Infrared Spectroscopy and its applications

Keshav Narayan PaiII MSc

DOS in BotanyManasagangotri

Mysore

• Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light .

• Infrared Spectroscopy is the analysis of infrared light interacting with a molecule.

Infrared Rays

Infrared (IR) is invisible electromagnetic radiation with longer wavelength  than those of visible light , extending from the nominal red edge of the visible spectrum at 800 nm to 1 mm(0.8μm to 1000μm).

Infrared radiation was discovered in 1800 by astronomer Sir William Herschel.

They are divided into 3 regions Near IR Region Middle IR Region Far IR Region

Most of the analytical applications are

confined to the middle IR region because absorption of organic molecules are high in this region.

• When infrared 'light' or radiation hits a molecule, the bonds in the molecule absorb the energy of the infrared and respond by vibrating.

IR IR radiationradiation

vanllivanllinn Molecular Molecular

vibrationsvibrations

Molecules are made up of atoms linked by chemical bonds. The movement of atoms and the chemical bonds look like spring and balls (vibration).

When internal vibrational energy of molecule matches with energy of externally applied IR, quantized.

Symmetry streching Asymmetry streching

In plane bending

Scissoring Rocking

Out plane bending

Twisting

• Molecules absorb IR. • A molecule can only absorb IR

radiation when its absorption cause a change in its electric dipole moment.

• Molecule excited from lower to the higher vibrational level.

i.e. Increases the amplitude of vibration .

Molecular vibrationsThere are 2 types of vibrations:1.Stretching vibrations2.Bending vibrations

1.Stretching vibrations: Vibration or oscillation along the line of

bond. Change in bond length. Occurs at higher energy: 4000-1250 cm-1

There are 2 Types of Stretching vibrations

a)Symmetrical stretching

a)Asymmetrical stretching

a) Symmetrical stretching:

2 bonds increase or decrease in length simultaneously.

H

H

C

b) Asymmetrical stretching

• One bond length is increased and other is decreased.

H

H

C

2. Bending vibrations• Vibration or oscillation not along the

line of bond.• These are also called as deformations .• In this, bond angle is altered.• Occurs at low energy: 1400-666 cm-1

• 2 types:a)In plane bending: scissoring, rockingb)Out plane bending: wagging, twisting

a) In plane bendingi. Scissoring:• This is an in plane bending• 2 atoms approach each other• Bond angles are decreased.

H

H

CC

ii. Rocking:• Movement of atoms take place in

the same direction.

H

H

CC

b) Out plane bendingi. Wagging:• 2 atoms move to one side of the

plane. They move up and down the plane.

H

H

CC

Twisting:• One atom moves above the plane

and another atom moves below the plane

H

H

CC

• We can also calculate an approximate value of the stretching vibrational frequency of a bond by treating the two atoms and their connecting bond, to first approximation, as two balls connected by a spring, acting as a simple harmonic oscillator for which the Hooke’s Law may be applied.

• According to Hooke’s Law , The Stretching frequency is related to the masses of the atom and the force constant(a measure of resistance of a bond to stretching) of a bond by the following equation

Hooke’s Law

• There are basically two types of spectrometers

1.single beam spectrometer 2.double beam spectrometer In a single beam spectrometer the

radiations emitted from the source are passed through a cell containing the sample and through the prism which disperses the light.

• Single beam spectrometers are simple,sensitive and versatile.

• The double beam spectrometers are so constructed that the light from the source is split into two beams of equal intensity, one passing through the sample and the other through the reference for compensation.

• The two beams are recombined on to a common axis and are alternatively focused on to the entrance slit of the monochromator.

• Dbs are very convenient and hence used in labs ,reserch work and in routine works

The main parts of IR spectrometer are as follows:radiation sourcesample cells and sampling of substances.monochromatorsdetectorsrecorder

IR instruments require a source of radiant energy which emit IR radiation which must be:

Sources of IR radiations are as follows:GLOBAR:

• NERNST GLOWER:

• For gas samples: The spectrum of a gas can be obtained by

permitting the sample to expand into an evacuated cell, also called a cuvette. Gas cell has NaCl windows at the end.

• For solution sample:Infrared solution cells consists of two

windows of pressed salt sealed. Samples that are

liquid at room temperature are usually analyzed in

pure form or in solution.

• For solid sample:Solids reduced to small particles (less than 2

micron) can be examined as a thin paste or mull. The mull is formed by grinding a 2-5 milligrams of the sample in the presence of one or two drops of a hydrocarbon oil (nujol oil). The resulting mull is then examined as a film between flat salt plates.

Another technique is to ground a milligram or less of the sample with about 100 milligram potassium bromide. The mixture is then pressed in an evaluable die to produce a transparent disk.

FORE OPTICS•Consists of Source, Mirrors, M1,M2 and a Rotating mirrors •M1,M2 divides the beam.•M alternately allows the sample beam and reference beam to pass through.

MONOCHROMATOR• Splits the polychromatic radiation

to component wavelengths.• Make use of prisms or grating or

both.• Resolution depends on slit width

and quality of mirrors.• Rock salt prism is generally used in

the range of 650-4000cm-1

Detector

Thermal Non-thermal

There are four types of thermal detector.BolometersThermocouple and thermopilePyro electric detectorGolay cell

THE RECORDER•The amplified signal is recorded by an Pen Recorder.

•This instrument optically balances out differential between 2 beams. This kind of Instrument is called Optical null recording Spectrometer.

•More sophisticated Instruments are called Ratio- recording Instruments. In these instruments the intensities of both sample and reference beams are measured and ratioed.

FT-IR• FT-IR stands for Fourier Transform Infrared

Spectrometer, the preferred method of infrared spectroscopy.

• Dispersive infrared spectrometers suffer from several disadvantages in sensitivity, speed and wavelength accuracy.

• An entirely different principle is involved in Fourier Transform infrared spectroscopy, which centres on a Michelson interferometer, so that the method can also be called Interferometric infrared spectroscopy.

Dispersive Spectrometer

FTIR

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.

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 dispersionspectrometer.)

• 1. Identification of Substances1. Identification of Substances

To compare spectrums.

No two samples will have identical IR spectrum.

Criteria: Sample and reference must be tested in identical conditions, like physical state, temperature, solvent, etc.

The “Fingerprint” Region (1200 to 700 cm-1) :

• Small differences in structure & constitution of molecule result in significant changes in the peaks in this region.

• Hence this region helps to identify an unknown compound.

Structural/Functional ComponentsStructural/Functional Components

Alkene

Alkane

• IR Spectrum in Alcohol

Computer Search Systems• Virtually all infrared instrument

manufactures now offer computer search systems to assist chemist in identifying compounds from stored infrared spectral data.

• The position and relative magnitudes of peaks in the spectrum of the analyte are determined and stored in memory to give a peak profile, which can then be compared with profiles of pure compounds stored.

Studying Progress of ReactionsStudying Progress of Reactions

• Observing rate of disappearance of characteristic absorption band in reactants; or

• Rate of increasing absorption bands in products of a particular product.

• Eg.: O—H = 3600-3650 cm-1

C=O = 1680-1760 cm-1

Detection of ImpuritiesDetection of Impurities

• Determined by comparing sample spectrum with the spectrum of pure reference compound.

• Eg.: ketone impurity in alcohols.

Measurement of Paints & Measurement of Paints & VarnishesVarnishes

• Measured by ‘reflectance analysis’.

• Advt: Measure IR absorbance of paints on appliances or automobiles without destroying the surface..

Examination of Old Paintings & Examination of Old Paintings & ArtifactsArtifacts

• Help to determine fake “masterpieces”.

• Varnish & paints from old items (statues, canvas, etc.) are analysed by IR spectroscopy.

• Presence of new paint traces implies the “masterpiece” is fake.

In IndustryIn Industry

1. Determine impurities in raw materials (to ensure quality products).

2. For Quality Control checks; to determine the % of required product.

3. Identification of materials made in industrial research labs,or materials of competitors.E.g.: Impurity in bees wax (with petroleum wax)

Medical applicationsHair: • Narcotics in hair• Effects of bleaching• Effects of UV-radiation

Skin:• Moisture content• Lipid content• Effects of cosmetics

SKIN

Difference IR spectrum of two healthy patients is very small

0

.2

.4

.6

.8

1

1.2

1.4

Abs

orba

nce

1750 1700 1650 1600 1550 1500 1450 Wavenumber (cm-1)

difference

Spectral deviations between “healthy” and “ill” patients,

exhibiting different RIAR values

- .15

- .1

- .05

0

.05

.1

.15

.2

Arb

itrar

y

1700 1600 1500 1400 1300 1200 1100 1000 900

Wavenumber(cm-1)

1550 RIAR 720 RIAR

20 RIAR

- .15

- .1

- .05

0

.05

.1

.15

.2

1600 1500 1400 1300 1200 1100 1000 900

HAIR

Infrared spectra of untreated (lower trace) and bleached (upper trace) female hair sample. The difference spectrum is given below (dashed line, with 5x ordinate expansion)

ADVANTAGES

• Detection (health condition)• Prevention (early diagnosis)• Monitoring• Diagnosis (under investigation)

• FT-IR Spectroscopy can be applied for the determination of a biochemical metabolite in biological fluids.

• FT –IR spectroscopy has been used for the determination of glucose, total protein, urea, triglyceride, cholesterol, and very low density lipoproteins in plasma and serum.

Reference• Stuart.B.2004.Infrared Spectroscopy:

Fundamentals and Applications, John Wiley and Sons Inc,New York, Pp: 223-240.

• Upadhyay.A,Upadhyay.K, and Nath.N. 2012. Biophysical Chemistry(Principles and Techniques). Himalaya Publishing House. Pvt. Ltd, Mumbai. Pp.175-186.

• Holme.D.J & Peck.H.1998.Analytical Biochemistry,Third Edition,Pearson Education Limited.Pp:37-39.

• http://www.chem.ucla.edu/~webspectra/irtable.html