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A method for determining ethinylestradiol (ETE) and levonorgestrel (LEV) in mixtures by first-derivative spectrophotometry is described. The procedure does not require any separation step. Measurements are made at the zero-crossing wavelengths and the calibration graphs are linear up to 26 and 33 micrograms ml-1 of ETE and LEV, respectively. The method was applied to the determination of both compounds in five different Spanish commercial low-dose oral contraceptives. Similar results were obtained by an HPLC method.

A micellar electrokinetic chromatograpliic method which permits the separation of very similar compounds such as ethinylestradiol (ETE), gestodene (GTD), (-)norgestrel (LEV) and p-estradiol valerate (EST) from synthetic mixtures is described. The study was carried out at 25°C and 20 kV, using a 15 mM borate •Tel. 2454 BERZAS, CASTANEDA, AND PINILLA buffer (pH 9.2), 15 mM sodium dodecylsulfate (SDS) and 26 % acetonitrile-water as background electrolyte. The analysis takes about 12 minutes.