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Fourier Transform InfraRed Spectroscopy (FTIR)
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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.
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Photo
courtesy of
Thermo Nicolet Corporation |
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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.
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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.
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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.
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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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