What is FT-IR?

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.

What information can FT-IR provide?

  • It can identify unknown materials
  • It can determine the quality or consistency of a sample
  • It can determine the amount of components in a mixture

Why Infrared Spectroscopy?

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.

How does Fourier Transform Infrared Work?

Fourier Transform Infrared (FT-IR) spectrometry was developed in order to overcome the slow scanning process of earlier dispersive infrared spectroscopy methods. FT-IR measures all of the infrared frequencies simultaneously, rather than individually as was done previously. A solution was developed that uses a 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.

Using a Beamsplitter to Create an Interferogram

Most interferometers employ a beamsplitter which divides it the incoming infrared beam into two optical beams. One beam reflects off of a flat mirror that is fixed in place. The other beam reflects off of a flat mirror on a mechanism that allows this mirror to move a very short distance (typically a few millimeters) away from the beamsplitter.

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 that exits the interferometer is the result of these two beams "interfering" with each other.

The signal that results from this interference is called an interferogram . It has the unique property that every data point (a function of the moving mirror position) that makes up the signal has information about every infrared frequency coming from the source.

Using Fourier Transformation to Decode the Spectral Signal

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 cannot 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.

The Sample Analysis Process

The normal instrumental process is as follows:

  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.
  3. The Laser: The laser beam also passes through the interferometer. It is used for wavelength calibration, mirror position control and data collection triggering of the spectrometer
  4. 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.
  5. The Detector: The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal.
  6. 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.

Background Spectrum

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.