The Effect of Particle Size on FT-IR-PAS Spectra

Abstract

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−1 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 paranitrophenol 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.