A Practical Guide to FT-IR Photoacoustic Spectroscopy
This chapter discusses Fourier Transform-Infrared/photoacoustic spectroscopy (FT-IR/PAS) signal generation, instrumentation, sample handling, and results obtained with various classes of samples. FT-IR/PAS spectra usually contain bands with infrared absorption coefficients in the ranges both before and after the onset of saturation, but spectral features due to reflectivity effects associated with full saturation are a very minor presence and often are not discernable. The signal-to-noise ratio of FT-IR/PAS measurements depends on the performance level of the FT-IR spectrometer as well as the detector sensitivity and noise level and signal generation efficiency of the sample. FT-IR/PAS allows spectra of polymers in powder, pellet, sheet, and chunk form to be directly measured and searched against standard commercial spectral libraries. The best FT-IR/PAS spectra of microparticles are obtained with low mirror velocities, a helium gas atmosphere, and adequate time for the desiccant to work. FT-IR/PAS has the unique capability of being able to vary sampling depth by changing the modulation frequency of the FT-IR beam.