Fig. 1.

Schematic illustration of the hair structure and different infrared spectral measurements. (a) The internal hair structure formed by three different parts: the inner medulla, the middle cortex layer, and the outmost cuticle layer. (b) The ATR reflection mode and (c) the transmission mode of FTIR experiments. The evanescent wave is generated in the total internal reflection of the ATR mode and penetrates into the sample (blue area) with a depth of 0.5 to $2\mu \mathrm{m}$, which makes the ATR-FTIR a surface-sensitive technique. The transmission mode of FTIR directly measures the infrared beam transmitting through the sample, which probes the overall bulk property. Gray trapezoid denotes the ATR prisms.