The middle spectrum of Fig. 25 is expanded so that the strongest band denoted by the star just above 1200 cm^-1 has the same height as this band in the bottom spectrum. Many of the absorbance band heights in the middle spectrum are now observed to exceed those in the bottom spectrum as, for example, the band at 1600 cm^-1. This observation should not, however, be interpreted as an increase in species concentrations with depth. It is instead due to inappropriate scaling, for comparative purposes, of one spectrum relative to the other.

2. Mylar-coated polycarbonate

The next example is a sample with a depth-varying composition consisting of a 2.5 micrometer thick coating of mylar on a polycarbonate substrate. The three top spectra of Fig. 26 show that as sampling depth is increased, more and more of the substrate spectra appear. As the FTIR mirror velocity is decreased from 1.5 cm/s to 0.5 cm/s and finally to 0.05 cm/s, the polycarbonate band at 1775 cm^-1 is seen to grow relative to the mylar band at 1725 cm^-1. Wherever there are bands of little or no spectral overlap, similar behavior is observed allowing differentiation between a number of substrate and coating features.

Fig. 26. FTIR-PAS spectra of a 2.5 m mylar coating on a polycarbonate substrate measured with increasingly deep-sampling depths going from mirror velocities of 1.50, 0.50, and 0.05 cm/s. Spectra of the mylar sheet and polycarbonate alone are shown for comparison with the upper spectra. Note the increasing prominence of the polycarbonate substrate band at 1775 cm^-1 as the sampling depth increases.