Fourier Transform Infrared Spectroscopy- Introduction and Priciple

INTRODUCTION

Fourier Transform Infrared Spectroscopy (FTIR) is a technique that is extremely useful for the characterization of Organic materials (including polymers) and certain inorganic compounds. Fourier Transform Infrared Radiation (FTIR) is a type of spectroscopy IR analysis that uses infrared radiation to record molecule movements via computer-based programs. It uses a formula called Fourier Transform and a scheme of conversion called Michelson Interferometer. FTIR is the most recent technology that uses IR in quantitative analysis. It is used by organic chemists to determine the components of organic compounds. Most of the organic compounds reveal their distinguishing component when exposed to infrared radiation. The revealed distinguishing component emits energy (wavelength) that is represented by a graph called spectrum. The spectra obtained by FTIR provide information about the presence of specific molecular structures. The electromagnetic spectrum is composed of energy that may behave both as a particle and as a wave. When we describe this energy as a particle, we use the word photon. When we describe this energy as a wave, we use the terms frequency (ν) and wavelength (λ). Frequency is the number of wave troughs that pass a given point in a second and wavelength is the distance from one crest of a wave to an adjacent crest. Frequency and wavelength are inversely related, according to the equation E = hν = hc / λ. Therefore, as frequency increases, wavelength decreases. When we discuss IR spectroscopy, we introduce a new unit of measurement called the wavenumber (νλ). The wavenumber is the number of waves in one centimeter and has the units of reciprocal centimeters (cm^-1). Since the wavenumber is inversely proportional to wavelength, it is directly proportional to frequency and energy which makes it more convenient to use.

PRINCIPLE

FT-IR stands for Fourier Transform Infrared, the preferred method of infrared spectroscopy. FTIR is based on the fundamental principles of molecular spectroscopy. This broad-ranging area of physics and chemistry covers a multitude of experimental techniques, some of which are found in other oil analysis tests, and others that are so sophisticated that they are of importance only in research laboratories.
The basic principle behind molecular spectroscopy is that specific molecules absorb light energy at specific wavelengths, known as their resonance frequencies. For example, the water molecule resonates around the 3450 wave number (given the symbol cm^-1), in the infrared region of the electromagnetic spectrum.
An FTIR spectrometer works by taking a small quantity of sample and introducing it to the infrared cell, where it is subjected to an infrared light source, which is scanned from 4000 cm^-1 to around 600 cm^-1. The intensity of light transmitted through the sample is measured at each wavenumber allowing the amount of light absorbed by the sample to be determined as the difference between the intensity of light before and after the sample cell.
When IR radiation is passed through a sample, some of the infrared radiation is absorbed by the sample and some of it is passed through (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. 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. To identify a component of certain compounds, they are exposed to high energy such as Infrared Radiation (IR). The reaction results to emission of energy showing the reactions of the molecules, which are automatically plotted to a graph by one of the programs embedded in spectroscopic instruments. Using the generated graph, organic chemists analyse the plot and detect distinctive peaks that can be attributed to the components of the compound.
For instance, a graph shows two distinctive peaks, and after analyzing the plot, one can find out that one peak corresponds to Hydrogen (H) and the other is Oxygen (O₂); thus, we can safely say that it is H₂O or water molecule. Molecules that react with IR always exhibit the same distinguishing peak of energy so they can easily be identified from the graph. In the infrared region of the spectrum, the resonance frequencies of a molecule are due to the presence of molecular functional groups specific to the molecule. A functional group is simply a group of two or more atoms, bonded together in a specific way. In the water molecule (H₂O), it is the O-H functional group that contributes to the resonance frequency around 3450 cm^-1.

The Sample Analysis Process

The normal instrumental process is as follows:
1. The Source: Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which controls the amount of energy presented to the sample (and, ultimately, to the detector).
2. The Interferometer: The beam enters the interferometer where the “spectral encoding” takes place. The resulting interferogram signal then exits the interferometer.
3. The Sample: The beam enters the sample compartment where it is transmitted through or reflected off of the surface of the sample, depending on the type of analysis being accomplished. This is where specific frequencies of energy, which are uniquely characteristic of the sample, are absorbed.
4. The Detector: The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal.
5. The Computer: The measured signal is digitized and sent to the computer where the Fourier transformation takes place. The final infrared spectrum is then presented to the user for interpretation and any further manipulation.

Figure: Block Diagram

Because there needs to be a relative scale for the absorption intensity, a background spectrum must also be measured. This is normally a measurement with no sample in the beam. This can be compared to the measurement with the sample in the beam to determine the “percent transmittance.” This technique results in a spectrum which has all of the instrumental characteristics removed. Thus, all spectral features which are present are strictly due to the sample. A single background measurement can be used for many sample measurements because this spectrum is characteristic of the instrument itself.

Bond Vibrations: The Basis of IR Spectroscopy
Spectroscopy is the study of matter and its interaction with electromagnetic radiation. All matter contains molecules; these molecules have bonds that are continually vibrating and moving around. These bonds can vibrate with stretch motions or bend motions. If we imagine two balls attached by a spring, representing a diatomic molecule, the movement of each ball toward or away from the other ball along the line of the spring represents a stretching vibration. Stretching can either be symmetric or asymmetric. A molecule with three or more atoms can experience a bending vibration, a vibrational mode where the angle between atoms changes.
In the following examples, a triatomic molecule ABC is considered.

Stretching Vibrations-

Symmetric Stretch: allows molecule to move through space.

Asymmetric Stretch: leads to an increase or decrease in bond length .

Bending Vibrations-

Figure: Bending Vibrations.

Each excited vibrational state is reached when a molecule is exposed to a specific frequency. In order for a bond to be promoted to the excited state, it must be exposed to radiation of the exact same frequency as the energy difference between ground and excited states. Determining these frequencies and representing them allows us to determine the bonds that exist in a molecule. These frequencies all lie within the infrared region of the electromagnetic region, a region of lower wavelength than visible light. A machine called an IR Spectrometer passes infrared radiation through a sample of an unknown compound and uses a detector to plot percent transmission of the radiation through the molecule versus the wavenumber of the radiation. A downward peak on the plot represents absorption at a specific wavenumber. In sum, IR spectroscopy is useful in determining chemical structure because energy that corresponds to specific values allows us to identify various functional groups within a molecule. An IR spectrum usually extends from radiation around 4000 cm-1 to 600 cm-1 and can be split into the functional group region and the fingerprint region. The fingerprint region is different for each molecule just like a fingerprint is different for each person. Two different molecules may have similar functional group regions because they have similar functional groups, but they will always have a different fingerprint region.

About the Author

Sukanta Goswami

Guest Author

Sukanta Goswami has done Graduation and Masters from Presidency College, Kolkata in Geology. He is also M. Tech. from Homi Bhabha National Institute, Mumbai. He is working as Scientific officer in Atomic Minerals Directorate for Exploration and Reserch. He is a good sportsman.