ATR FT-IR Imaging of Human Hair Cross-Section

Human hair is typically 50-100 μ in diameter, previous transmission and ATR studies of its cross section1,2 reveal three major identifiable regions. The core, or medulla, typically 5-10 μ in diameter, is surrounded by the cortex, making up the bulk of the hair and finally an outer layer or cuticle, typically less than 5 μ in thickness. Biochemical compositional studies may be performed by examining the IR spectra in the –C-H stretching region (ca. 3000 cm-1), and amide (I) and (II) regions like (ca. 1700-1400 cm-1). Using single point IR transmission microspectroscopy however, neglecting sample and instrumental effects, spatial resolution is ultimately limited by the wavelength of the IR radiation ca. 6-11 μ in the fingerprint region. Furthermore, to obtain relatively pure transmission spectra of the core area, very small (< 10 μ) apertures are required. Signal-to-noise and alignment problems can become a significant issue, and consequently, this study has been subject to IR microspectroscopy using a synchrotron as the radiation source3. However, this is clearly a highly specialized option.