Photoacoustic spectroscopy effective sampling depth

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. 

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