has been plotted as a transmission spectrum using the procedure discussed in Section V.A.1. Many of the small-and medium-sized features of the conventional transmission spectrum are so seriously distorted that they are not discernible for a qualitative analysis and that observable features are not useful for quantitative analyses. For instance, it is not possible to observe the small band at 2330 cm^-1 in the transmission spectrum and the larger band at 1100 cm^-1 is not suitable for quantitative analysis due to the fringe bands.

Fig. 33. Conventional transmission and FTIR-PAS spectra of a 32 micrometer thick polyethylene film. The PAS spectrum has been converted to transmittance for the purpose of comparison. Note the absence of interference fringes in the PAS spectrum.

2. Polarized beam measurements on oriented films

In production, polymer fibers are oriented by drawing in order to improve tensile strength. Polarized FTIR-PAS measurements are useful in monitoring the orientation process. A polarizer is placed in front of the photoacoustic detector so that the infrared beam incident on the sample in the detector's sample holder is polarized. It may be useful to initially rotate the polarizer until the signal is maximized with a carbon black reference in the detector. This procedure will ensure that the polarizations imposed by the FTIR optics and the polarizer are in coincidence resulting in maximal intensity in the polarized beam.
After this adjustment, the polarizer should be left fixed and the sample should be rotated in the sample cup. Figure 34 shows spectra of a PET film with the infrared beam polarized parallel and perpendicular to the direction of draw. A number of spectral changes are observed as a function of orientation particularly in the 800-900 cm^-1 and 1300-1400 cm^-1 ranges. The absence of interference fringes in FTIR-PAS spectra is obviously also an advantage in this application.