186 JOURNAL OF COSMETIC SCIENCE 0.32 0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00 1935 1200 1400 1600 1800 2000 2200 Wavelength (nm) Figure 1. NIR absorption spectrum of human (Piedmont) hair and the effect of water. The solid lines represent the spectrum at 22øC and 50% RH. This hair sample was then heated at 110øC for 90 rain and the dotted-line spectrum results. Notice the significant changes in the 1440-nm and 1935-nm regions, which are associated, respectively, with the harmonic of the O-H stretch and a combination of O-H stretch and H-O-H bend. The other bands are likely due to protein. water. The small residual "bump" at 1935 nm after heating may either be due to small residual water that is very tightly bound, water uptake during measurement, some underlying protein band, or some combination of the above. Clearly the 2051-nm band is due to protein, since it is unaffected by heating. The band at 1984 nm is also likely due to protein. The data clearly show that of the two bands assigned to water, the 1935-nm band will be clearly more sensitive for moisture content studies since the interference from protein will be minimal at this wavelength and the absorption cross section is higher. It should be noted that the bands associated with water in hair are very similar to the corresponding bands observed in the NIR spectrum of the epidermis component of skin with respect to their positions and relative intensities (11). The above assignment of bands at ca. 1450 and 1935 nm to water is further substan- tiated by the results of the experiment in which the water in hair was replaced with D20 , the isotopic analogue. Since the mass of deuterium is twice that of hydrogen, the vibrational frequencies of O-D are different and lower in energy than the corresponding O-H vibrations. Thus, when hair treated with water is soaked in D20 , due to mass action the bound water will be replaced by D20. This, in turn, will be reflected in the NIR spectrum as a frequency shift of the water-related bands. The effect was most pronounced in the 1400-1500-nm and in the 1900-2000-nm regions. For example, the

NEAR-INFRARED SPECTROSCOPY 187 1935-nm water band decreased in intensity and a new band appeared at 2023 nm, as seen in Figure 2. This latter band has been assigned to the stretch/bend vibrations associated with deuterium oxide. The ratio of the intensities of the bands at 2023 nm to 1935 nm increased with increasing temperature and the time of incubation of the tress in D20 (data not shown). This suggests that this ratio could be used as an index of D20/H20 exchange in hair. This line of experimentation can be used to distinguish between the various types of bound water and the bulk water. The D20 for water exchange depends on the diffusion of D20 into the hair followed by the replacement of water already present in the hair. Since hair damage, either due to weathering or chemical processing, affects the hair integrity and damages the constituent protein, both .40 .35 .30 .25 .20 .15 .10 .05 .00 i [ 1935 1900 1950 2000 (2) 2023 2050 2100 Wavelength (nm) Figure 2. Deuterium oxide (D20)/water exchange in human hair. Spectrum 1 is due to hair soaked in water and towel dried. This was then soaked in D20 for 10 rain at 80øC, towel dried, and remeasured (spectrum 2). These data clearly suggest that the 1935-nm band in the hair spectrum is due to water. The ratio of the bands at 1935 nm and 2023 nm is an index of H20/D20 exchange and may be related to the structural integrity of the fibers.