Phase references and cell effects in photoacoustic spectroscopy

In Fourier transform infrared (FT-IR) photoacoustic spectroscopy, advanced scanning and data-handling techniques have placed increasing emphasis on th e phase of the photoacoustic signal. Unfortunately, there is no agreement o n the best material to use as a phase reference. We have examined the frequ ency dependence of the signal from several candidate phase references and f ound that cell effects dominate the absolute phases and magnitudes observed . The absolute phase is exceptionally fast at low frequencies and exception ally slow at high frequencies because of the cell effects. Accordingly, det ails such as sample position must be scrupulously controlled to achieve acc urate, reproducible results. Because of the cell effects, no candidate mate rial behaves like an ideal phase reference. If relative phases are used, ho wever, glassy carbon comes closest to the ideal, differing from theory by n o more than 8 degrees at any frequency examined.