A comparison of infrared spectra collected in both a macro sample configuration and on a microscope accessory was conducted. The results of this comparison demonstrated that as the microscope aperture was reduced, absorbance values increased for a given polymer film by as much as 33% as compared to the same data collected in the corresponding macro experiment. A detailed evaluation of the optical pathlength for both the micro and macro configurations was conducted. Results of this evaluation and that of detector nonlinearity showed that the latter effect was most problematic. During the course of the investigation the average angle of light traversing through the objective was determined for several infrared microscopes. Further analysis indicated that changing the aperture size of the microscope can have significant influence on the absorbance values obtained. In addition, extinction coefficients of poly(methyl methacrylate), polystyrene, poly(vinyl acetate), polycarbonate, and poly(ethylene terephthalate) were determined using macro FT-IR spectroscopy.
Louis G. Tisinger and André J. Sommer, "Extinction Coefficient Determination of Polymeric Materials and Their Use in the Optical Characterization of Infrared Microscopes: A Preliminary Investigation into Quantitative Infrared Microspectroscopy," Appl. Spectrosc. 56, 1397-1402 (2002)