Reading Notes for FTIR Articles

Fourier Transform-Infrared Spectroscopy, Part 1. Instrumentation (Perkins, W.D. J. Chem. Educ.,63, 1986, A5-10.)

  1. Introduction. Provides a historical background on the development of FT-IR. Note the date AA Michelson constructed the first interferometer! Also, the development of the FFT by Cooley and Tukey is very significant. This algorithm makes it possible to calculate the spectrum (frequency vs absorbance) from the interferogram (mirror movement vs intensity).

  2. Instrument design. This section is critical. It describes how the interferometer works. Remember the important point here is that the interferometer "encodes" the radiation from the source. This discussion shows how the moving mirror makes the source radiation oscillate like a cosine wave. The most important concept here is that the frequency of this wave depends upon the wavelength of the light. Each wavelength is "encoded" with a different frequency.
    1. Equation 1. The intensity of the signal as the mirror moves (delta), it is basically the formula for a cosine wave where B(v) is the amplitude, and 2 pi, delta/lamba is the frequency.
    2. Equation 2. Rearranges equation one in terms of the frequency of the light (instead of the wavelength).
    3. Equation 3. This just expands equation 2 (which is for a single frequency of light) to say that the interferogram is the sum of all the individual frequencies.
    4. Equation 4. Modification of equation 3. Instead of adding the individual frequencies, integrate the spectrum of the source (This is like the difference between Calculus I where you find the area by adding the rectangles, and Calculus II where you integrate).
    5. Equation 5. This one is REALLY IMPORTANT because it has practical applications. It says that the resolution (delta v) is inversely proportional to the total optical displacement. If the optical displacement is 1 cm, the maximum theoretical resolution is 1 cm^-1 (this is typical for most instruments). If the displacement is 100 cm, the resolution is 0.01 cm^-1 (which is pretty high). The longest displacement I have seen is 10 m. What resolution will this provide?

  3. Computation of the spectrum.
    1. The FT. The section starts with the equation for the fourier transform. Equation 6 lets you go from the interferogram (which is the form of the raw data) to the spectrum (which is the form that chemists are used to). Note, equation 4 lets you go back from the spectrum to the interferogram (this is called the inverse fourier transform) which is essentially what the interferometer (the device with the mirrors) does.
    2. Single Beam. Another significant point in this section is that the FTIR is a "single beam" instrument. To obtain a "spectrum" you must first take a "background", then acquire your sample, and finally subtract the "background" from your sample to obtain a final "spectrum". This is shown nicely by figure 4.
    3. Signal Average. Next the author points out that it is possible to average the signal from multiple scans. This is a valuable technique for enhancing the signal to noise ratio (S/N).

  4. Design variations. This discusses several important differences in instruments. The selection of a detector is an important part of an instrument and has a big effect on price and performance. Also significant is the mechanical tolerance as the mirror moves. Remember we are looking at distances on the order of the wavelength of light. At 1000 cm^-1, this corresponds to a movement of 5 micrometers.

  5. Apodization. This is another important section. Review equation 6, which requires integration to infinity. Since it is "impractical" to have the mirror move this far, it causes some distortion of the signal that is observed in Figure 6. These "sinc wiggles" may be suppressed by multiplying the interferogram by various functions. The selection of an apodization function involves tradeoff between resolution and S/N. This is nicely shown in figure 7.

Part II, Vibrational Spectroscopy (Lambert, J.B.; Shurvell, H.F.; Lightner, D.A.; Cooks, R.G. Organic Structural Spectroscopy; Prentice-Hall: Upper Saddle River, NJ, 1998.)

Chapters 7, 8 and 9 focus on Vibrational Spectroscopy. This includes both Infrared and Raman Spectroscopy. We will be discussing Infrared, which is much more widely used. Raman spectroscopy is another technique that probes vibrational states of a molecule. The difference is in the selection rules (based upon molecular symmetry) for the types of transitions that each can observe.
  1. Chapter 7. This chapter focuses on the experimental methods. This should be read carefully. Skip sections 7 & 8 on Raman.
  2. Chapter 8. This chapter provides an overview of IR absorption bands. Skim the text, tables 8-2 (p 189) and 8-3 (p 193) will be very useful for assigning IR bands.
  3. Chapter 9. This chapter provides details of IR features found for different structural groups. Skim through the text and then use this chapter extensively as a reference for identification using IR.

The Stable World of FT-IR (Anal. Chem. 70, 1998, 273A-276A).

This article is included to help you see what options are available on commercial instruments and to give you some idea of their price. Also look at the specifications that are available from different vendors.

Theory and Interpretation of Infrared Spectra, Nicolet Interpretation of Infrared Spectra Training Workshop

These are notes for an IR training workshop presented by Nicolet Instruments. It provides an intrododuction to why IR spectroscopy is useful, a little information about interpretation, and lots of practical information about sample handling.

Additional Reading

  1. Chapter 3, Infrared Spectrometry (Silverstein, R.M.; Bassler, G.C.; Morrill, T. Spectrometric Identification of Organic Compounds; Wiley: New York, 1991.
    This chapter focuses on interpretation of IR spectra. There is a brief introduction section, a nice discussion of IR theory (you should read section 3.2). Skip section 3.3 (it is outdated). Section 3.5 is a nice overview of how to approach interpretation of IR spectra. Section 3.6 discusses characteristic IR features for different functional groups, this section will be very useful for interpretation. The appendix includes:

  2. Fourier Transform-Infrared Spectroscopy, Part 3, Applications (Perkins, W.D. J. Chem. Educ., 64, 1987, A296-A305).
    1. Aqueous Solutions. These are almost impossible with dispersive instruments. The S/N advantages of FT-IR make this possible with several different techniques.

    2. Low Transmission. S/N advantage of FT-IR

    3. Interesting experiments possible with FT-IR because of S/N enhancement
      1. Circular Internal Reflection. (Now usually called Attenuated Total Reflection or ATR). Place a drop of liquid or a film against a crystal to get a spectrum. Depends on absorbance as the beam bounces off the crystal/sample interface. Fast and easy to use. Measure liquid samples without using salt plates.
      2. Diffuse Reflectance. Measures spectrum of solids without making a KBr pellet.
      3. Photoacoustic Spectroscopy. When something absorbs IR radiation it heats up. When it heats it expands. This creates an acoustic wave that is measured with a small microphone.
      4. IR emission.
      5. IR microscope

    4. GC/FT-IR. The speed advantage of FT-IR makes it possible to obtain an IR spectrum on each peak from a GC run.

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