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j. Cosmet. Sci., 50, 231-248 (July/August 1999) Influence of matrix compounds on the analysis of oxidalive hair dyes by HPLC URSULA VINCENT, GUY BORDIN, and ADELA R. RODRIGUEZ, European Commission, Joint Research Centre, Institute for Re•rence Materials and Measurements, Retieseweg, B-2440 Geel, Belgium. Accepted for publication June 30, 1999. Synopsis Oxidative hair dyeing products consist of a mixture of a broad spectrum of organic compounds including the hair dye's so-called active compounds and the matrix-forming compounds. Forty-seven dye intermedi- ates commonly used in cosmetic formulations have thus been analyzed by RP-HPLC, and their chromato- graphic characteristics have been recorded. Since the matrix compounds could interfere with the quantitative analysis of the active compounds, eighteen matrix compounds commonly used in cosmetic formulations have been tested for their influence on dye intermediate determination. Since some of them do affect the chromatographic behavior of the dye intermediates, an isolation procedure, for separating matrix compo- nents from the dye-forming compounds, consisting of a liquid-liquid extraction by n-heptane, has been set up. In most cases, this procedure is effective for the extraction of the matrix products. Moreover, the dye intermediates are not extracted by n-heptane, and their chromatographic behavior is not altered by the extraction procedure. In summary, this study has shown that a reference method for the analysis of oxidative hair dyes should include a compulsory extraction step before submission of samples to RP-HPLC. INTRODUCTION Among the different methods of changing the color of human hair, oxidative hair dyeing plays an important role. Formulations consist of a wide range of organic compounds of two distinct types, i.e., the hair dye intermediates and the matrix-forming compounds. Some of the hair dyes, which are available for use in hair dye formulations, have a toxicological or sensitizing potential and are prohibited or restricted in concentration by the 6th Amendment of the European Union Council Directive 93/35/EEC. To identify and quantify substances used in hair dye formulations, with the aim of implementing the European Union Cosmetic Directive, there is need for a reliable analytical method. Initially, a high-performance liquid chromatography method was developed that enables the identification of a broad range of dye-forming compounds in standard solutions (1,2). The next step for the setup of a reliable analytical method, besides additional optimization steps, involves the investigation of possible effects of matrix components. 231