NEAR-INFRARED SPECTROSCOPY 189 4. It compares the results obtained with the untreated (control) half of a hair tress with the other half of the tress treated with a leave-in conditioning product. After the treatment the tress was heated at about 90øC for 30 min, following which the mea- surements were made (see Experimental section for details). As is clearly seen, there appears to be more water in the treated portion of the tress right after heating and for short water-take times, after which the two seem to converge. The data obtained after 24 h (infinite time) were indistinguishable from those obtained after 4 h, and have not been shown in the figure for clarity. The data obtained at 4 h at this humidity may, therefore, represent the equilibrium amount of water under these conditions. Thus, these data show that this product allowed more moisture to be retained in hair under the drying conditions of high temperature, similar to those encountered during blow-drying 100 - • 90 •- 70 o 60 o 0• 50 40 r'Y 30 0 50 100 150 200 Time (min) Figure 4. The water regain by hair as a function ooe time. The hair samples were heated to 93øC for 30 min, following which they were measured in a room maintained at 20øC and 50% RH (see text for details). The 1935-nm band was normalized against a water-insensitive protein band. The open circles represent the treated site, while the open squares represent the untreated control. Notice that at early times the treated sample contains more water than the untreated control, but that the two converge as the system approaches equilibrium.

190 JOURNAL OF COSMETIC SCIENCE and heat setting. At this time we do not have a mechanistic interpretation for these findings. It should be noted that water uptake measurements are typically performed gravimetri- cally under isothermal conditions with RH as the only variable. The gravimetric method, by its very nature, is susceptible to interference by the convection currents set up during changes in temperature. Volumetric methods, which are less frequently used, are also performed under isothermal conditions. Furthermore, these methods cannot be performed on live heads. Our measurements, on the other hand, are not "real" equilib- rium uptake measurements. The heated sample is allowed to relax to a new final RH and temperature. Two variables, temperature and RH, are changed simultaneously, so in that sense it is a composite of a temperature-jump and an RH-jump experiment, both of which affect the water content of hair. Superimposed on this complexity is the hysteresis associated with the heating of hair, which is well documented in the literature (1,2). Nevertheless, this protocol models the real-life situation better than the isothermal measurements. Also, the "infinite time" measurements represent true equilibrium. The data presented above show that this method has the sensitivity to allow identifi- cation of materials that would alter the water-binding property of hair. In the later stages of product development, such experiments can be performed either on hair tresses or directly on live heads in a controlled-humidity environment, for product optimization and for claims substantiation. We have also found this technique to be extremely useful in the characterization of oxidative hair-coloring products. The measurement of "lift" (melanin bleaching) during oxidative coloring, particularly with dark brown and black shades, is not possible with a typical colorimeter due to interference from the dyes themselves. A way around this problem is to use a dyeless base. This strategy does not work when evaluating com- petitive products. As noted in the Introduction section above, the synthetic hair dyes do not affect the reflectance properties of hair beyond 750 nm, while the natural hair pigment also absorbs the NIR radiation. This difference in the light absorption char- acteristics of the natural hair pigment and the synthetic dyes can be exploited to measure bleaching or "lift" produced during oxidative hair coloring, without interference from the deposited hair color, using NIR spectroscopy. Figure 5 shows the tail end of the absorption of human hair. Spectrum 1 is from black hair from one of the authors (C.P., Asian Indian) while spectra 2 and 3 are from commercial dark brown and blended gray hair, respectively. The effect of pigmentation in this region of NIR wavelengths is clear when one compares the above spectra with spectrum 6, which is due to Piedmont hair that lacks melanin pigment.* These data reveal that the darker the natural hair color, the higher the absorption in this region. It would follow that this region of the spectrum could be used to follow changes in the natural pigment concentration in hair. Indeed, spectra 4 and 5 represent blended gray hair (spectrum 3) dyed with dark brown shades of two oxidative hair-coloring products. It is clear from the spectra that oxidative coloring reduced the intrinsic melanin con- centration in the blended gray hair sample. Furthermore, the product corresponding to * The band at ca. 1285 nm, seen clearly in the Piedmont hair spectrum and only as a shoulder in the other spectra, is due to protein backbone.