ABSTRACT
FTIR spectroscopy technique is usually one of the most preferred techniques used to give a correct assignment of the observed spectral characteristic of functional groups corresponding to different absorption bands which are responsible of the absorption. FTIR is the characterization technique which is both rapid, nondestructive and requires small sized samples. In the material to be analysed, chemical bonds vibrate at a characteristic frequency representative of their structure, bond angle and length.
INTRODUCTION Fourier Transform Infrared Spectroscopy (FTIR) technique is usually one of the most preferred techniques used to give a correct assignment of the observed spectral characteristic of functional groups corresponding to different absorption bands which are responsible of the absorption. The characterization of the material by using FTIR spectroscopy consists in making a spectrum of radiation energy absorbed by
material molecules and interpretation of the obtained spectrum. FTIR is the characterization technique which is both rapid, non-destructive and requires small sized samples. In the material to be analysed, chemical bonds vibrate at a characteristic frequency representative of their structure, bond angle and length. Certain frequencies of IR can interact with the dipole moments of various functional groups of the polymer. These frequencies are dependent on the bond strength, length, and the mass of the atoms in the bonds. IR spectral transmittance of an unknown molecule can be collected and correlated to known values to determine the functional groups present within the molecule. Their individual molecules have the ability to interact with incident radiation by absorbing the radiation at specific wavelengths. To individual absorption peaks, individual chemical bonds can be identified and assigned in order to identify qualitatively or quantitatively individual compounds in complex systems. FTIR spectroscopy is measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample. Mid-infrared light (4000 - 200 cm-1) is energetic enough to excite molecular vibrations to higher energy states. The wavelength of many IR absorption bands is characteristic of specific types of chemical bonds, and IR spectroscopy finds its greatest utility for qualitative analysis of organic and organometallic molecules. IR spectroscopy is used to identify particular compound or a newly synthesized molecule. For FTIR spectroscopy, IR radiation is passed through a sample. A part of the radiation is absorbed by the sample and the other part is transmitted. The FTIR spectrum represents the absorption and transmission of the molecular in creating a spectral fingerprint of the sample. To overcome the slow scanning speeds of older dispersion infrared instrument, the method that measures all of the infrared frequencies simultaneously, rather than individually is used.
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. FTIR spectrometer operates on principle called Fourier transform. The interferogram is determined experimentally in FTIR spectroscopy, and the
corresponding spectrum (frequency against intensity plot), is computed using Fourier transform. This transformation is carried out automatically and the spectrum is displayed. The core of FTIR spectrometers is the Michelson interferometer that is used to split one beam of light into two so that the paths of the two beams are different. The Michelson interferometer recombines the two beams and conducts them into the detector. In the detector, the difference of the intensity of these two beams is measured as a function of the difference of the paths. FTIR is a valuable tool to assess the functional groups present in an organic compound or polymer. However, to fully elucidate the structure of an unknown compound, the use of other qualitative spectroscopic methods, such as NMR, MS, UV-Vis, is needed to complement the results gathered from the IR Spectroscopy. Different compounds and polymers were gathered for structural elucidation. However, the IR spectroscopy is not an omnipotent tool in elucidating structures, thus a library, a collection of IR spectra, was used to determine the structure of the unknown compounds and polymers. The preparation of the sample of different compounds and polymers is the most crucial part of the method since some solvent or sample holder can absorb IR frequency that can ruin the spectra of the samples gathered. The background noise was also determined to correct the spectra gathered. The objective of the experiment is to elucidate and to check the structure of the compounds and polymers gathered. Also, to determine the compound and elucidate its structure.
EXPERIMENTAL METHODS A. Gathering of Different Compounds and Polymers The compound were provided by the instructor. Also, the polymers were provided by the author using common household plastics. The compounds used were pentanol, acetone, hexane, benzene, and benzoic acid. The polymer samples were gathered from the plastic sources: Pride Fabric conditioner packaging, transparent disposable plastic cup, cloudy disposable plastic cup, thick plastic folder, clearbook cover, WATsons plastic packaging, Gaisano plastic packaging, LDPE plastic packaging, Premiere plastic bottle packaging, COKE Sakto label, SkyFlakes wrapper, Ariel detergent plastic pack, and SMART dishwashing paste label.
B. Preparation of the Sample The sample holder was properly cleaned through wiping. The liquid samples were inserted into the sample holder ensuring that the compound properly covered the window of the sample holder. The sample holder was properly shut tight. For the polymers, an approximately one in 2 of the plastic was cut from its source. It was made sure that the polymer is free of contaminants before putting inside the sample holder. C. Gathering of Data and Spectra The parameters such as resolution were set on the spectrometer operating software.We had made sure that the surface of the ATR prism was cleaned. The background was then collected by operating the spectrometer. Furthermore, the instrument was tested for its performance, whether the instrument passed or failed the standard conditions. The instrument passed the performance verification, thus, making it eligible to use to determine the spectra of the compound and polymers. Also, the polystyrene calibration was done to calibrate the instrument. The sample holder was placed into the instrument then determined the spectrum. The spectrum was compared to the data in the library. The data with the highest percentage of match was assumed to be the identity of the certain compound or polymer.
D. Determination of Functional Groups Using the spectra gathered, the functional groups of the compound and polymers were checked with the help of the table which coordinates the peaks to its specific functional groups.
RESULTS AND DISCUSSIONS
Hexane
Pentanol
Benzene
Acetone
Benzoic Acid
Figure 1. List of compounds used and its corresponding structure. Peak Position
