Fourier Transform Infrared, FTIR spectroscopy is a method of infrared spectroscopy utilizing infrared radiation to produce a finger print spectrum of a specific molecular species present in the subject of the analysis (ThermoNicolet 2). This spectroscopic type is generally employed in the compound identification, functional group determination, molecular conformation and stereochemistry investigations, and molecular orientation studies (Hsu 247). Also, analysis of polymers, plastics, copolymers and resins are done through this method (Hsu 248).
In this experiment, FTIR will be utilized in the characterization of several polymeric materials such as polystyrene, plastic package material, baggie, and silica film. Infrared radiation interacts with the sample resulting to the bending, stretching, and contraction of its molecular constituents particularly the functional groups (Hsu 249). A functional group adsorbs infrared energy of specific frequency or wave number regardless of molecular structure. For instance, the carbon-oxygen stretching of the carbonyl group in various molecular types is observed at 1700cm-1 (Hsu 266).
Moreover, infrared adsorption of functional group is found to be consistent amidst the changes in intra-molecular structure, pressure, sampling, and temperature (Hsu 266). Hence, the identification of a specific functional group is possibly done by correlation between the chemical structure and the wave number (Hsu 249). The FTIR System Figure 1. Schematic Diagram of FTIR system (Keck Interdisciplinary Surface Science Center-NUANCE). The FTIR system is primarily consisted of the source of infrared radiation, interferometer, and detector (Hsu 254).
In splitting the incoming infrared into the two optical beams, a beam splitter is designed for every interferometer (ThermoNicolet 4). The infrared beams reflected off by the two mirrors are then meet and again combined at the beam splitter. This re-combined beam serves as a signal or interferogram bearing encoded infrared frequency (ThermoNicolet 4). As the interferogram passes through the sample, the functional groups of the molecular constituents adsorb energy of specific frequencies.
After interaction with the sample, the signal is transmitted to and measured by the detector. The measured transmitted interferogram is decoded through mathematical technique known as “Fourier Transformation” (Hsu 256). The computer performs the Fourier transformation and plots adsorbance or transmittance against wave number producing a single beam spectrum (ThermoNicolet 5). Meanwhile, the single beam spectrum of the sample is similar with the background spectrum, thus, the latter should always be determined.
Background spectrum is obtained, as induced by instrumental and environmental conditions, in the absence of the sample (ThermoNicolet 5). The single spectrum produced after analysis contains sample peaks superimposed on instrumental and environmental interferences (Hsu 255). In correcting this, the single beam spectrum of the sample is normalized with respect to background spectrum (Hsu 255). Detectors and Sensitivity Mercury cadmium telluride, MCT and deuterated triglycine sulfate, DTGS are commonly used detectors in FTIR spectroscopic analysis.
Although DTGS detector has a linear response, it is less sensitive than MCT detector (Hsu 256). While MCT detector has a less linearity response as compared with DTGS detector, it must be cooled with liquid nitrogen or thermo electric cooling which leads to some problems while doing a glovebox work (Hsu 256). In terms of sensitivity, solids, liquids or gaseous samples can be analyzed through FTIR in microgram level (Hsu 248). Also, impurities in as low as 1% to 0. 01% levels can be traced.
Further, analysis per sample can be done form one to ten minutes depending on the type on FTIR instrument and the required resolution (Hsu 248). Works Cited Hsu, C. -P. Sherman. “Infrared Spectroscopy. ” Handbook of Instrumental Techniques for Analytical Chemistry. Ed. Settle, Frank A. New Jersey: Prentice Hall, 1997. Keck Interdisciplinary Surface Science Center, NUANCE. n. d. “FT-IR. ” Northwestern University. 29 January 2009 <http://www. nuance. northwestern. edu/KeckII/ftir1. asp>. ThermoNicolet. Introduction to Fourier Transform Infrared Spectrometry. Wiscosin: Thermo Electron Business, 2001
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