Determination of furosemide in plasma and urine using monolithic silica rod liquid chromatography

In the present study we developed a fast and reliable HPLC assay for the determination of the loop diuretic furosemide in plasma and urine, using a Chromolith^® RP 18e (100 mm × 4.6 mm) monolithic silica rod HPLC column. After liquid-liquid extraction with diethylether, plasma or urine samples were separated with a gradient consisting of solvent A (20% acetonitrile) and solvent B (80% acetonitrile), both in 0.25% acetic acid. The flow rate was 3.5 ml/min and the effluent was monitored by fluorescence with excitation at 230 nm and emission at 410 nm. The retention times for the internal standard (naproxen) and for furosemide were 2.1 and 3.7 min, respectively, and total run time was 8 min. The calibration curves were linear between 7.8 and 1000 ng/ml, and within-assay and between-assay coefficients of variation were <6.5% and <10%, respectively. The proposed assay for furosemide in plasma and urine using monolithic silica rod chromatography is fast, sensitive, and reliable, and, thus, well suited for pharmacokinetic studies.