Experimental and statistical validation of HPLC analysis of hydroquinone and its 4-methoxyphenol, 4-ethoxyphenol and 4-benzyloxyphenol ethers in cosmetic products

The isocratic HPLC analysis of hydroquinone and its derived ethers in cosme tic preparations, as described in the sixth Directive 95/32/EC of the Europ ean Commission, was investigated with a view to determining the fundamental validation parameters of the technique. The calibration lines are highly l inear with coefficients of determination > 0.9996; the slopes of the calibr ation equations as well as the slopes normalised to the standard deviations indicate increased sensitivities for all compounds. The method is very rep roducible; both the repeatability and the intermediate precision confirm a system performance of < 2% variation; the limits of repeatability and inter mediate precision are consistently < 6 % for every compound studied. The de tection limits correspond to concentrations of 0.03 and 0.02 % (m:m) for hy droquinone and the derived ethers, respectively. Application of the respons e factor calculation gives reliable results and signifies a practical and e conomic advantage when compared to laborious calibration with several repli cate standard injections. Comparison of the response factor and calibration techniques did not reveal significant differences. The obviously insuffici ently efficient extraction preceding chromatographic analysis might constit ute the only drawback of the method, especially when the concentrations app lied bolder the allowable.