Infrared Spectroscopy

Nicolet FTIR
IR Activity: Bonds must have dipoles

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. 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.
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

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.