Photoacoustic opaque materials

The photoacoustic effect was first discovered by Alexander Graham Bell in the early 1880s (27), but not applied to FTIR spectroscopy until a century later (28,29). Significant advantages of FTIR photoacoustic spectroscopy (PAS) include (a) spectra may be acquired on opaque materials (commonly found in pharmaceutical formulations), (b) minimal sample preparation is necessary, and (c) depth profiling is possible. 

The measurement of very small absorption coefficients (down to lO-5 cm-1) of optical materials has been carried out by laser calorimetry. In this method, the temperature difference between a sample illuminated with a laser beam and a reference sample is measured and converted into an absorption coefficient at the laser energy by calibration [13]. Photoacoustic spectroscopy, where the thermal elastic waves generated in a gas-filled cell by the radiation absorbed by the sample are detected by a microphone, has also been performed at LHeT [34]. Photoacoustic detection using a laser source allows the detection of very small absorption coefficients [14]. Photoacoustic spectroscopy is also used at smaller absorption sensitivity with commercial FTSs for the study of powdered or opaque samples. Calorimetric absorption spectroscopy (CAS) has also been used at LHeT and at mK temperatures in measurement using a tunable monochromatic source. In this method, the temperature rise of the sample due to the non-radiative relaxation of the excited state after photon absorption by a specific transition is measured by a thermometer in good thermal contact with the sample [34,36]. 

Two FT-IR sampling methods are commonly employed to measure IR spectra of mixture components separated by thin layer chromatography (TLC). Diffuse reflectance and photoacoustic IR spectroscopy are techniques that can be employed when sample materials are opaque. 

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