Fourier Transform Infrared (FT-IR) Spectroscopy

Theory and Applications

THE ELECTROMAGNETIC SPECTRUM

INFRAREDGAMMA RAYS X RAYS UV VISIBLE

Prof. V. Krishnakumar

Professor and Head

Department of Physics

Periyar University

Salem – 636 011, India

Introduction to FTInfrared Spectroscopy

• What is infrared spectroscopy?

• Theory of FT-IR

• FT-IR Advantages?

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.

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.

Infrared region

 

LIMIT OF RED LIGHT: 800 nm, 0.8 m, 12500 cm-1

NEAR INFRARED: 0.8 -2.5 m, 12500 - 4000 cm-1

MID INFRARED: 2.5 - 25 m, 4000 - 400 cm-1

FAR INFRARED: 25 - 1000 m, 400 - 10 cm-1

Divisions arise because of different optical materials and instrumentation.

Principle of Infrared spectroscopy

E-vector in EMR has frequency

Energy transferred to molecule

by resonance

Oscillating electric field begins in molecule

 Vibration must have change in DM

 Vibration frequency

Vibrations in molecules

 

Energy in range 20-4000 cm-1

vibrational levels quantized,

labelled by v.

molecule vibrational energy

E = h(v+1/2)

Vibrating Diatomic molecule

When two atoms combine to form a stable covalent molecule – internal electronic arrangement

Repulsion – Positively charged two nucleus and between two electron clouds

Attraction – Between Positively charged nucleus and electron clouds

Spring – compression and extensionSpring obeys Hooke’s law f = - k(r-req)F – restoring forceK – force constantR – internuclear distance Req- equilibrium distance

Simple harmonic oscillator

0

3

1

2

5

4

21

2 eqE K r r

1

2osc

k

1

2osc

k

Energy

Oscillation

Wavenumbers

1

2 oscE h

1

2 osc

Eh

hc

Permitted energy levels ofthe harmonic oscillator

- Vibrational quantum number

If = 0,1

2 osc

Eh

hc

Zero-point energy

Selection rule for the harmonic oscillator undergoing vibrational changes = ± 1

11

1 11

2 2osc osc osccm

Applying the selection rule

For emission

11 osccm

The vibrating molecule will absorb energy only from radiation which it can coherently interact and this must be radiation of its own oscillation frequency

• Bond – not a perfect Elastic nature

• Does not obey exactly simple harmonic motion

• Dissociates

• Not a ideal parabola

2

1 expeq eqE D a r r

A – constant; Deq – Dissociation energy

Morse curve

Anharmonic Oscillator

Anharmonic Oscillator

1 11

2 2e ex

11

2osc e ex

Allowed vibrational energy levels

- oscillation frequency; xe – anharmonicity constant

Anharmonic oscillator frequency

10

11

2e ex cm

Ground state energy

Selection rule = ±1, ±2, ±3…

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

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.

h

Excited states

Ground (vibrational)

states

h(1 - 0 )

h(1 - 0)

h(2 - 1) (overtone)

Infrared Absorption and Emission

1

2

0

3

Number of vibrations

 

Vibration: centre of gravity unchanged

N nuclei, each move along x,y,z

Nonlinear molecule: 3N-6 modes

Linear molecule: 3N-5 modes

Infrared Spectroscopy

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

• All molecules can be identified on the basis of their characteristic absorption spectrum (except diatomic elements such as O2 and noble gases)

• Each molecule absorbs infrared radiation at its characteristic frequencies

• IR absorption spectrum is a fingerprint unique to each molecule

• Beer’s law: Absorption strength i.e absorbance is directly proportional to concentration

IR spectrum of HCl

Wavenumbers

Abs

orba

nce

All gases except O2, N2, H2, Cl2, F2, H2S, and noble gases can be measured

HCl molecule stretching vibration at 2880 cm-1

Interferogram is made by an interferometer.

Interferogramis transformedinto a spectrum using a FT.

BKG

SB

3000 2000 1000

[cm-1]

Sample

SB

Sample

3000 2000 1000

[cm-1]

Sample/BKG

IR spectrum

%T

3000 2000 1000 [cm-1]

The Principles of FTIR Method

FTIR seminar

Interferometer

He-Ne gas laser

Fixed mirror

Movable mirror

Sample chamber

Light source

(ceramic)

Detector

(DLATGS)

Beam splitter

FT Optical System Diagram

Fourier transformation

Fourier transformationpair

• The interferogram signal is recorded as a function of optical path difference• The interferogram is comparable to a time domain signal (eg. a recorded sound) and the spectrum

represents the same information in frequency domain (eg. the frequency of the same sound)• Fourier transformation is the mathematical relation between the interferogram and the spectrum

(in general, between time domain signal and frequency signal)• A pure cosine wave in the interferogram transforms to a perfectly sharp narrow spike in the

spectrum

OPD / cm

Inte

nsi

ty

Inte

nsi

ty

Wave number / cm-1

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.

Infrared Spectroscopy

For isopropyl alcohol, CH(CH3)2OH, the infrared absorption bands identify the various functional groups of the molecule.

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.

1.Better sensitivity and brightness- Allows simultaneous measurement over the entire wavenumber range- Requires no slit device, making good use of the available beam2.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-13. Resolution- Provides spectra of high resolution4. Stray light- Fourier Transform allows only interference signals to contribute to spectrum. Background light effects greatly lowers.- Allows selective handling of signals limiting intreference5. Wavenumber range flexibility- Simple to alter the instrument wavenumber rangeCO2 and H2O sensitive

FT-IR Advantages and Disadvantages