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.

188 JOURNAL OF COSMETIC SCIENCE these processes are likely to be affected. It is, therefore, interesting to speculate that the kinetics and equilibrium parameters of this exchange may provide information on the structural integrity of the fibers. Figure 3 shows the effect of moisture on the NIR spectrum of hair in the 1900-nm region. This figure clearly shows that the intensity of the band at 1935 nm can be used to measure both the dehydrating effects of heat, as in blow-drying or upon using a curling iron, as well as in water uptake by dried hair. It should be noted that heating the hair to 110øC for 90 min did not cause any apparent irreversible change in the water-binding properties of hair and that the process was totally reversible. Also, the bands due to protein that are used to normalize the data were also unaffected. This suggests that no apparent damage to protein, as judged by these markers, occurs under these conditions. A previous study on the use of NIR to measure water content of hair used second-derivative methods (12). We have chosen to use baseline subtraction of the raw spectra as the method of choice because it is simple, straightforward, and as the data show, not influenced significantly by the protein bands. The derivative method, we feel, is complicated when deconvolving bands of differing bandwidths. Hair treatments, particularly the "leave-in" kind, may affect the water-binding proper- ties of hair. Mechanistically, this may result from one or more of a variety of effects, such as the interaction of the product with the hydrophilic sites in hair, or be due to the hydrophobicity of the product, etc. The result of such an experiment is seen in Figure .40 .30 .20 .10 .00 (6) O) (5) (4) (2) i L • 1900 1950 2000 2050 2100 Wavelength (nm) Figure 3. Changes in the ambient hair spectrum (1) upon heating at 110øC for 90 min (2), followed by water regain in a room, maintained at 20øC and 50% RH, after 5 min (3), 10 min (4), and 75 min (5). The hair tress was then soaked in water and dabbed with a paper towel, and the spectrum was measured (6).