Infrared Spectrophotometry

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Infrared Spectrophotometry

INTRODUCTION

It is defined as the measurement and interpretation of the absorbed infrared radiation. This method is mainly used for the identification of chemical substances. The absorption is mainly based on the vibrations of the chemical substances after absorption of the infrared radiation.

The spectrum bands present in the IR spectrum represent the functional groups and bonds present in the chemical substance. Hence, the IR spectrum is commonly known as the finger print of the chemical substance.

The IR region is in between 0.8 and 1,000 μm. Then these regions are again classified into three subclasses. They are as follows:

• Near infrared region: 0.8-2.5 μm or 12,500–4,000 cm−1.
• Middle infrared region: 2.5-50 μm or 4,000-200 cm−1.
• Far infrared region: 50-100 μm or 200-10 cm−1.

The main IR region, mostly the vibrational bands, occurs in 2.5-25 μm.

THEORY

The requirements for the IR absorption are as follows:

• A molecule should have the electric dipole.
• It should be in the correct wavelength of radiation.

The frequency of the vibration is given by the following formula:

where v is the frequency; f is the force constant; μ is the reduced mass and is given by the formula:

where w1 and w2 are masses of the individual atoms.

TYPES OF VIBRATIONS

Generally these vibrations are classified into the following:

• Stretching vibrations: it is obtained by changing the bond length.
• Bending vibrations: it is obtained by changing the bond angles.

The stretching vibrations are again divided into the following:

Symmetric stretching: Both bond lengths are increased or decreased.

Symmetric stretching

Asymmetric stretching: one bond length is increased and another bond length is decreased.

Asymmetric stretching

The bending vibrations are again divided into four classes:

1. Scissoring: decrease in the bond angle.
3. 
5. Scissoring
6. Rocking: the bonds moves together in one direction.
8. 
10. Rocking
11. Twisting: the charges are same on both bonds.
13. 
15. Twisting
17. Wagging: the charges are different on the bonds.
19. 
21. Wagging

INSTRUMENTATION

The components of IR spectrometer are the following:

1. Radiation source.
2. Monochromators.
3. Sample cells.
4. Sampling techniques.
5. Detectors.

1. Radiation source: Generally, heated solids are used as sources. The common temperature required to emit the IR radiation is 1,500–2,000 K. Most commonly used radiation sources are as follows:
• Incandescent lamp: It is prepared by using the glass which consists of nichrome wire. This is most suitable for near infrared measurements. It has long life time when compared to other radiation sources.
• Nernst glower: It is composed of cylinder with rare earth oxides such as zirconia, yttria and thoria. Platinum wire is inserted in the cylinder through which the current is passed.
• Globar source: Silicon carbide is generally used as a globar source. The only precaution is water cooling is necessary to prevent arcing of the circuits.
• Mercury arc: This radiation source mainly used in the far infrared measurements.
2. Monochromators: The radiation source emits the polychromatic light which contains the wide range of frequencies. It is used to convert the polychromatic light into monochromatic light. The most commonly used monochromators are prism and grating monochromators.
3. Prism monochromators: When polychromatic light passes through the prism, it is refracted. For IR, the prisms are made up of sodium chloride, potassium bromide, lithium fluoride and cesium bromide. Based on the material used for the construction of the prisms, they are divided into two types:
• Metal halide prisms: These are made up of KBr (12–25 μm) or LiF (0.2–6 μm) or CeBr (15–38 μm).
• NaCl prisms: These are made up of NaCl mainly used in the overall range of IR region. The main disadvantage of this prism is that it is thermally unstable above 20 °C temperatures.
• Grating monochromators: Grating is nothing but the lines made on the glass which is previously coated with aluminium. Rotation of these gratings converts the polychromatic light to monochromatic light more efficiently when compared to prism monochromators.
4. Sample cells: Based on the sample to be handled, the selection of sample cells is done. Generally, cells made up of salts such as sodium chloride (NaCl) and potassium bromide (KBr) are commonly employed.
5. Sampling techniques: Based on the state of the sample, the sampling techniques are employed.
• Sampling of solids: The solid samples are generally handled by the four sampling techniques.
• Solids dissolved in solvent: The sample is dissolved in the suitable solvent and sample solution is taken into the cells made up of NaCl and KBr.
• Solid films: The sample is dissolved in the solution and it is evaporated on the surface of a NaCl and KBr cells.
• Mull technique: The sample is mixed with the equal amount of mineral oil (nujol) to form a thick paste and is spread on the IR sample cells made up of NaCl or KBr.
• Pressed pellet technique: The sample mixed with 100 times of its weight of KBr powder and is pressed under high pressure to form small pellets. This technique is more advantageous than other methods are these pellets can be stored for a long time. The main disadvantage of this method is by using higher pressure for the formation of pellets leads to changes in the polymorphic state of the crystals.
• Sampling of liquids: Normally the liquid samples are analysed in the rectangular cells made up of NaCl or KBr. In the case of organic liquids, it must be dried before taking into the sample cells.
• Sampling of gases: The gaseous samples are measured in the special sample cells which are made up of NaCl.
6. Detectors: In the IR spectrometry, generally the thermal detectors are used for attaining accurate values. The commonly employed thermal detectors are as follows:
• Thermocouples
• Bolometer
• Golay detector
• Pyroelectric detector
• Thermocouples: It is made by two metal wires which are welded through a joint which is maintained at different temperatures. This thermocouple is closed with KBr in an evacuated steel casing.
• Bolometer: It is made by inserting the platinum strip in an evacuated glass vessel and one arm is connected to the Wheatstone bridge.
• Golay cell: It is made up of gas-filled chamber which under goes a pressure rise. This detector is more efficient than other detectors.
• Pyroelectric detector: These detectors work mainly based on the principle of polarisation which shows the electrical signal.

Selection of the conditions for the IR spectra:

1. Frequency range: The frequency should be in the range of 600–4,000 cm−1.
2. Band width and the scan speed: The slit width limits the scan speed effectively.

Factors Affecting the Vibrational Shift

• Vibrational coupling is observed in the compounds containing the –CH2 and –CH3 groups such as carboxylic acids, amides and aldehydes.
• Hydrogen bonding shows the absorption shift towards the lower wave length. This is observed in the alcohols, phenols and enols.
• Electronic affects such as conjugation, mesomeric affects and inductive affects lower the absorption frequencies.

INTERPRETATION OF THE IR SPECTRA

Interpretation of the IR spectra is mainly based on the frequencies at which a band occurs within a molecular structure. The region 4,000–1,500 cm−1 easier is to interpret than 1,500–650 cm−1.

The vibrations of the alkanes and the alkyl groups are mainly because of the C–C stretching and C–H bending motions of the molecules. The C–C stretching produces the bands in 800–1,200 cm−1 region. The C–H bending vibrations are observed in 720–790 cm−1. Methylene and the methyl groups are observed in the bands in the 1,460–1,468 cm−1.

Alkenes show the vibrations of C=C and C–H stretching. The C=C bands are observed in the 1,580–1,680 cm−1. C–H stretching bands are observed in the 3,000 –3,080 cm−1.

The absorptions in this range do not apply only to bonds in organic molecules. IR spectroscopy is useful when it comes to the analysis of inorganic compounds (such as metal complexes or fluoromanganates) as well.

Phosphates could be also characterised by two middle-sized bands between 2,300 and 2,400 cm–1.

LIMITATIONS OF IR SPECTROSCOPY

• It is not possible to determine pure substance present in the mixture of substances.
• It does not give the information about the positioning of the functional groups.

ADVANTAGES

• Cheap.
• High acceptability.
• Wide applicability.

DISADVANTAGES

• It is time consuming for the sample preparation.
• It is a destructive method.

APPLICATIONS OF IR SPECTROSCOPY

• Used in the identification of functional groups in the organic compounds.
• Example: –NH, –OH, –CO, etc., functional groups analysis.
• Used for the determination of geometric isomers.
• Used in the determination of water in the samples.
• Example: Moisture content determination.
• While studying, the progress of the chemical reaction can be determined by examining the small portion of the reaction mixture withdrawn from time to time. The rate of disappearance of a characteristic absorption band of the reactant group and/or the rate of appearance of the characteristic absorption band of the product group due to formation of product are observed.
• Example: Kinetics study.
• Used in the detection of impurities.
• Example: Additives and solvents used in the manufacturing process may remain as impurities in final products.

FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR)

INTRODUCTION

This is a technique which is used to obtain an infrared spectrum of absorption of a solid, liquid or gas. This offers a significant advantage over a dispersive spectrometer which measures intensity over a narrow range of wavelengths at a time. Fourier transform is required to convert the data into the actual spectrum.

PRINCIPLE

The main principle involved in Fourier transform spectroscopy is when a monochromatic beam of light is passed through the sample, the intensity of the light is measured which is absorbed by the sample. Then the beam is modified to obtain a different number of frequencies, giving a second data point.

This process is repeated for many times. Then this is amplified with the help of amplifier and recorded with a recorder at each wavelength. Generally the beam of light is generated by the light sources used in the IR spectroscopy. Then the light is focused with configuration of mirrors which is generally called as the Michelson interferometer. The main principle of the Michelson interferometer is to allow the desired wavelengths and block the undesired wavelengths. Then the computer processes the data obtained at each mirror to desired result that is absorbance at each wave length. This conversion is generally known as Fourier transform and the data obtained at the interferometer are called as interferogram.

Schematic diagram for the Michelson interferometer

INSTRUMENTATION

In this instrumentation, the light from the general IR radiation source is collimated and directed to a beam splitter. From this, half of the radiation is reflected to the fixed mirror and the remaining half is transmitted to the moving mirror. Then the light is reflected from the mirrors to the beam splitter then to the sample compartment. The light absorbed by the sample is determined by the general IR detectors. Finally, an interferogram is obtained by varying the retardation which is obtained from the difference in the optical path length between the two arms of the interferometer.

There are two principal advantages for a FT spectrometer compared to a dispersive spectrometer.

1. Fellgett's advantage: The wavelengths are collected simultaneously and result in a higher signal-to-noise ratio for a given scan time or a shorter scan time for a given resolution.
2. Jacquinot's advantage: The interferometer throughput is determined by the diameter of collimated beam coming from the radiation source.
3. Better wavelength accuracy.
4. Less sensitivity to stray light.

ADVANTAGES OF FTIR

• Capable of identifying organic functional groups and often specific organic compounds.
• Extensive spectral libraries for compound identification.
• Ambient conditions (not vacuum; good for volatile compounds).
• Typically non-destructive.
• Minimum analysis area: ∼15 μm.
• Improved frequency resolution.
• Improved frequency reproducibility (older dispersive instruments must be recalibrated for each session of use).
• Higher energy throughput.
• Faster operation.
• Computer based (allowing storage of spectra and facilities for processing spectra).
• Easily adapted for remote use (such as diverting the beam to pass through an external cell and detector, as in GC-FT-IR).

LIMITATIONS OF FTIR

• Limited surface sensitivity (typical sampling volumes are ∼0.8 μm).
• Minimum analysis area: ∼15 μm.
• Limited inorganic information.
• Typically not quantitative (needs standards).

APPLICATIONS

• Used in the separation of mixture of components to individual components which is not be possible by general IR spectra.
• Example: Isomers separation.
• Used in the analysis of minute fractions of samples.
• Example: Elemental analysis.
• Used in the characterisation of artistic materials in old master paintings.
• Example: Aging of paintings.
• Used in the identification of chemicals from spills, paints, polymers, coatings, drugs and contaminants.
• Example: Pollutants, Evolved gases.
• Used in the quantitation of silicone, esters, etc., as contamination on various materials.
• Used in the identification of the molecular structure of organic compounds for contamination analysis.
• Example: Purity studies.
• Used in the identification of organic particles, powders, films, and liquids (material identification).
• Example: Particle analysis.
• Used in the quantification of O and H in Si, and H in SiN wafers (Si–H vs. N–H).

REVIEW QUESTIONS

1. Name the types of vibrations occur in the molecules?
2. Why water cannot be used as a solvent in the IR?
3. Explain how sample cells made in the IR?
4. What is the difference between the bolometer and thermocouple detectors?
5. What is basic need for the molecules to absorb the IR radiation?
6. What type of vibration does SO2 show?