The most valuable application of FT-IR microscopy is in the chemical mapping of a sample of varying composition. For example, it can be used to study food materials, such as the chemical composition of wheat grains, and different layers in laminated plastic packaging.
FT–IR Spectroscopy is an extremely versatile analytical tool which can be used to analyse a wide range of organic materials, and some inorganics as well.
A particularly important application is the identification of foreign bodies reported from food, such as this fragment of plastic. The plastic is placed over the diamond window of the device and pressed down to achieve very close contact with the diamond: this close contact is essential for the system to work. Powders can be analysed by crushing them against the diamond crystal. Liquids are analysed by placing a small drop of the test liquid on the diamond crystal.
Results are displayed on the PC monitor. The computer then searches its library of known spectra of different materials to make an identification.
When pouches leak or lidding film fails to seal properly to a food tray, it is important to identify which layer of the material has failed. FT-IR microscopy can identify each of the plastic layers in order to trace the source of the problem.
A Focal Plane Array Detector [cooled by liquid nitrogen], enables us to analyse the different layers simultaneously and create a chemical map of the cross–section. The sample is first focussed under the beam, when the different layers can be seen. A germanium crystal is then placed in front of the microscope lens and the sample pressed gently against it.
The microscope then generates a chemical map of the cross–section which is seen here as a 3–D image, the different colours representing different layers of the plastic laminate. The infra-red spectrum of each layer can then be compared with library spectra to make an identification.