Improved enantioselective assay for the determination of fluoxetine and norfluoxetine enantiomers in human plasma by liquid chromatography

A simple and innovative assay is described which allows the chiral separation of the four enantiomers of fluoxetine and norfluoxetine, with performance characteristics adequate for therapeutic drug monitoring. The assay requires liquid–liquid extraction into acetonitrile/n-hexane/isopropylic alcohol and re-extraction into phosphoric acid for clean-up. The acidic layer is injected onto the HPLC system after filtering. Separation of the analytes is achieved with a Chiralcel ODR column and a mobile phase consisting of potassium hexafluorophosphate/acetonitrile. Detection is made by ultraviolet absorbance at 227 nm. Standard curves are linear for each enantiomer (r2≥0.992) over the range of 10–1000 ng/ml with a limit of quantification of 10 ng/ml for each enantiomer. Within-day and between-day CV% are ≤10% for each enantiomer.