Study on photoacoustic signals from powdered samples

Photoacoustic (PA) signals (amplitude and phase) from powdered samples are examined as a function of the effective absorption coefficient (k) of powder. Variation of the k value of a sample is accomplished by dilution of the sample in a diluent (BaSO4) with various degrees of dilution or by variation of the excitation wavelength for undiluted samples. It is determined that every powder studied shows a minimum in its PA phase vs k curve. A current theory of powder PA spectroscopy explains the presence of the phase minimum using a composite mechanism where the periodic expansion of buffer gas by heat transferred from the sample and the mechanical vibration of the sample ore assumed as sources of the PA signal. The authors propose a method of estimating the relative value of k in a bulk sample from the phase minimum observed using the dilution method for the powdered sample.