Determination of hydroquinone in cosmetic emulsion using microdialysis sampling coupled with high-performance liquid chromatography

A method based on microdialysis sampling combined with high-performance liquid chromatography (HPLC) has been developed for the determination of hydroquinone in cosmetic emulsion. In microdialysis sampling, using deionized water as perfusate, a probe length of 10mm and dialysis flow-rate of 5 microl min(-1) were found to be the optimum conditions for the analysis. The accuracy (% bias) for intra-day (n=3) and inter-day (n=12, four consecutive days) ranged from -9.5 to 13.0% with a precision less than 7.55% relative standard deviation (R.S.D.). Recovery obtained between 89 and 112% by adding hydroquinone standards into medicated cosmetics and the coefficients of variance between 0.5 and 3.3%. The linearity of calibration curve is in the range of 2 microM-2 mM with an R(2) value for the linear regression of 0.9981. The detection limit is 0.2 microM derived from the three times of signal-to-noise ratio. This method has been applied to determine the hydroquinone content in medicated and non-medicated cosmetics. The contents of hydroquinone in the medicated cosmetics are 5.36% (0.12% R.S.D., n=3) and 3.85% (0.26% R.S.D., n=3), respectively; in contrast to the percentages given by the manufacturer are 5 and 4%, the accuracy (% bias) are 7.20 and 3.75%, respectively. No hydroquinone was detected in non-medicated cosmetics. The proposed method has the advantages of easy sample pretreatment, rapid isolation and lower organic solvent consumption than in other methods for the determination of hydroquinone in cosmetics.