Fourier Transform Infrared

Spectroscopy

Presentation by

Muhammad Sarfraz Akram M.Sc. 19-06

What is FTIR?

FTIR stand for Fourier-Transform Infrared Spectroscopy.

It may also called infrared or IR spectroscopy.

FTIR is a chemically-specific analysis technique. It can be used to identify

chemical compounds and substituent groups.

What is Fourier-Transform? In FTIR

The Fourier Transform is a mathematical technique which

decomposes a function into a continuous spectrum from its

frequency components.

What is Infrared?

Infrared light is a light that occurs between 0.78- 1000 μm (13000-10 cm-1), in the

electromagnetic spectrum, between the visible and microwave regions.

What is Spectroscopy?

Spectroscopy is the study of the interaction between radiation

(electromagnetic radiation, or light, as well as particle radiation) and matter.

Spectrometry is the measurement of these interactions and a machine which

performs such measurements is a spectrometer or spectrograph.

A plot of the interaction is referred to as a spectrogram, or, informally, a

spectrum.

Why Infrared Spectroscopy?

• Infrared spectroscopy has been a workhorse technique for materials analysis in the laboratory for over seventy years.

• An infrared spectrum represents a fingerprint of a sample with absorption peaks which correspond to the frequencies of vibrations between the bonds of the atoms making up the material.

• Because each different material is a unique combination of atoms, no two compounds produce the exact same infrared spectrum. Therefore, infrared spectroscopy can result in a qualitative analysis of every different kind of material.

• The size of the peaks in the spectrum is a direct indication of the amount of material present.

• With modern software algorithms, infrared is an excellent tool for quantitative analysis.

So, what information can FT-IR provide?

• It can identify unknown materials

• It can determine the quality or consistency of a sample

• It can determine the amount of components in a mixture

Working Principle of FTIR

Michelson or Fourier Transform Spectrometer

Light from the source is split into two beams by a half-silvered mirror, one is reflected off a fixed mirror and one off a moving mirror. The beams interfere, and by making measurements of the signal at many discrete positions of the moving mirror, the spectrum can be reconstructed.

Background

Chemistry Involved

Molecular Dynamics

Total energy of a molecule: Etotal = Etranslation + Eelectronic + Evibration + Erotation

Etranslation = kinetic energy, 3-D movement

Eelectronic = electronic energy, excitation of electrons: ground state excited state

Evibration = vibrational energy, bending and stretching of covalent bonds

Erotation = rotational energy

Molecular Vibration

All the motions of molecule can be described in terms of two types of molecular vibrations.

• Stretch Vibration A stretching vibration involves a continuous change in the interatomic distance

along the axis of the bond between two atoms. A stretch is a rhythmic movement along the line between the atoms so that the interatomic distance is either increasing or decreasing.

• Bend Vibration

A bending vibrations involves a continuous change in the angle between two bonds. These are also sometimes called scissoring, rocking, or wig wag motions.

Bond stretching

In covalent bonds, atoms aren't joined by rigid links - the two atoms are held together

because both nuclei are attracted to the same pair of electrons. The two nuclei can

vibrate backwards and forwards - towards and away from each other - around an

average position.

The energy involved in this vibration depends on the length of the bond and the mass of

the atoms at either end.

That means that each different bond will vibrate in a different way, involving different

amounts of energy.

Bonds are vibrating all the time, but if you shine exactly the right amount of energy on a

bond, you can kick it into a higher state of vibration.

The amount of energy it needs to do this will vary from bond to bond, and so each

different bond will absorb a different frequency (and hence energy) of infra-red radiation.

Bond bending

As well as stretching, bonds can also bend. The diagram shows the bending of the

bonds in a water molecule. The effect of this, of course, is that the bond angle between

the two hydrogen-oxygen bonds fluctuates slightly around its average value.

Scissoring Rocking Wagging Twisting

Again, bonds will be vibrating like this all the time

And again if you shine exactly the right amount of energy on the bond, you

can kick it into a higher state of vibration.

Since the energies involved with the bending will be different for each kind of

bond, each different bond will absorb a different frequency of infra-red

radiation in order to make this jump from one state to a higher one.

Dipole Change During Vibrations and Rotations

• Infrared radiation is not energetic enough to bring about the electronic

transitions that encounter in U.V, Visible and X-ray radiation.

• Absorption of infrared radiation is confined to molecular species that have

small energy differences between vibrational and rotational states.

• In order to absorb infrared radiation, a molecule must undergo a change in

dipole moment as a consequence of its vibrational or rotational motion.

• Under these condition alternating electrical field of the radiation interact with

the molecule and cause changes in the amplitude of one of its motions.

Example of HCl

• Charge distribution around HCl is not symmetric.

• Chlorine has a higher electron density then the hydrogen.

• Thus, HCl has a significant dipole moment and is said to be polar.

• So, HCl molecule vibrates, a regular fluctuation in dipole moment occurs, and a field is

established that can interact with the electrical field associated with radiation.

• If the frequency of the radiation exactly matches a natural vibrational frequency of the

molecule, a net transfer of energy takes place that results in a change in the amplitude of

the molecular vibration; results absorption of the radiation.

Dipole Change During Vibrations and Rotations

Example of homonuclear species

• The vibration or rotation of homonuclear species such as O2, N2 and Cl2 cannot

absorb in the infrared.

• With the exception of few compounds of this type, all other molecular species

absorb infrared radiation.

Dipole Change During Vibrations And Rotations

Mechanical Model of Stretching Vibration in a

Diatomic Molecule • The characteristics of an atomic stretching vibration can be approximated by a mechanical

model consisting of two masses connected by a spring.

• A disturbance of one of these masses along the axis of the spring results in a vibration called a simple harmonic motion.

• First consider, the vibration of a single mass attached to a spring is given by Hooke’s Law. That is,

F = -ky ---------------------- ( 1 )

Where

k = force constant depend upon stiffness of the spring.

y = displacement of the mass from its equilibrium position.

F = restoring force

Potential Energy of a Harmonic Oscillator • Potential energy E of the mass and spring is zero when the mass is in its rest or equilibrium position.

• As the spring is compressed or stretched, the potential energy of the system go on increasing.

• If the mass is moved from some position y to y+dy, the work and hence the change in potential energy dE is equal to the force F times the distance dy.

dE = -Fdy --------------- ( 2 )

combining equation (1) and (2)

dE = kydy

Integrating between the equilibrium position (y=0) and y gives

o∫E

dE = k o∫y

ydy

E = 1 ky2 ---------------- ( 3 )

2

Vibrational Frequency

According to Newton’s II law

F = ma

Acceleration is the second derivative of distance with respect to time. Thus,

a = d2y dt2

Putting the values of F and a in (1)

m d2y = -ky ------------- ( 4 ) dt2

At instantaneous displacement of the mass at time t is

y = A cos 2π vmt ------------- ( 5 )

where vm is the natural vibrational frequency and A is the maximum amplitude of the motion.

The second derivative of equation ( 5 ) is

d2y = - 4 π2 v2m A cos 2π vmt ---------- ( 6 )

dt2

Substituting eq. (5) and (6) into eq. (4)

A cos 2π vmt = - 4 π2 vm2

A cos 2π vmt - k/m

The natural frequency of the oscillation is then

vm = 1 √k/m ----------------- ( 7) 2π

For a system consisting of two masses m1 and m2 connected by spring.

μ = m1m2 ----------------- (8) m1 + m2

Thus, the vibrational frequency for such a system is given by

vm = 1 √k/μ = 1 √k(m1 + m2) -------- (9) 2π 2π m1m2

Vibrational Frequency

Molecular Vibrations

• The behavior of a molecular vibration is analogous to the mechanical model just

described.

• So the frequency of the molecular vibration can calculated using equation

vm = 1 √k/μ = 1 √k(m1 + m2) 2π 2π m1m2

where

m1 and m2 = masses of two atoms

k = force constant for the chemical bond, which is measure of its stiffness

Quantum Treatment of Vibrations

For simple harmonic oscillator to develop the wave equation of quantum

mechanics, the equation for potential energy have the form

E = (v + ½) h √k/μ -------- ( 10 ) 2π

where

h = planck constant

v = vibrational quantum number

Substituting eq (9) into eq (10)

E = (v + ½) hvm -------- ( 11 )

Assume that transitions in vibrational energy levels can be brought about by absorption of radiation, provided the energy of the radiation exactly matches the difference in energy levels ΔE between the vibrational quantum states and provided also that the vibration causes a fluctuation in dipole. This difference is identical between any pair of adjacent levels.

ΔE = hvm = h √k/μ -------- ( 12) 2π

At room temp, the majority of molecules are in the ground state (v=0), thus from eq (11)

Eo = ½ hvm

Promotion to the first excited state (v=1) with energy

E1 = 3/2 hvm

Requires radiation of energy

(3/2 hvm – 1/2 hvm) = hvm

Quantum Treatment of Vibrations

The frequency of radiation v that will bring about this change is identical to the classical vibrational frequency of the bond vm. That is,

Eradiation = hv = ΔE = hvm = h √k/μ 2π or v = vm = 1 √k/μ ------------- (13) 2π

The vibrational frequency v is expressed in term of wavenumber as;

v = 1 √k/μ = 5.3 x 10-12 √k/μ 2πc

Where v is the wavenumber of an absorption peak in cm-1, k is the force constant for the bond in newtons per meter (N/m), c is the velocity of light in cm/s, and reduced mass μ.

Quantum Treatment of Vibrations

v = 1 √k/μ = 5.3 x 10-12 √k/μ ---------- (14) 2πc

Where

v is the wavenumber of an absorption peak in cm-1

k is the force constant for the bond in newtons per meter (N/m)

c is the velocity of light in cm/s, and reduced mass μ

• Generally k has been found to lie in the range between 3 x 102 and 8 x 102 N/m for most single bond, with 5 x 102 as a reasonable average value

• For double bond k = 1x103

• For triple bond k = 1.5x103

• With these avg exp. values, we can find wavenumber of the absorption peak due to the transition from the ground state to the first excited state, for a variety of bond type.

Quantum Treatment of Vibrations

Case Study

Lets calculate the approximate wavenumber and wavelength of the

fundamental absorption peak due to the stretching vibration of a

carbonyl group C=O. • The mass of carbon atom in kg is given by

m1 = 12 x 10-3 kg/mol x 1 atom 6.0 x 1023 atom/mol

= 2.0 x 10-26 kg

• Similarly for oxygen

m2 = 16 x 10-3 kg/mol x 1 atom 6.0 x 1023 atom/mol

= 2.7 x 10-26 kg

• The reduced mass μ

μ = m1m2 = 2.0 x 10-26 x 2.7 x 10-26 m1 + m2 2.0 x 10-26 + 2.7 x 10-26

μ = 1.1 x 10-26 kg

• The force constant for typical double bond 1 x 103 N/m

• substituting these values in eq (14)

v = 5.3 x 10-12 √ k/μ = 1.6 x 103 cm-1

The carbonyl stretching band is found experimentally to be in the

region of 1600 to 1800 cm-1 ( 6.3 – 5.6 μm)

Carboxylic acid

1-propan-ol

CH3-CH2-CH2-OH

Infrared Spectral Regions

Sample Type Analysis

Type

Measurment

Type

Wavenumber

(v) Range, cm-1

Wavelength (λ)

Range, μm Region

Solid or liquid material Quantitative Diffuse reflectance

Absorption 12,800 – 4000 0.78 – 2.5 Near

Pure solid, liquid or gaseous

compounds

Complex gaseous, liquid or solid

mixtures

Pure solid or liquid compounds

Atmospheric samples

Qualitative

Quantitative

Qualitative

Quantitative

Absorption

Reflectance

Emission

4000 – 200 2.5 – 50 Middle

Pure inorganic or metal organic

species Qualitative Absorption 200 – 10 50 – 1000 Far

4000 – 670 2.5 – 15 Mostly

Used

FTIR Instrumentation

The Sample Analysis Process

The normal instrumental process is as follows:

1. The Source: Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which controls the amount of energy presented to the sample (and, ultimately, to the detector).

2. The Interferometer: The beam enters the interferometer where the “spectral encoding” takes place. The resulting interferogram signal then exits the interferometer.

3. The Sample: The beam enters the sample compartment where it is transmitted through the sample. This is where specific frequencies of energy, which are uniquely characteristic of the sample, are absorbed.

4. The Detector: The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal.

5. The Computer: The measured signal is digitized and sent to the computer where the Fourier transformation takes place. The final infrared spectrum is then presented to the user for interpretation and any further manipulation.

• Because there needs to be a relative scale for the absorption intensity, a background spectrum must also be measured.

• This is normally a measurement with no sample in the beam. It is then compared to the measurement with the sample in the beam to determine the “percent transmittance.”

• This technique results in a spectrum which has all of the instrumental characteristics removed.

• Thus, all spectral features which are present are strictly due to the sample.

• A single background measurement can be used for many sample measurements because this spectrum is characteristic of the instrument itself.

How FTIR Works

• Infrared beam strikes the beamsplitter which takes the incoming infrared beam and divides it into two optical beams.

• One beam reflects off of a flat mirror which is fixed in

place. The other beam reflects off of a flat mirror which

is on a mechanism which allows this mirror to move a

very short distance (typically a few millimeters) away

from the beamsplitter.

• The two beams reflect off of their respective mirrors

and are recombined when they meet back at the

beamsplitter.

• Because the path that one beam travels is a fixed

length and the other is constantly changing as its mirror

moves, the signal which exits the interferometer is the

result of these two beams “interfering” with each other.

• The resulting signal is called an interferogram which

has the unique property that every data point (a function

of the moving mirror position) which makes up the

signal has information about every infrared frequency

which comes from the source.

How FTIR Works • This means that as the interferogram is measured, all frequencies are

being measured simultaneously. Thus, the use of the interferometer results in extremely fast measurements.

• Because the analyst requires a frequency spectrum in order to make an identification, the measured interferogram signal can not be interpreted directly. A means of “decoding” the individual frequencies is required.

• This can be accomplished via a well-known mathematical technique called the Fourier transformation. This transformation is performed by the computer which then presents the user with the desired spectral information for analysis.

• The FTIR is a single beam spectrometer. That is, it does not automatically measure a sample. Background spectrum is measured before sample. As the sample and background are collected separately, a laser (usually HeNe at 632 nm) is used to ensure the background and sample frequencies are identical.

FTIR Instrument

Sample

Compartment

IR Source Detector

FTIR

Sample

Port

salt plates positioned

on sample holder

calcium chloride

(CaCl2) desiccant

Infrared

radiation

to detector

organic sample

selectively absorbs

infrared radiation

IR

Perkin-Elmer

1600 FTIR

To run an IR spectrum of a liquid sample, a drop or two of

the liquid sample is applied to a salt plate.

A second salt plate is placed on top of the first one such that the

liquid forms a thin film “sandwiched” between the two plates

The two plates are then secured in a sample holder that is compatible

with the particular instrument being used

The cell holder is then placed in the beam of the instrument

Light Path (shown by red line)

The light beam traverses the sample compartment, as illustrated by the red line

Slide 17

Sample of a

printout of

an IR

spectrum.

Source of Infrared Radiation

Infrared sources consist of an inert solid that is heated electrically to a

temperature between 1500 and 2200 K.

Continuum radiation approximating that of a blackbody results.

The maximum radiation (5000-5900 cm-1) occur at these temperature.

The Globar Source

A Globar is a silicon carbide rod.

It is usually about 50mm in length and 5mm in diameter.

It is heated 1300 to 1500K.

Source of Infrared Radiation

The Nernst Glower

• The Nernst Glower is composed of rare earth oxider.

• It have a diameter of 1-2 mm and length of 20mm.

• Platinum leads are sealed to the ends of the cylinder to permit electrical connecion .

• As current passes through the device, temperature between 1200K and 2200K results.

The Mercury Arc

• This device consists of a quartz-jacketed tube containing mercury vapor at a pressure greater than one atmposphere.

• Passage of electricity through the vapor forms an internal source that provides continuum radiation in the far-infrared radiation.

The Tungsten Filament Lamp

• An ordinary tungsten filament lamp is a convenient source for the near-infrared region of 4000 to 12,800 cm-1.

Source of Infrared Radiation

Advantages of FTIR

Some of the major advantages of FT-IR over the dispersive technique include:

• Speed: Because all of the frequencies are measured simultaneously, most measurements by FTIR are made in a matter of seconds rather than several minutes. This is referred to as the Felgett Advantage.

• Sensitivity: Sensitivity is dramatically improved with FT-IR for many reasons. The detectors employed are much more sensitive, the optical throughput is much higher (referred to as the Jacquinot Advantage) which results in much lower noise levels, and the fast scans enable the coaddition of several scans in order to reduce the random measurement noise to any desired level (referred to as signal averaging).

• Mechanical Simplicity: The moving mirror in the interferometer is the only continuously moving part in the instrument. Thus, there is very little possibility of mechanical breakdown.

• Internally Calibrated: These instruments employ a HeNe laser as an internal wavelength calibration standard (referred to as the Connes Advantage). These instruments are self-calibrating and never need to be calibrated by the user.

Thank You

Comments or Questions?