The research is based on a scientific experiment developed, to provide undergraduate students a solid understanding of Fourier transform (FT) infrared (IR) spectroscopy. Practically, Fourier Transform Infrared Spectroscopy being an Industrial and Laboratory Chemical Analysis presents the Fourier Transform Infrared Spectroscopy (FT-IR) as a valuable analytic tool in solving industrial and laboratory chemical problems. Apart from its application to measuring the mid-IR spectra of organic molecules, the experiment introduces several techniques with wide applicability in physics, including interferometry, the FT, digital data analysis, and control theory. This report covers both the basic theory of FT-IR and how it works as well as discussing some the practical aspects of FT-IR use in the science laboratory.
1.1 BACKGROUND OF STUDY
Fourier spectroscopy is a general term that describes the analysis of any varying signal into its constituent frequency components, while Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-spectral-resolution data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time.
FT-IR stands for Fourier Transform InfraRed, 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 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.
Record has it that, the original infrared instruments were of the dispersive type. These instruments separated the individual frequencies of energy emitted from the infrared source. This was accomplished by the use of a prism or grating. An infrared prism works exactly the same as a visible prism which separates visible light into its colors (frequencies). A grating is a more modern dispersive element which better separates the frequencies of infrared energy. The detector measures the amount of energy at each frequency which has passed through the sample. This results in a spectrum which is a plot of intensity vs. frequency.
The term Fourier-transform infrared spectroscopy originates from the fact that a Fourier transform (a mathematical process) is required to convert the raw data into the actual spectrum.
The first low cost spectrophotometer capable of recording an infrared spectrum was the Perkin-Elmer Infracord produced in 1957 (Giddings, J. C.: 1985). This instrument covered the wavelength range from 2.5 μm to 15 μm (wavenumber range 4000 cm−1 to 660 cm−1). The lower wavelength limit was chosen to encompass the highest known vibration frequency due to a fundamental molecular vibration. The upper limit was imposed by the fact that the dispersing element was a prism made from a single crystal of rock-salt (sodium chloride), which becomes opaque at wavelengths longer than about 15 μm; this spectral region became known as the rock-salt region. Later instruments used potassium bromide prisms to extend the range to 25 μm (400 cm−1) and caesium iodide 50 μm (200 cm−1). The region beyond 50 μm (200 cm−1) became known as the far-infrared region; at very long wavelengths it merges into the microwave region. Measurements in the far infrared needed the development of accurately ruled diffraction gratings to replace the prisms as dispersing elements, since salt crystals are opaque in this region. More sensitive detectors than the bolometer were required because of the low energy of the radiation. One such was the Golay detector. An additional issue is the need to exclude atmospheric water vapour because water vapour has an intense pure rotational spectrum in this region. Far-infrared spectrophotometers were cumbersome, slow and expensive. The advantages of the Michelson interferometer were well-known, but considerable technical difficulties had to be overcome before a commercial instrument could be built. Also an electronic computer was needed to perform the required Fourier transform, and this only became practicable with the advent of mini-computers, such as the PDP-8, which became available in 1965. Digilab pioneered the world’s first commercial FTIR spectrometer (Model FTS-14) in 1969 (D. M. Halland and E.V. Thomas 1988)
(Digilab FTIRs are now a part of Agilent technologies’ molecular product line after it acquired spectroscopy business from Varian).
1.2 STATEMENT OF PROBLEM
Infrared spectroscopy has been a workhorse technique for materials analysis in the laboratory for over seven decades now. The original infrared instruments were of the dispersive type. These instruments separated the individual frequencies of energy emitted from the infrared source. This was accomplished by the use of a prism or grating. An infrared prism works exactly the same as a visible prism which separates visible light into its colors (frequencies).
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.
A grating is a more modern dispersive element which better separates the frequencies of infrared energy. The detector measures the amount of energy at each frequency which has passed through the sample. This results in a spectrum which is a plot of intensity vs. frequency. With modern software algorithms, infrared is an excellent tool for quantitative analysis. Today, Fourier transform infrared spectroscopy is preferred over dispersive or filter methods of infrared spectral analysis as it provides a precise measurement method which requires no external calibration. It can also increase speed, collecting a scan every second. Thus, a discrete Fourier transform is needed. The fast Fourier transform (FFT) algorithm will be used in this present experiment.
1.3 AIMS/OBJECTIVES OF STUDY
The major aim of examining Fourier Transform Infrared (FT-IR) using some laboratory technique is to overcome the limitations encountered with dispersive instruments during experiments in the science laboratory. The specific objective of the research is;
i. To showcase how Fourier-transform infrared is used to measure the mid-IR spectra of organic molecules.
ii. To examine the application of Fourier transform infrared spectroscopy in headspace gas volatile organic compound and methane analysis.
iii. To demonstrate how Fourier-transform infrared (FTIR) can be used as detector in chromatography.
1.4 RESEARCH HYPOTHESIS
H0: Fourier-transform infrared (FTIR) cannot determine the amount of components in a mixture.
H1: Fourier-transform infrared (FTIR) can determine the amount of components in a mixture.
H0: Fourier-transform infrared cannot determine the quality or consistency of a sample.
H2: Fourier-transform infrared can determine the quality or consistency of a sample.
1.5 SIGNIFICANCE OF STUDY
Fourier Transform Infrared (FT-IR) spectrometry was developed in order to overcome the limitations encountered with dispersive instruments. The main difficulty was the slow scanning process. A method for measuring all of the infrared frequencies simultaneously, rather than individually, was needed. A solution was developed which employed a very simple optical device called an interferometer. The interferometer produces a unique type of signal which has all of the infrared frequencies “encoded” into it. The signal can be measured very quickly, usually on the order of one second or so. Thus, the time element per sample is reduced to a matter of a few seconds rather than several minutes. Therefore this report will serve as a reference material to students from science department, wishing to embark on a project of this nature in their project research. The results obtained from this report will be an instrument of validation to lab scientists in the faculty of science.
1.6 LIMITATIONS OF STUDY
A research of this nature is bound to experience some of limitations such as:
Finance: The finance at the disposal of the researcher in the course of the study could not permit wider coverage as resources are very limited as the researcher has other academic bills to cover.
Availability of Research Material: The research materials available to the researcher at the time of this study were insufficient, thereby limiting the study.
Availability of equipments: most of the required equipments for the experiments were not available at the lab as the time of this report, while some of the available ones were malfunctioning thus limiting the indebt level of the research.
1.7 DEFINITION OF TERMS
Fourier Transform Techniques; Fourier-transform spectroscopy is a measurement technique whereby spectra are collected based on measurements of the coherence of a radiative source, using time-domain or space-domain measurements of the electromagnetic radiation or other type of radiation.
Laboratory technique: Laboratory techniques and procedures are performed on patient specimens to detect biomarkers and diagnose diseases. Blood, urine, semen or tissue samples can be analysed using biochemical, microbiological and cytological methods
Analytical Chemistry: Analytical chemistry studies and uses instruments and methods used to separate, identify, and quantify matter. In practice separation, identification or quantification may constitute the entire analysis or be combined with another method.
Computer-Based Learning: sometimes abbreviated CBL, refers to the use of computers as a key component of the educational environment. While this can refer to the use of computers in a classroom, the term more broadly refers to a structured environment in which computers are used for teaching purposes.
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