Depth Profile Analysis of Keratin Fibers by FT-IR Photoacoustic Spectroscopy

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Abstract
The depth profiling capability of Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) has been used to distinguish spectroscopically between the surface layer (cuticle) and the interior (cortex) of intact keratin fibers. The thermal diffusion lengths of wool and hair fibers, calculated as a function of the wavenumber and optical scanning velocity of the interferometer, indicate a depth resolution of the order of a micrometer. Shifts in the amide I and II bands to lower wavenumbers were observed with increasing velocity of the interferometer, consistent with differences in protein structure between the bulk and surface regions of the fiber. On the basis of these spectral changes, the location of the cuticle/cortex interface below the fiber surface can be determined.