The intense peak at 1540 cm-1 is most likely the optical thin film peak corresponding to the inner coating on the paper sample, as this peak was not observed in the ATR – IR measurements (compare to figure 2).The three peaks at 1180 cm-1, 1120 cm-1
and 1076 cm-1 are in the fingerprint region. Therefore, one has to take into account that here an overlap between chemical peaks and optical thin film phenomena might
occur. The peak at 1076 cm-1 corresponds best to the Berreman peak observed in ATR – IR (compare to figure 2), taking the overall shift of the IR spectra between the
ATR – IR and IRRAS measurements into account.