The deep-sampling spectrum measured at an OPD velocity of 0.05 cm/s reveals additional absorbance bands of the paper such as the feature at 900 cm^-1. The DRIFTS spectrum, plotted in Kubelka-Munk units probes still deeper into the paper and shows absorbance bands including the one at 3700 cm^-1 that are not observed in the photoacoustic spectra. The DRIFTS spectrum also contains artifacts as, for instance, the dip at approximately 1260 cm^-1. This feature is due to absorbance by the plastic coating and should be a peak rather than a dip. In spite of some artifacts, the information in the DRIFTS spectrum gives spectral information from deeper within the sample than FTIR-PAS probes at a mirror velocity of 0.05 cm/s. New step- scan FTIR interferometers provide considerably lower mirror velocities which will extend FTIR-PAS sampling depth by over a factor of 3. This advance will yield sampling depths comparable to or greater than DRIFTS on many classes of samples.

Fig. 29. FTIR spectra of a plastic-coated spectra measured by PAS and DRIFTS sampling. The sampling depth of the spectra increases from the bottom to the top of the figure.