VI. CONCLUSION

PAS provides the FTIR spectroscopist with a rapid non-destructive means of directly obtaining spectra of materials without traditional sample preparation to reduce opacity. The PAS method is applicable to all types of samples in macro and micro forms. FTIR-PAS spectra have the same absorbance peak wavenumber locations as classic transmission spectra but usually have truncation of strong absorbance bands due to photoacoustic signal saturation. The presence of band truncation, however, has not been found in applications explored to date, to limit the capability of FTIR-PAS in either qualitative or quantitative determinations based on commercial FTIR search and factor analysis software, respectively.
FTIR-PAS has the unique capability of being able to vary sampling depth by changing the modulation frequency of the FTIR beam. This capability allows, for instance, measurement of spectra with either high surface specificity to analyze a coating, or with bulk specificity to observe the absorbance bands of a substrate.
Other important aspects of FTIR-PAS measurements include operation over all spectral regions, absence of interference fringes, elimination of microsample pressing and aperturing, and capability to switch between sampling modes by interchanging photoacoustic detector sampling heads for DRIFTS, PAS, and transmission analyses.

VII. ACKNOWLEDGEMENTS

This work was funded by MTEC Photoacoustics, Inc.; by the Center for Advanced Technology Development, which is operated by Iowa State University for the U.S. Department of Commerce under Grant No. ITA 87-02; and by Ames Laboratory, which is operated by Iowa State University for the U.S. Department of Energy under Contract No. W-7405-ENG-82, and supported by the Assistant Secretary for Fossil Energy.
We wish to thank the following people for supplying samples used in this work: J.B. Callis (Univ. of Washington), L. Bright (DuPont), H.L.C. Meuzelaar (Univ. of Utah), M.W. Tungol and E.G. Bartick (FBI), P. Milne (Univ. of Miami), P.J. Codella (General Electric), B.J. Slomka and W.H. Buttermore (Ames Laboratory), and R. Kellner (Technical University of Vienna).