A Novel Approach for Study of in Situ Diffusion in Human Hair Using Fourier Transform Infrared Spectroscopic Imaging

Diffusion of chemicals into human hair fibers has been of interest to the cosmetic industry due to the process of various hair treatments and hair damage assessments, which often involve the diffusion of active compounds into the hair fibers. Various approaches have been applied to study the diffusion of substances into the hair fibers, such as electron microscopy and fluorescence microscopy. Complementary to these approaches is vibrational spectroscopy, which provides chemical information about the sample based on its molecular vibrations. Photoacoustic and confocal Raman has been applied to obtain the depth profile of hair and has been used to study the diffusion of chemicals into hair fibers. The advantage of this method is that depth profiles can be readily obtained without any physical damage (cross-sectioning) to the hair fiber. However, the color of the hair fiber that can be studied is limited to very pale colored or white due to fluorescence of the pigment of colored hair. Despite little differences between the physical properties of gray and black hair, previous studies have shown that the color of hair fibers could be an important factor in studying of the sorption of explosives and incorporation of some drugs into hair. Furthermore, depth profile spectra are measured non-simultaneously in these types of measurements, requiring a relatively long time to collect (up to one hour per depth profile). This limits study to essentially static systems. Infrared spectroscopic imaging using a focal plane array (FPA) detector, on the other hand, provides a means by which to measure all spectra simultaneously in less than one minute (depending on the size of the array detector and the number of scans used), permitting the study of dynamic systems in situ. Fourier transform infrared (FT-IR) imaging has been applied to study the diffusion of solvents in polymer films. Previous studies have shown that FT-IR imaging measurement can be applied both in the micro-attenuated total reflection (ATR) and transmission modes to study hair samples. Despite the necessity of cross-sectioning the samples, the advantages of having in situ measurements with FT-IR imaging could outweigh this disadvantage. This work demonstrates the opportunity to measure the diffusion of different chemicals in cross-sections of hair fibers using FT-IR imaging. The diffusion of four substances, acetonitrile, 4cyanobenzoic acid, 4-cyanophenol, and 1-pentanenitrile, into the cross-sectioned hair fiber were imaged as a function of time. The study of these chemical compounds allows the comparison of the diffusion phenomenon in human hair with molecules of different sizes and functional groups.