FT-IR-PAS has been used to study minerals, monodisperse polymer beads, and organic crystals of size ranges varying from 5 μm up to several hundred μm diameter. In results that are to be expected, it has been found that the FT-IR-PAS spectra show a fairly complicated dependence on size at a resolution of 8 cm<sup>−1</sup> for polymer beads up to a diameter of 35 μm. AC Fine Test Dust samples in cuts 5 μm up to 35 μm were found to have FT-IR-PAS spectra that varied with diameter of the sample by as much as 5% of the intensity of the bands. This was thought to be due to chemical inhomogeneity exaggerated by the process of aerodynamic sizing of the samples. The FT-IR-PAS spectra of para-nitrophenol samples showed a decrease in band intensity and an increase in band width which was most dramatic when the average crystal diameter exceeded about 75 μm. It has been concluded that FT-IR-PAS spectroscopy suffers from the same limitations of spectral distortion with particle size as has been observed for visible photoacoustic spectra and for diffuse reflectance (DRIFT) spectra.
N. L. Rockley, M. K. Woodard, and M. G. Rockley, "The Effect of Particle Size on FT-IR-PAS Spectra," Appl. Spectrosc. 38, 329-334 (1984)