FTIR
Description of
Fourier Transform (FT) Infrared (IR) Method
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.
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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