The Advantages of an Attenuated Total Internal Reflection Infrared Microspectroscopic Imaging Technique for the Analysis of Polymer Laminates

Abstract

Until recently, the analysis of polymer laminates using infrared microspectroscopy involved the painstaking separation of individual layers by dissection or by obtaining micrometer thin cross-sections. The latter usually requires the expertise of an individual trained in microtomy and even then, the very structure of the laminate could affect the outcome of the spectral results. The recent development of attenuated total internal reflection (ATR) infrared microspectroscopy imaging has provided a new avenue for the analysis of these multilayer structures. This report compares ATR infrared microspectroscopy imaging with conventional transmission infrared microspectroscopy imaging. The results demonstrate that the ATR method offers improved spatial resolution, eliminates a variety of competing optical processes, and requires minimal sample preparation relative to transmission measurements. These advantages were illustrated using a polymer laminate consisting of 11 different layers whose thickness ranged in size from 4-20 μm. The spatial resolution achieved by using an ATR-FTIR (Fourier transform infrared spectroscopy) imaging technique was diffraction limited. Contrast in the ATR images was enhanced by principal component analysis.

Analysis of Counterfeit Coated Tablets and Multi-Layer Packaging Materials Using Infrared Microspectroscopic Imaging

“…A portion of the tablets and an area of packing generally 1 × 3 mm was dissected and embedded in a 5-min two-part epoxy (Devcon, Danvers, MA, USA) using a process described by Lanzarotta et al (Lanzarotta et al, 2009, 2011Ling & Sommer, 2015) and later by Chen et al The medicinal tablet was dissected so that a portion of the outer coating and the tablet core could be imaged simultaneously. After setting, generally overnight, the sample blocks could be trimmed and microtomed (Reichert UltraCut, Leica Microsystems Inc., Buffalo Grove, IL, USA).…”

“…Although being close to the spatial resolution limit of the microscope, they were still able to image and obtain spectra from thin adhesive layers which they concluded were between 3 and 6 μm. Ling & Sommer (2015) recently demonstrated the benefits of the technique for the analysis of polymer laminates with up to 11 layers.…”