Simple and sensitive assay of torasemide in human plasma by high-performance liquid chromatography using a monolithic silica column

A rapid and sensitive method to assay torasemide in plasma was developed using a simple liquid-liquid extraction technique followed by high-performance liquid chromatography. Torasemide and the internal standard furosemide were extracted from 0.5 mL of plasma using ethyl acetate in the presence of 0.1M HCl. The analysis of the extracts was performed on a monolithic silica column with ultraviolet spectrophotometric detection. The calibration curve was linear over the concentration range of 0.05-5 μg mL^-1 in plasma. Recoveries were reasonable for routine analyses (>80%); the limit of quantification was 0.05 μg mL^-1 with a signal-to-noise ratio of 5. The coefficient of variation of the assay precision was less than 6.1%, and the accuracy exceeded 98%. This method was used to measure the torasemide concentration in plasma from healthy subjects after a single 20-mg oral dose of torasemide. This method provides a very simple, sensitive, and accurate way to determine torasemide concentrations in plasma.