Fig. 13. Spectra of an adhesive-coated polyethylene material, of bare polyethylene and of the adhesive obtained by spectral subtraction. The upper subtraction spectrum was obtained by a direct subtraction and the lower by interactive subtraction to remove the negative features at the starred locations.

The lower adhesive spectrum of Fig. 13 was obtained by interactive spectral subtraction with a small scale change between the two top spectra to correct for the quenching phenomenon prior to subtraction. The correction to the starred features of the lower adhesive spectrum is not exact, probably due to a phase shift associated with the transit time of the thermal-wave crossing the coating. These small remaining artifacts have a derivative-like character associated with slight band shifts. Such small spectral distortions, however, do not prevent a spectral search from predicting the primary adhesive component to be vinyl acetate.
Interactive subtractions may also be necessary when non-layered samples of varying composition are analyzed if their thermal properties vary significantly with composition. When interactive FTIR-PAS spectral subtraction is used, it is necessary with many FTIR data systems to change the data file ordinate label from transmittance to absorbance units prior to performing the interactive subtraction.