252 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS .35 1200 1400 1600 1800 2000 2200 2400 Fim Figure 1. Apparent absorbance spectra of skin and water. intensities, plus a stronger band at 1879 nm. These wavelengths suggest slightly different assignments from those of Luck. The band at 1879 nm is due to weakly bonded water (free water), and may be the water giving rise to the evaporative flux across the SC. The 1890-nm band of skin corresponds to the strongest band of liquid water and will be referred to as bulk water, while the longer wavelength bands at 1909 and 1927 nm are due to more strongly bound water, which may be that associated with protein. The free water and protein-bound water are expected to be found within the stratum corneum, while bulk water, and possibly some protein-bound water, are expected to be found in the epidermal layer just below the SC. These assignments are summarized in Table I. EFFECT OF HUMIDITY ON SKIN WATER CONTENT Figure 3 compares the second derivative spectra of untreated skin at two humidities. At an absolute humidity of 3.7 g/m 3 (corresponding to 19% relative humidity at 22øC), the water bands near 1879, 1909, and 1927 nm shift to longer wavelength (stronger hydrogen bonding) compared to the spectrum at 8.2 g/m 3 (42% RH at 22øC) and become less intense, while the band at 1890 nm (bulk water) does not shift but becomes more intense. At lower humidity, therefore, the free and protein-bound water molecules are somewhat more strongly hydrogen-bonded. Figure 4 shows the correlation between absolute humidity and the second derivative intensities of free water (1879 nm) from the untreated control sites using data pooled from five separate studies. The absolute humidity in these studies is limited to the range corresponding to 3%-50% RH at 22øC. The increase in intensity at 1879 nm with

MOISTURE MEASUREMENT BY NIR SPECTROSCOPY 253 c nm Figure 2. Second derivative spectra of skin and water in the water combination region. increasing humidity indicates a greater concentration of free water and supports the assignment that this is water that is readily transported across the skin barrier and thus gives rise to the flux across the SC. In his review on measurements of water in skin, Potts noted that the SC thickens with hydration, providing a more substantial barrier to water loss (3). The maximum flux found in those studies, as measured by TEWL, was found at 30%-40% relative humidity (6.9-9.2 g/m 3) at 25øC. Figure 4 seems to correlate with this observation. The increases in more tightly bound water (1909 and 1927 nm) are shown in Figures 5 and 6. These parallel the increase observed for free water, suggesting that there may be free exchange between the two types of water. Bulk water (1890 nm), on the other hand, decreases with increasing humidity, as shown in Figure 7. Although the data for water at 1879, 1909, and 1927 nm were pooled from five studies without discontinuity, differences between the five clinical studies are apparent in the data at 1890 nm. The reason for the discontinuity may be related to minor changes in the thickness of the cushion between the detector port and the Table ! Assignments in Water Combination Region of Skin nm Assignment 1879 Free water 1890 Bulk water 1909 Protein-bound water 1927 Protein-bound water