D. Variation of Sampling Depth

In Section II signal generation considerations related to sampling depth variation were discussed and an approximate expression for the sampling depth of FTIR-PAS measurements was introduced. In this section, examples are presented where spectra are acquired with different sampling depths by varying FTIR mirror velocity. The first example deals with a homogeneous sample of polycarbonate in order to show that some spectral changes which occur with varying sampling depth are due solely to changes in the degree of absorbance band truncation that appears when the sampling depth is changed. The other examples deal with samples which do have varying compositions with depth.

1. Homogeneous polycarbonate

The top and bottom polycarbonate spectra of Fig. 25 are measured at deep- and shallow-sampling depths, respectively. The top spectrum is reduced so that the heights of the starred small absorbance band just above 2200 cm^-1 are equal in both the top and the bottom spectra. This band is so weak that it is expected to have little, if any, truncation in either spectrum and consequently is a good feature to use in establishing a consistent scale for these two spectra. After this scaling, it is easy to observe the very significant truncation of strong bands that occurs in FTIR-PAS spectra measured with deep sampling.

Fig. 25. FTIR-PAS spectra of polycarbonate measured at OPD mirror velocities of 0.05 cm/s (deep sampling depth) and 0.5 cm/s (shallow sampling depth).