Infrared Spectroscopy

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.

A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer.

Region Wavelength range (µm)

Near 0.78 - 2.5Middle 2.5 – 50Far 50 -1000

Wavenumber range (cm-1)

12800 – 40004000 - 200200 - 10

Bond Absorption region, cm–1

C–C, C–O, C–N 800–1300C=C, C=O, C=N, N=O 1500–1900C C, C N 2000–2300C–H, N–H, O–H 2700–3800

PRINCIPLE OF INFRARED SPECTROSCOPYExcitation of molecule from a lower to the higher vibrational levelOnly those bonds which are accompanied by a change in dipole moment will absorb in the IR Region.C=O, N-H, O-H etc bond are IR activeCarbon carbon bonds in symetric alkenes and alkynes etc are IR inactive

Theory of molecular vibration Absorption in the infra-red region is due

to the changes in the vibrational and rotational levels.

A single energy vibrational energy change is accompanied by a large number of rotational energy changes.

Thus the vibrational spectra appear as a vibrational rotaional bonds.

Theory of molecular vibration In Infra-red spectroscopy the absorbed energy

brings about predominant chnages in the vibrational energy which depends upon:

1. Masses of the atoms present in a molecule

2. Strength of the bond and

3. The arrangement of atoms within the molecule|

It has been found that no two compounds except the enantiomers can have similar Infra-red spectra.

Theory of molecular vibration When infra-red light is passed through the

sample , the vibrational and rotational energies of the molecules are increased. Two kinds of fundamental vibrations are:

a) Stretching : The distancre between the two atoms increases or decreases but the the original bond axis remains same.

b) Blending: The position of the atoms change with respect to the original bond axis.

Stretching Vibrations

Bending Vibrations

Mode of Vibration

Degree of freedom is the number of variables required to describe the motion of a particle completely.  For an atom moving in 3-dimensional space, three coordinates are adequate so its degree of freedom is three.  Its motion is purely translational.  If we have a molecule made of N atoms (or ions), the degree of freedom becomes 3N, because each atom has 3 degrees of freedom.  Furthermore, since these atoms are bonded together, all motions are not translational; some become rotational, some others vibration.  For non-linear molecules, all rotational motions can be described in terms of rotations around 3 axes, the rotational degree of freedom is 3 and the remaining 3N-6 degrees of freedom constitute vibrational motion.  For a linear molecule however, rotation around its own axis is no rotation because it leave the molecule unchanged.  So there are only 2 rotational degrees of freedom for any linear molecule leaving 3N-5 degrees of freedom for vibration.

Light Source

Silicon Carbide Rod (Globar) The most ubiquitous IR source used in FTIR is a resistively

heated silicon carbide rod (see image below). This device is commonly and somewhat simply referred to as a Globar.

An electric current is passed through the bar which become very hot, prducing large amounts of IR raidiation.

A Globar can reach temperatures of 1300K, and in the past required water cooling to keep from damaging the electrical components. Advances in ceramic metal alloys have lead to the production of Globars that no longer require water cooling. However these newer Globars are typically not operated at as high a temperature as 1300K.

Silicon Carbide Rod (Globar)

Infrared Instruments

An infrared spectrophotometer is an instrument that passes infrared light through an organic molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. In infrared spectra the absorption peaks point downward because the vertical axis is the percentage transmittance of the radiation through the sample.

Absorption of radiation lowers the percentage transmittance value. Since all bonds in an organic molecule interact with infrared radiation, IR spectra provide a considerable amount of structural data.

Infrared Instruments

Infrared Instruments

A common FTIR spectrometer consists of a source, interferometer, sample compartment, detector, amplifier, A/D convertor, and a computer. The source generates radiation which passes the sample through the interferometer and reaches the detector. Then the signal is amplified and converted to digital signal by the amplifier and analog-to-digital converter, respectively. Eventually, the signal is transferred to a computer in which Fourier transform is carried out. 

Michelson Interferometer

A typical Michelson interferometer consists of two perpendicular mirrors and a beamsplitter. One of the mirror is a stationary mirror and another one is a movable mirror. The beamsplitter is designed to transmit half of the light and reflect half of the light. Subsequently, the transmitted light and the reflected light strike the stationary mirror and the movable mirror, respectively. When reflected back by the mirrors, two beams of light recombine with each other at the beamsplitter. 

Infrared Instruments

1. Beam Splitter The beam splitter is made of a special material

that transmits half of the radiation striking it and reflects the other half. Radiation from the source strikes the beam splitter and separates into two beams. 

2. Stationary Mirror The stationary mirror in an FTIR interferometer is

nothing more than a flat highly reflective surface

Infrared Instruments

3. Moving Mirror The beauty of the FTIR spectrometers design lies

in its simplicity. There is present only one moving part in an FTIR spectrometer, its oscillating mirror

4. Detectors Infrared detectors are classified into two

categories; thermal, and quantum models. A thermal detector uses the energy of the infrared beam as heat, while the quantum mechanical detector uses the IR beam as light and provides for a more sensitive detector

Development of IR Spectrometers

Up till FTIR spectrometers, there have been three generations of IR spectrometers. 

The first generation IR spectrometer was invented in late 1950s. It utilizes prism optical splitting system. The prisms are made of NaCl. The requirement of the sample’s water content and particle size is extremely strict. Further more, the scan range is narrow. Additionally, the repeatability is fairly poor. As a result, the first generation IR spectrometer is no longer in use.

The second generation IR spectrometer was introduced to the world in 1960s. It utilizes gratings as the monochrometer. The performance of the second generation IR spectrometer is much better compared with IR spectrometers with prism monochrometer, But there are still several prominent weaknesses such as low sensitivity, low scan speed and poor wavelength accuracy which rendered it out of date after the invention of the third generation IR spectrometer.

The invention of the third generation IR spectrometer, Fourier transform infrared spectrometer, marked the abdication of monochrometer and the prosperity of interferometer. With this replacement, IR spectrometers became exceptionally powerful. Consequently, various applications of IR spectrometer have been realized.

Working Procedure

Step 1: The first step is sample preparation. The standard method to prepare solid sample for FTIR spectrometer is to use KBr. About 2 mg of sample and 200 mg KBr are dried and ground. The particle size should be unified and less than two micrometers. Then, the mixture is squeezed to form transparent disc which can be measured directly. For liquids with high boiling point or viscous solution, it can be added in between two NaCl pellets. Then the sample is fixed in the cell by skews and measured. For volatile liquid sample, it is dissolved in CS2 or CCl4 to form 10% solution. Then the solution is injected into a liquid cell for measurement. Gas sample needs to be measured in a gas cell with two KBr windows on each side. The gas cell should first be vacuumed. Then the sample can be introduced to the gas cell for measurement.

Working Procedure

Step 2: We obtain the background spectrum because the solvent in which we place our sample will have traces of dissolved gases as well as solvent molecules that contribute information that are not our sample. The background spectrum will contain information about the species of gases and solvent molecules, which may then be subtracted away from our sample spectrum in order to gain information about just the sample. 

Working Procedure

Step 3: Next, we collect a single-beam spectrum of the sample, which will contain absorption bands from the sample as well as the background (gaseous or solvent).

Step 4: The ratio between the single-beam sample spectrum and the single beam background spectrum gives the spectrum of the sample.

Sample Spectrum

Step 5: Data analysis is done by assigning the observed absorption frequency bands in the sample spectrum to appropriate normal modes of vibrations in the molecules.

General Application• Night vision• Thermography• Hyperspectral imaging• Tracking• Heating• Communications• Spectroscopy• Meteorology• Climatology• Astronomy• Art history• Biological systems• Photobiomodulation

Qualitative Analysis

• Identification • Determination of molecular structure• Studying progress of reaction• Detection of impurities• Isomerism in organic chemistry• Functional group isomerism• Shape of symmetry of molecule• Identification of functional group• Presence of water in sample

Qualitative Analysis

• Measurement of paint and vernishes• Examination of old paintings and

artifacts• Analysis of petroleum hydrocarbons, oil

and grease content

http://www.slideshare.net/wonderingsoul114/applications-of-ir-infrared-spectroscopy

Other Application

• Determination of unknown contaminant• Determination of cell wall of mutant and

wild type plant• Biomedical study of human hair to

identify disease state• Identify odour and test component of

food• Determine atmospheric pollutant from

atmosphere itself.