NEAR-INFRARED SPECTROSCOPY 185 surements on skin have revealed that NIR radiation can easily penetrate the thickness of hair, which is approximately 50 pm in diameter (10,11). It should be noted, however, that since natural hair pigment absorbs NIR radiation (see below), it will attenuate the incident beam, thereby affecting the apparent absorption due to water, compared to the unpigmented hair. Thus comparative measurements should be made only on hair of similar color. Measurements were either made on human heads or on hair tresses. The tresses were typically 6" in length and were made from Piedmont hair, dark brown hair, or from blended gray hair, all obtained commercially (DeMeo Brothers, New York). Hair drying at precise temperatures was carried out in a convection-air oven. Moisture uptake was studied by allowing hair tresses that had been partially dehydrated in an oven to pick up moisture in a humidity- and temperature-controlled room. Deuterium oxide/water exchange in hair was studied by incubating hair in a 99.9% D20 solution at various temperatures and for various times in an oven. The experiment, involving evaluation of the effect of a leave-in treatment on the water- binding properties of hair, was performed on a Piedmont hair tress. This test product contained the following ingredients: amodimethicone, cyclomethicone, panthenol, to- copherol, hydrolyzed protein, polyquaternium 37, polyquaternium 11, dicaprylate/ dicaprate, tallowtrimonium chloride, nonoxynol-10, phenoxyethanol, propylene glycol, and water. A tress was shampooed and dried with a towel. The lower half of the tress was dipped in the product, and the excess was squeezed out. This tress was then blow dried, combed, and heated in an oven at 93øC for 30 min. Subsequently this was transferred to a humidity-controlled room and measurements were made on the treated and un- treated sites. Two measurements were made on each site, and the data collection was done in a manner that scrambled any sampling preference. Care was taken to insure that the measurement sites on the treated and untreated portions of the tress were not separated by more than 1.5", to minimize intrinsic differences in hair properties due to weathering. An average value for each site was obtained and the data were collected periodically for 4 h. An "infinite time" measurement was made the following day, after which the sample was heated at 120øC for 30 min to provide a baseline ("no water") reading. Hair dyeing was performed on tresses using the dark brown shades of two commercial permanent hair-coloring products according to package instructions. RESULTS AND DISCUSSION The NIR spectrum of human hair under ambient conditions of 22øC and 50% RH is shown in Figure 1. The shoulder at 1450 nm (6896 cm -•) in the spectrum most likely is the first overtone of the O-H stretching vibration of water observed in the mid-IR spectra at about 3450 cm -•. The doublet at about 1740 nm is the overtone of the methylene C-H stretch of protein side chains and lipids. In the 2000-nm region, the hair spectrum shows a strong band at 1935 nm, a shoulder at about 1984 nm, and an overlapping band at 2051 nm. The 1935-nm band has been assigned to a combination of the O-H stretch and H-O-H bending vibrations of water (12). The main changes in the hair spectrum upon heating occur at about 1450 nm and 1935 nm, due to loss of

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