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J. Cosmet. Sci., 60, 519–525 (September/October 2009) 519 High-performance liquid chromatography with dual- wavelength ultraviolet detection for measurement of hinokitiol in personal care products YASUHIKO HIGASHI, MASATOSHI SAKATA, and YOUICHI FUJII, Department of Analytical Chemistry, Faculty of Pharmaceutical Sciences, Hokuriku University, Ho-3, Kanagawa-machi, Kanazawa 920-1181, Japan. Accepted for publication March 26, 2009. Synopsis Hinokitiol is found in the heartwood of several cupressaceous plants and is frequently added to cosmetic products such as hair restorers, skin lotions, and body soaps because of its potent and broad-spectrum anti- bacterial activity. In this study, we established a simple method of hinokitiol determination by high-performance liquid chromatography (HPLC) with dual-wavelength ultraviolet detection at 240 and 345 nm, using a reversed-phase C4 column (RP-4). The retention time of hinokitiol was 7.1 min at both wavelengths. The value of the symmetry coeffi cient of the hinokitiol peak was close to 1 when the RP-4 column, not an RP-8 or RP-18 column, was used. With the RP-4 column, the regression equation for hinokitiol showed good linearity in the range of 0.05–5 μg/ml, with a detection limit (signal-to-noise ratio of 3) of 0.005 μg/ml at 240 nm and 0.01 μg/ml at 345 nm. The coeffi cients of variation at 240 and 345 nm were less than 8.2% and 8.7%, respectively, and the recovery was good. The proposed method was used for the determination of hinokitiol in commercial hair restorers, skin lotions, and body soaps. INTRODUCTION Hinokitol (Figure 1, β-thujaplicin, 4-isopropyl-2-hydroxycyclohepta-2,4,6-trien-1-one) is a naturally occurring toxic compound belonging to the class of tropolones that contain an unsaturated seven-membered carbon ring. The compound is found in the heartwood of several cupressaceous plants, such as western red cedar (Thuja plicata), eastern white cedar (Thuja occidentalis), hinoki cypress (Chamaecyparis obtusa), and hiba (Thujopsis dola- brata) (1,2). Since hinokitiol has potent antibacterial and antifungal activities (minimum inhibitory concentration of 0.2 μg/ml for Staphylococcus epidermidis and Daedalea dickinsii) (2,3), it is added to products such as hair lotions, skin lotions, and body soaps, among other cosmetics. Little work has been reported on the gas chromatography (GC) or high-performance liquid chromatography (HPLC) of hinokitiol because the tropolone ring has a chelating Address all correspondence to Yasuhiko Higashi.