Determination by liquid chromatography–mass spectrometry of clomiphene isomers in the plasma of patients undergoing treatment for the induction of ovulation

A rapid, sensitive and selective LC–MS method is described for the simultaneous determination of zuclomiphene and enclomiphene in plasma from patients undergoing treatment with clomiphene citrate for the induction of ovulation. Samples spiked with N-didesmethyltamoxifen, the internal standard, were extracted into methyl tertiary butyl ether. The compounds were separated on a Luna C18 analytical column, and a mobile phase of methanol–water (70:30 v/v) containing 0.05% trifluoroacetic acid at a flow rate of 1 ml/min. The limits of determination were 35 pg/ml and 7 pg/ml for zu- and enclomiphene, respectively. Within-day coefficients of variation ranged from 2.1% to 7.2%.