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Fourier Transform InfraRed Spectroscopy (FTIR)

Most materials absorb IR energy at different wavelengths depending upon their chemical nature. This phenomenon provides a method for characterizing many materials. IR energy is passed through the sample and the absorbance and/or transmittance versus IR wavelength is measured. Output is in the form of a graph, which is called the IR spectrum. The spectrum is a "fingerprint" of the material. It can be compared to those of known materials (reference spectra) to identify the unknown material.

 
Photo courtesy of
Thermo Nicolet Corporation

The spectra of long-chain hydrocarbons (mineral oils, waxes, polyethylene) will be very different from the spectra of esters (vegetable oils, synthetic oils, acrylates). Different class groups are usually easy to identify. To characterize materials within a class group more subtle differences in the spectra can be used to narrow the identity of the substance (Spectral Interpretation). For very closely related materials like vegetable oils (corn, cotton, linseed) the method can only characterize the material as a vegetable oil, but not identify individual oils.

If spectra of two materials are the same with respect to both IR band position (wavelength) and relative band intensity, then the substances are chemically similar or closely related. If the spectra differ in any way, then the materials are not the same. A number of specifications (Mil-Spec, USP, ASTM) use the IR method for material identification.

Mixtures of materials (commercial products, contaminants, additives) are more difficult to characterize, since bands from all of the components overlay one another in the spectrum. Also components at low concentration are difficult to detect, because the major component overlays the bands from the minor component. In these cases the components must be separated from one another for identification.

If a reference match cannot be found, the location and band intensity give some information about the chemical nature of the material. For example, the spectra of esters always have a carbonyl (C=O) band, while those of pure long-chain hydrocarbons will not.

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