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Attenuated Total Internal Reflection Infrared Microscopy of Multilayer Plastic Packaging Foils
Volume 61, Number 6 (June 2007) Page 593-602
van Dalen, Gerard; Heussen, Patricia C.M.; den Adel, Ruud; Hoeve, Robert B.J.
Multilayer plastic foils are important packaging materials that are used to extend the shelf life of food products and drinks. Fourier transform infrared (FT-IR) spectroscopic imaging using attenuated total internal reflection (ATR) can be used for the identification and localization of different layers in multilayer foils. A new type of ATR crystal was used in combination with a linear array detector through which large sample areas (400 × 400 μm2) could be imaged with a pixel size of 1.6 μm. The method was tested on laminated plastic packing materials containing 5 to 12 layers. The results of the identification of the different materials using ATR-FT-IR were compared with differential scanning calorimetry (DSC) and the layer thickness of the individual layers measured by ATR-FT-IR was compared with polarized light microscopy (LM) and scanning electron microscopy (SEM). It has been demonstrated that individual layers with a thickness of about 3 μm could be identified in multilayer foils with a total thickness ranging from 100 to 150 μm. The results show a spatial resolution of about 4 μm (measured at wavenumbers ranging from 1000 to 1730 cm−1), which is about a factor of two better than can be obtained using transmission FT-IR imaging. An additional advantage of ATR is the ease of sample preparation. A good correspondence was found between visible and FT-IR images. The results of ATR-FT-IR imaging were in agreement with those obtained by LM, SEM, and DSC. ATR-FT-IR is superior to the combination of these techniques because it delivers both spatial and chemical information.