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Fourier Transform Infrared Spectrometer (FT-IR)

What Does FT-IR Do?

Fourier Transform Infrared (FT-IR) Spectrometer uses infrared light to identify chemical compounds via vibrational spectroscopy. FT-IR is a technique that utilizes the absorption of infrared radiation as a way of obtaining infrared spectra. The spectrometer is comprised of an interferometer which produces an interferogram, and software that digitally executes the mathematical Fourier Transform function, upon which the final IR absorption spectra is displayed. This instrumentation is a valuable asset as it provides fast acquisition of spectra with high S/N ratio, high spectral accuracy and resolution all while allowing the researcher to work with weak signals.

How FT-IR Works

The basic science behind the process is infrared absorption which occurs when infrared radiation interacts with a molecule under dipole change. When exposed to infrared radiation molecules in the sample solution absorb radiation of specific wavelengths. This absorption will only happen when the infrared photon has enough energy to enable the transition to the next vibrational energy state. This causes the molecules to transfer from ground state to excited state. Analyzing this activity can reveal information regarding the structure of the molecules. As the dipole moment changes and transitions of energy levels occur the absorption peaks. The number of such peaks is determined by the number of vibrational freedom of the molecule in question. It is important to note that several homonuclear diatomic molecules are not infrared active and therefore not identifiable via this method.

The physical structure of the average FTIR spectrometer includes an infrared source, interferometer, sample compartment, detector, amplifier, A/D convertor, and a computer. The source, appropriately called such, generates radiation. This radiation interacts with the sample in the interferometer and once passed through reaches the detector. The amplifier then intensifies the signal and converts it to digital output. This digital signal is the interpreted by a computer utilizing the Fourier Transform function.

How FT-IR is Used

The basics steps for performing an FTIR analysis are outlined below along with a more thorough explanation of the instrumentations functionality.

1.0 Sample Preparation. This step simply involves correctly preparing the sample to be analyzed by the FTIR spectrometer. The standard method of sample preparation is to use KBr.

2.0 Background Spectrum. Collect a interferogram and the resulting frequency data for the prepared solvent prior to testing the sample. This provides data on what traces of molecules are in the solvent that will need to be accounted for in the final results.

3.0 Single-Beam Spectrum. Next collect a single-beam spectrum of the sample. This spectrum will included absorption bands from the sample as well as the background and must be adjusted.

4.0 Sample Ratio. Adjust the spectrum by taking the ratio between the single-beam sample spectrum and the background spectrum. This gives you the spectrum of the sample.

5.0 Data Analysis. By observing the absorption frequency bands in the sample, assign them to the appropriate normal vibration modes in the molecules.

The first step of the FTIR spectrometer’s process happens within the interferometer. The interferometer’s purpose is to create specified interference of the infrared waves. This is made possible by the physical properties of the mechanism as depicted below:


Figure 1.0


As seen in Figure 1.0, the interferometer consists of two mirrors, one fixed and the other which oscillates at a controlled rate, and a beam splitter. The beam splitter , as it is aptly called, separates the radiation source beam into two beams. The first beam continues on, striking the moving mirror while the second strikes the immobile mirror. These beams are then reflected back to the beam splitter which combines the beams, splits this combined beam, and transmits two new beams through the sample to the detector and the other being back to the source.

When the distance traveled by the two beams to and from the mirrors are equivalent, it means the distance between the two mirrors are the same. This is called zero path difference (ZPD) and happens when the mobile mirror is equidistant from the beam splitter. When the moveable mirror is at a greater distance from the beam splitter than the immobile mirror, the beam striking the mobile mirror will travel a greater distance than the beam striking the immobile mirror. The delta in the mobile mirrors’ placement we will represent by ∆. The extra distance travelled by the beam striking the mobile mirror is then 2∆. This extra distance is called the optical path difference (OPD) and is used to amplify the interferometer’s results.

This amplification happens when the OPD is a multiple of the signal’s wavelength from the beam sent to the stabilized mirror as this creates constructive interference overlapping the signal’s crests with crests and troughs with troughs. Destructive interference happens when the OPD is half the wavelength or the half wavelength plus multiples of the wavelength as this overlaps crests with troughs and vice versa. This destructive interference causes a signal of minimum intensity to be picked up by the detector.

This interference, whether constructive or destructive, is picked up by the detector, digitized and formatted into what is referred to as an interferogram. This report plots the signal with the x-axis in units of time and the y-axis measuring the signal or volts. As stated earlier the maximum will appear at the point when the mirrors are equidistant from the beam splitter. The raw data as presented on the interferogram can be helpful in troubleshooting but must be transformed into the IR absorption spectrum where the absorption intensity is plotted against the measured wavelength or wavenumber. A wavenumber is the number of full waves per cm of length for a specific wavelength. The reason for using the wavenumber instead of the wavelength is that wavenumbers are directly related to energy levels.

Due to the complexity of the signal and derived graph the detected signal must be transformed into a spectrum. This is accomplished using the Fourier Transform. This mathematical formula transforms a given function into a new equivalent function. While the interferogram is a plot of intensity versus OPD a Fourier transform is viewed as the inversion of OPD. This would make the units inverse centimeters, known as wavenumbers. This new plot is referred to as an IR spectrum.

In calculating the IR spectrum steps must be taken to account for instrumental imperfections and the known background data discovered in the pre-screening analysis. If not addressed these imperfections can created flawed readings. Once addressed the spectrum should provide the necessary information for conclusive molecular identification.

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