The photoacoustic spectra were measured at FTIR OPD mirror velocities from 0.1 cm/s to 0.5 cm/s. A wider range should also be suitable. The spectra were searched against the Perkin-Elmer SEARCH^TM library using the following procedure:

1. After the ratio of sample spectrum to carbon black spectrum is calculated, most FTIR data systems erroneously label the ordinate of the resulting spectrum as "transmittance". Scale this spectrum so that its maximum and minimum values are around 90% and 5% transmission, respectively.
2. Command the computer to relabel the ordinate in absorbance units.
3. Convert the spectrum to transmission via the FTIR software.
4. If necessary, rescale the spectrum so that the maximum and minimum are around 90% and 5% transmission, respectively.
5. Set the search threshold value at 5%.
6. Activate the search which should be based on peak positions only.

The success of this procedure was evident by higher search rankings that the Perkin-Elmer SEARCH^TM software assigned to searches of the photoacoustic spectra than to searches of the library spectra shown in Figs. 9 through 12.
Procedures similar to the one above should be used with other FTIR systems. A suite of known polymer samples should be used to provide a basis for any refinements that might be necessary for a particular FTIR software package.

9. Library and photoacoustic spectra of polycarbonate.