fourier Transformation)
• Prior to FTIR, grating and prism spectrometers were
used
22. What is FTIR
• Fourier-transform infrared spectroscopy is a vibrational
spectroscopic technique, meaning it takes advantage of
asymmetric molecular stretching, vibration, and rotation of
chemical bonds as they are exposed to designated
wavelengths of light.
• Fourier transform is to transform the signal from the time
domain to its representation in the frequency domain
23. FTIR seminar
Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light
source
(ceramic)
Detector
(DTGS)
Beam splitter
FT Optical System Diagram
24. The Interferometer
Simplest interferometer design
Beamsplitter for dividing the incoming IR beam into
two parts
Two plane mirrors for reflecting the two beams back
to the beamsplitter where they interfere either
constructively or destructively depending on the
position of the moving mirror
Position of moving mirror is expressed as Optical Path
Difference (OPD)
An Interferometer
Albert Abraham Michelson
(1852-1931)
25. OPD = Distance travelled by red beam
minus distance travelled by yellow beam
OPD = 0 at
the white line
Interference signal Interference signal
EM waves with same
amplitude and frequency,
out of phase
EM waves with same
amplitude and frequency, in
phase (OPD = 0)
A A
A A
• When EM-waves interact, interference is
observed
• Depending on the relative phase of the
waves, interference is either destructive or
constructive
destructive interference constructive interference
Michelson interferometer producing Interference
26. • Collect data in the time domain and convert to the frequency domain by
Fourier Transform.
Multiplexing (FT) Spectrometers
• Detectors are not fast enough to respond to power variations at high frequency
(1012 to 1015 Hz) so the signal is modulated by a Michelson interferometer to a
lower frequency that is directly proportional to the high frequency.
27. 1. Michelson Interferometer
B. Multiplexing (FT) Spectrometers
• The source beam is split into two
beams.
• One beam goes to a stationary
mirror and the other goes to a
moveable mirror.
• Movement of the mirror at a
constant rate and recombination of
the two beams results in a signal
that is modulated by constructive
and destructive interference
(Interferogram).
28. Multiplexing (FT) Spectrometers
• The frequency of the
radiation (n) is directly
related to the frequency
of the interferogram (f).
n
n
c
f m2
n = frequency of radiation
f = frequency of inteferogram
nm = velocity of the mirror
c = speed of light (3.00 x 1010
cm/s)
• FT-IR spectrometers use a polychromatic source and collect the entire
spectrum simultaneously and decode the spectrum by Fourier Transform.
29. Fourier Transform
Time domain: I vs. δ Frequency domain: I vs. v
FT
δ
I
v
I
Fourier transform defines a relationship between a signal in time domain and its
representation in frequency domain.
Being a transform, no information is created or lost in the process, so the original
signal can be recovered from the Fourier transform and vice versa.
Fourier transformation is the mathematical relation between the interferogram
and the spectrum (in general, between time domain signal and frequency signal)
30. Mathematics
• Optical path difference is
• Intensity of the detector has maxima at
and minima at
)(I
)(I 2,1,0, nnl
l )2/1( n
31. Mathematics (contd.)
• The resulting interferogram is described as an infinitely long cosine
wave
• where =intensity as F(v)
• For non-monochromatic source treat each frequency as if it resulted
in a separate cosine train.
)2cos()(
l
nB )(nB
32. Mathematics (contd.)
• An infrared source is typically approximated as a black body radiator
and the summation can be replaced by an integral.
• At = 0, signal always has a strong maxima called Centreburst
• Outwards from the centreburst the cosine waves cancel and reinforce
and the amplitude of the interferogram dies off.
nnn dBI )2cos()()(
0
33. Mathematics (contd.)
• Spectroscopists are interested in the spectrum in the
frequency domain i.e intensity versus wavenumber
• If the mathematical form of the interferogram is
known,spectrum in the frequency domain can be
calculated by Fourier Transformation
nn dIB )2cos()()(
For mathematical validity this integration must be carried out over all
possible values of delta i.e + to –
34. Measurement Techniques
Though some emission spectroscopy is done most of the
work is in the absorption spectra area using FTIR.
• 1. If there is no sample present the spectrum is that of
a black body radiator modified by any transmission
characteristics of the interferometer components.
• 2. If the interferometer chamber is not evacuated or
purged with dry gas some absorption from the
atmospheric Co2 and H20 is observed. This is called the
background spectrum.
• 3. When the sample is introduced the spectrum now is
a superposition of the absorption bands of the sample
on an uneven background.
• 4. To obtain %T with wavenumber we ratio the single
beam sample spectrum with the background spectrum.
36. FT IR Detectors:
The two most popular detectors for a FTIR spectrometer are:
• deuterated triglycine sulfate (DTGS):
Is a pyroelectric detector that delivers rapid responses because it
measures the changes in temperature rather than the value of
temperature. It operates at room temperature,
• mercury cadmium telluride (MCT).
It must be maintained at liquid nitrogen temperature (77 °K) to
be effective.
In general, the MCT detector is faster and more sensitive than
DTGS detector.
Thermal Detectors are not used in FT IR:
• The response times of thermal detectors (for example,
thermocouple and thermistor) used in dispersive IR
instruments are too slow for the rapid scan times (1 sec or less)
of the interferometer.
37.
38. 2. FT-IR instrument
Multiplexing (FT) Spectrometers
• Mirror length of travel ranges
from 1 to 20 cm.
• Use multiple scans and signal
averaging to improve S/N.
• Scan rates from 0.1 to 10 cm/s
• Detectors are usually pyroelectric
or photoconducting.
• Cost $10,000 - $20,000
• Have virtually replaced
dispersive instruments.
39. Performance Characteristics
• Range: 7800 to 350 cm-1 (less expensive)
25,000 to 10 cm-1 (Near to far IR, expensive)
• Resolution: 8 cm-1 to 0.01 cm-1
• Qualitative: Very good, functional groups are identifiable
• Quantitative: Dispersive – poor
FTIR - fair
40. FT-IR Advantages
• Fellgett's (multiplex) Advantage (High S/N ratio comparing with dispersive
instruments)
• All frequencies are measured at the same time
• Hard to do samples having low transmission and weak spectra can be done with
FTIR.
• High resolution, reproducibility and highly accurate frequency determination
• Technique allows high speed sampling with the aid of laser light interference
fringes
41. Better sensitivity.
In the interferometer, the radiation power transmitted on to the detector is
very high which results in high sensitivity.
No Stray light
Fourier Transform allows only interference signals to contribute to
spectrum.
Background light effects greatly lowers.
Precision Advantage
Internal laser control the scanner – built in calibration
42. Disadvantages of FTIR compared to Normal IR:
1) single-beam, requires collecting blank
2) can’t use thermal detectors – too slow
3) CO2 and H2O sensitive
4)Too sensitive that it would detect the smallest contaminant
43. APPLICATIONS
• Identification of inorganic compounds and organic
compounds
• Measurement of toxic gas in fuels
• Identification of components of an unknown mixture
• Analysis of solids, liquids, and gases
• In remote sensing
• Can also be used on satellites to probe the space
44. 02 January 2006 Introduction to FTIR
Advantages of FTIR spectroscopy
• Speed (Felgett advantage): All the frequencies are recorded
simultaneously; a complete spectrum is measured in less than a second.
• Sensitivity (Jacquinot or Throughput advantage): In the interferometer,
the radiation power transmitted on to the detector is very high which
results in high sensitivity.
• Internally Calibrated (Connes advantage): FTIR spectrometers employ a
HeNe laser as an internal wavelength calibration standard, no need to be
calibrated by the user.
• Multicomponent capability: Since the whole infrared spectrum is
measured continuously, all infrared active components can be identified
and their concentrations determined.
45. 02 January 2006 Introduction to FTIR
REFERENCES
•Introduction to Spectroscopy ,
Donald L. Pavia
• Instrumental Analysis; Skoog, Holler and Crouch
•Stuart; IR Spectroscopy-Fundamentals and Applications.
• Instrumental methods-Willard, Merrit and Dean
•Infrared Spectroscopy in Conservation Science,
Michele R Derrick,Dusan Stulik,James M. Landry
•http://resources.yesicanscience.ca/trek/scisat/final/grade9/sp
ectrometer2.html
•http://www.health.clinuvel.com/en/uv-light-a-skin
•http://mmrc.caltech.edu/FTIR/FTIRintro.pdf
