184 JOURNAL OF COSMETIC SCIENCE properties such as hold and feel. It is, therefore, very important to be able to measure the effects of hair care products on their ability to effect changes in the kinetic and equi- librium water-binding properties of hair. Human hair fibers derive their natural color from melanin pigment. Unlike synthetic hair colors that are mixtures of small molecules dispersed throughout the fiber, includ- ing the surface, the natural pigment is polymeric and exists as discreet granules only in the hair cortex. This structural difference between these two types of coloring matter has a profound influence on their optical properties. While the synthetic hair dyes do not absorb above 750 nm (red light), the natural pigment shows semiconductor-like optical properties and absorbs (scatters) light from the UV region all the way into the near- infrared, at least up to 1400 nm. NIR energy is relatively weak in causing electronic transitions to excited states in all but the highly delocalized electronic systems. The hair pigment melanin is unusual in this regard, and it has been suggested that its absorption does not fit the classical definition of chromophoric absorption and is more semiconductor-like (7). Typically, NIR absorp- tion by molecules results from overtones and combinations of the characteristic bond vibrations. For biological tissue, this region is dominated by bands due to O-H, mainly from water, and N-H from the protein backbone. NIR spectroscopy has been used extensively in skin research (8-11). Use of NIR second-derivative methodology to monitor the water content of hair has also been reported (12). Here we report the results of our study aimed at evaluating the potential of NIR spectroscopy as a tool in hair research. We find that this technique allows measurement of the water content of hair under hair-grooming conditions. Although these measure- ments provide relative water concentrations, they can be converted to absolute values by appropriate calibration. We have also exploited the difference in the light absorption characteristics of the natural hair pigment and the synthetic dyes to measure "lift," or the bleaching of the natural pigment due to the oxidative hair coloring process in the presence of the deposited dye. This latter aspect is particularly important when testing competitive products for which information on the ingredient concentrations is often lacking. EXPERIMENTAL Spectra were collected with a Magna 760 FT-IR system with NIR capability (Nicolet Instrument Co.), using the SAB-IR accessory. The latter is a bifurcated fiber-optic probe consisting of a ca. 3-mm fiber-optic bundle in which the fibers that bring in the incident light and those that take the reflected light to the detector are randomly arranged. The assembly tip is encased in a screw-in cover with a Sapphire angled window to cut down the specularly reflected light. The system uses a PbS detector and runs under the OMNIC © operating system. Background correction was performed using Spectralon ©. Typically 32 scans (38 sec) were collected for each sample at a resolution of 8 cm-•. The data were collected in the reflectance mode. Absorption curves were generated as log (l/R). Baseline correction was performed on the spectra using the packaged routine. In reflectance measurements, both absorption and diffuse scattering of the incident light contribute to the observed signal. The depth of penetration by light depends on the wavelength and the nature of the sample. In biological materials, for example, the longer wavelength NIR radiation will penetrate more deeply than the ultraviolet light. Mea-

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