FTIR �Working Principle� and� Instrumentation

Prepared by :

SALONI SHARMA

PG Deptt. of Physics

Various regions in the Infrared spectrum

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�FTIR SPECTROSCOPY�

• FT-IR stands for Fourier Transform Infra-Red, the preferred method of infrared spectroscopy.
• In infrared spectroscopy, IR radiation is passed through a sample. Some of the infrared radiation is absorbed by the sample and some of it is passed transmitted.
• The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample.
• Like a fingerprint no two unique molecular structures produce the same infrared spectrum. This makes infrared spectroscopy useful for several types of analysis.

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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

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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 positive identification (qualitative analysis) of every different kind of material.

In addition, 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.

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Requirements for the absorption in the IR region by matter

• First requirement for absorption:
• The radiation must have precisely the correct energy to satisfy the energy of the material. A given frequency in IR region corresponds exactly to a fundamental vibration frequency of a given molecule.

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• Second requirement for absorption:
• There must be a coupling (interaction) between the radiation and the matter.

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Condition for IR activity

• For a vibration to be IR active, the molecule must undergo a change in dipole moment when the fundamental vibration occurs.
• If no change in dipole moment occurs, when the molecule vibrates, there will be no interaction between the EMR and the molecule and no absorption will take place regardless of energy compatibility.
• Such a vibration is said to be IR-inactive.

Dipole moment

The IR Spectroscopic Process

• As a covalent bond oscillates – due to the oscillation of the dipole of the molecule – a varying electromagnetic field is produced
• The greater the dipole moment change through the vibration, the more intense is the EM field that is generated.
• When a wave of infrared light encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed.
• The coupled wave now vibrates with twice the amplitude

Types of molecular Vibrations

The fundamental vibrations of a molecule are

• Stretching vibration- The stretching vibration in which the distance between two atoms around a bond varies with time are of two types, symmetrical and unsymmetrical.

• In the symmetrical stretching vibration, the side atoms move away from the central atom along the molecular axis and, after reaching maximum displacement, move back toward the central atom.

• In the asymmetrical stretching vibration , one side atom approaches the central atom while moving back from the other.

• Bending vibrations: These cause changes in the bond angle. These involve: d (in plane), and p (out of plane) vibrations

The bending vibrations can be classified as

• Wagging, where the structural unit swings back and forth in the plane of the molecule

• Rocking, where the structural unit swings back and forth out of the plane of the molecule

• Twisting, where the structural unit rotates about the bond which joins it to the rest of the molecule

• Scissoring, where, for example, the two hydrogens of a methylene group move toward each other

Various Bending Modes

PRINCIPLE OF IR SPECTROSCOPY

• Infrared spectroscopy works on principle that the molecules vibrate at specific frequencies. These frequencies (~ 4000 to ~ 200 cm⁻¹) fall in IR portion of electromagnetic spectrum.
• When IR radiation is incident on a sample, it absorbs radiation at frequencies similar to its molecular vibration frequencies, and transmits other frequencies.
• Frequencies of absorbed radiation are detected by infrared spectrometer, and a plot of absorbed energy against frequency, called ‘infrared spectrum’, can be obtained.

• A nonlinear molecule, which has N atoms, exhibits 3N-6 vibrational motions for its atoms, also called fundamental vibrations or normal modes.
• Only those molecular vibrations appear in the IR spectrum which are IR active (or absorb IR light).
• For a mode to be IR active, the vibration must induce a change in dipole moment of the vibrating molecule. Therefore, symmetric vibration do not appear in IR spectrum. However, molecular vibrations which are asymmetric are IR active and thus appear in the spectrum. This results in simultaneous detection of all chemical groups present in the material.

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• Interestingly, this technique can readily detect the amino acids as well as water molecules which are difficult to detect by other spectroscopies.
• Chemical groups which possess permanent dipole (e.g., polar bonds) exhibit strong absorption in infrared. Thus, IR spectra of proteins include absorption peaks caused by carbonyl groups of polypeptide chains.

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Theoretical number of fundamental modes of vibration

• It is useful and fairly accurate to visualize a molecule as an assembly of balls and springs in constant motion, the balls representing nuclei and the springs representing chemical bonds.
• The theoretical number of fundamental modes of vibration of a molecule containing N atoms is 3N-6 modes a nonlinear molecule or 3N-5 modes for a linear molecule.
• Each normal vibration is associated with a characteristic frequency.

• For a molecule with N atoms, a total of 3N coordinates is required, and the molecule is said to have 3N degrees of freedom of motion.
• Of these, three degrees must be subtracted for translational motion, since the molecule as a whole can drift off in a straight- line (transitional) motion in any of three directions in space.
• For a nonlinear molecule, three additional degrees must be subtracted out for rotational motion about the three Cartesian coordinates, leaving 3N - 6 fundamental modes of vibration for a nonlinear molecule.
• For a linear molecule only two degrees of rotational motion need be subtracted out, since a linear molecule can be placed along one Cartesian coordinate and rotation about this axis cannot be recognized as a spatial movement. Hence, a linear molecule will have 3N - 5 fundamental modes of vibration.

• The bond strength decides the vibration frequencies (v) such that triple or double bonds cause higher frequency than the single bonds.
• Frequencies are highly sensitive to the type of chemical group, electronegativity of adjacent atoms or molecules, and the interactions involving hydrogen bonds. If one atom is involved in hydrogen bonding, the bond will be weakened, causing a downshift in the stretching mode frequency of chemical moiety. Therefore, the stretching mode frequencies can provide structural details of certain chemical groups, such as carbonyl or carboxylic moieties in proteins. This can be achieved by monitoring the formation or disruption in hydrogen bonds.
• Another factor affecting the vibration frequency is mass of the atoms involved in vibrations. The vibration frequency can also be altered by isotope labelling of the atoms involved.

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Factors effecting the bond strength

• Depending upon the type of atoms present, a specific chemical/functional group exhibits vibrational frequencies in particular region.
• The frequencies in these regions are affects by the environment (solvents) of the chemical group. Thus, a detailed analysis of simplified model compounds in varying environments is required for establishing a clear relationship between the IR mode frequency and structure of the compound.
• Moreover, from absorption frequencies the presence or absence of various chemical groups in a chemical structure can be ascertained. Other than the characteristic position of the absorption peaks, their magnitude is concentration dependent.
• Thus both quantitative and qualitative information can be obtained using FTIR Spectroscopy

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Components of FTIR System

• The essential component of an FTIR system are include:
• The Source:- A glowing black body Usually a SiC ceramic heated around 1550 Kelvin is used as IR source. This emits a broad band radiation having various IR wavelengths This beam is passed to an aperture that limits amount of energy incident on sample.

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• Interferometer:- Interferometer performs “spectral encoding” on incoming radiation and exiting signal contains all the IR frequency components.
• Michelson interferometer which has two mirrors, IR source and IR detector, and a beam splitter is often employed to analyse the infrared beam after passing through sample.

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• Beam-splitter, the main component of interferometer, is a half-silvered mirror, is central to the functioning of the interferometer.
• When light is incident on it, half of the incident light reflected while the remaining half is transmitted.
• The rotating mirror and stationary mirror of the interferometer, both receive each half of these beams. These beam again reach the beam-splitter after being reflected from the two mirrors.
• The beam-splitter again splits each half beam into reflected and transmitted half beams. Two output beams result: one travels to the source and other to the detector.
• When these two beams come back to the beam-splitter, an interference pattern is generated, which is called interferogram.

• The displacement of the moving mirror causes difference in path length in the two arms of the interferometer, which in turn varies the interference pattern.
• The interference pattern is detected by the IR detector as variations in IR energy level and ultimately yields spectral information.
• An ideal beam-splitter has the following charateristics: 
• It does not generate or absorb radiation
• Half of the incident light is transmitted by it
• The remaining half of all the incident light is reflected.

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• Sample:- Beam is either transmitted through or reflected off of the sample surface, depending on type of investigations being performed. Sample absorbs energies at frequencies characteristic to it.
• The Detector:- Detector measures the interferogram signal.
• The Computer:-It performs Fourier transformation to obtain final IR spectrum for examination.

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Internal view of FTIR

Working of FTIR Interferometer

• The light entering the interferometer from the source, is either sent back to the source or directed towards IR detector.
• The moving mirror, while scanning back and forth, causes greater or less than half of total light, entering the interferometer, to reach the detector. The light which does not go to the detector is sent to the source. There is no other sink of the light.
• If the mirror in interferometer are equidistant from beam-splitter, both light beams travel the same distance. When these beams return to beam-splitter, each of them is again split and recombined with half of the light from the other arm of the interferometer.
• The two component beams which are sent to the detector as output, are entirely in phase with each other, leading to constructive interference. Whereas, the two component beams sent to the source are completely out of phase to each other, resulting in destructive interference. Thus, the entire incident light reaches the detector and none reaches the source.

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• On the contrary, if moving mirror is displaced by λ/4 (where λ is the wavelength of the incident light), the light from moving mirror coming to beam-splitter is λ/2 out of phase with the light from fixed mirror. This causes destructive interference in the component lights reaching the detector, and constructive interference in the components reaching the source. Thus, no light reaches the detector and entire light is sent back to the source.
• Thus, with the back and forth movement of the moving mirror, the detector perceives light and dark bands resulting in total or partial constructive/destructive interference.
• In this case, the interferogram is a cosine function, having full intensity when both mirrors are equidistant from beam splitter, to zero intensity when there is λ/4 path difference between them.

• If the moving mirror is displaced by λ/2 or multiples of λ/2, then also full intensity in output is obtained. Likewise, a displacement of λ/4 or multiples of λ/4 result in zero intensity.
• Since the IR source is polychromatic, it emits light with a wide frequency range. Each emitted frequency produces a separate cosine signal, and the resulting interferogram is the sum of all cosine waves produced by each individual IR frequency. In this case, complete constructive interference occurs only when both mirrors are equidistant from the beam splitter, as at this point all waves simultaneously interfere in constructive manner. At all other points, there is only apartial constructive interference. The frequency as well as intensity of each cosine wave present in the interferogram is resolved by Fourier transform. Thus, a plot of intensity as a function of frequency is obtained in the form of a spectrum

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�Advantages of FT-IR�

• Speed: Because all of the frequencies are measured simultaneously, most measurements by FT-IR are made in a matter of seconds rather than several minutes. This is sometimes 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.

• These advantages, along with several others, make measurements made by FT-IR extremely accurate and reproducible. Thus, it a very reliable technique for positive identification of virtually any sample. The sensitivity benefits enable identification of even the smallest of contaminants. This makes FT-IR an invaluable tool for quality control or quality assurance applications whether it be batch-to-batch comparisons to quality standards or analysis of an unknown contaminant. In addition, the sensitivity and accuracy of FT-IR detectors, along with a wide variety of software algorithms, have dramatically increased the practical use of infrared for quantitative analysis. Quantitative methods can be easily developed and calibrated and can be incorporated into simple procedures for routine analysis.
• Thus, the Fourier Transform Infrared (FT-IR) technique has brought significant practical advantages to infrared spectroscopy. It has made possible the development of many new sampling techniques which were designed to tackle challenging problems which were impossible by older technology. It has made the use of infrared analysis virtually limitless.

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