Fig. 12. Library and photoacoustic spectra of polyethylene.

2. Adhesive spectrum by spectral subtraction Spectral subtraction

Spectral subtraction is a very useful procedure in qualitative analyses in order to separate one component from a background spectrum. If the sample is a two-layer system such as a coating on a substrate, spectra can be measured separately of the coated and bare sides followed by a spectral subtraction to isolate the coating spectrum. The FTIR mirror velocity should be set high enough to perform shallow sampling in order to increase the intensity of coating bands in the spectra. Fig. 13 shows spectra of an adhesive-coated polyethylene film material, the bare polyethylene and the adhesive spectra obtained by spectral subtraction. This spectral subtraction approach should be used when it is not possible to isolate the coating sufficiently by increasing mirror velocity alone.
The upper adhesive spectrum of Fig. 13 was obtained by a straight spectral subtraction of the two top spectra without scaling one relative to the other. This results in small negative pointing features denoted by stars in the upper adhesive spectrum due to a weakening of the polyethylene substrate photoacoustic signal by the adhesive coating. The weakening is a consequence of the decay of thermal-waves as they cross the coating before reaching the gas. This reduces the amplitude of the polyethylene bands in the top spectrum of the coated-side relative to the bare-side spectrum.