Fourier-Transform Infrared Spectrometer (FTIR)

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Nicolet FTIR [SOP]
FTIR-052621.png
IR Activity: Bonds must have dipoles
Sample Type: Solids (bulk, powder, fiber, film), Liquids (solution, paste, gel), Gas (transmission only, needs proper cell)
Detector: DTGS (deuterated triglycine sulfate)
Accessories: iD7 ATR (attenuated total reflectance), iD1 Transmission

Fourier Transform Infrared Spectroscopy uses infrared radiation to determine the chemical bonds present in the sample. A spectrum of wavenumber and absorbance can be analyzed for the chemical composition of a sample. Organic molecules are most readily identified, making the FTIR an ideal tool for polymer identification and characterization.  In addition to a standard transmission attachment, an attenuated total reflectance head allows for direct testing of solids and liquids without any sample preparation.  The OMNIC software connected to the spectrometer allows for single and multi-component identification of the molecules and polymers in a sample by comparing the spectra gathered to stored libraries of data.

Principle

The normal instrumental process is as follows:[1]

  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. The interferometer uses a reference laser for precise wavelength calibration, mirror position control and data acquisition timing.
  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.
    FTIRSetup.PNG
  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.

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

Applications

A reference chart with some of the most common peaks seen in FTIR [2]

Infrared spectroscopy has been a workhorse technique for materials analysis in the laboratory for over seventy years. 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.

Dispersive vs Fourier Transform IR

Simplified layout of a FTIR spectrometer


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, resulting in a spectrum which is a plot of intensity vs. frequency.

Fourier transform infrared (FTIR) 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. Most interferometers employ a beamsplitter which takes the incoming infrared beam and divides it into two optical beams. One beam reflects off of a flat mirror which is fixed in place. The other beam reflects off of a flat mirror which is on a mechanism which allows this mirror to move a very short distance (typically a few millimeters) away from the beam splitter. The two beams reflect off of their respective mirrors and are recombined when they meet back at the beamsplitter. Because the path that one beam travels is a fixed length and the other is constantly changing as its mirror moves, the signal which exits the interferometer is the result of these two beams “interfering” with each other. The resulting signal is called an interferogram which has the unique property that every data point (a function of the moving mirror position) which makes up the signal has information about every infrared frequency which comes from the source. This means that as the interferogram is measured, all frequencies are being measured simultaneously. Thus, the use of the interferometer results in extremely fast measurements. Because the analyst requires a frequency spectrum (a plot of the intensity at each individual frequency) in order to make an identification, the measured interferogram signal can not be interpreted directly. A means of “decoding” the individual frequencies is required. This can be accomplished via a well-known mathematical technique called the Fourier transformation. This transformation is performed by the computer which then presents the user with the desired spectral information for analysis.

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Advantages of FT vs. Dispersive IR

  • Non-destructive technique
  • Provides a precise measurement method which requires no external calibration
  • Can increase speed, collecting a scan every second
  • Can increase sensitivity – one second scans can be co-added together to ratio out random noise
  • Greater optical throughput
  • Mechanically simple with only one moving part

References

  1. Thermo Scientific. (n.d.). Introduction to FTIR. http://tools.thermofisher.com/content/sfs/brochures/BR50555_E_0513M_H_1.pdf.
  2. Bradley, M., says, B. B., & says, R. katole. (2021, June 11). Free FTIR Basic Organic Functional Group Reference Chart. Advancing Materials. https://www.thermofisher.com/blog/materials/a-gift-for-you-an-ftir-basic-organic-functional-group-reference-chart/.