Quantitative analysis of the loop diuretic torasemide in tablets and human urine by HPLC-EC

A High Performance Liquid Chromatographic method with amperometric detection for the determination of the loop diuretic 1-Isopropyl-3-[4-(3-methylphenylamino)-3-pyridinesulphonyl] urea, torasemide, is reported using a mu-Bondapak C-18 column and a mobile phase water:acetonitrile (65:35), 5 mM in potassium dihydrogenphosphate/dipotassium hydrogenphosphate at pH 5.3. Eluant is monitored at +1300 mV with an amperometric detector equipped with a glassy carbon working electrode. The method showed a determination limit of 8 ppb and a reproducibility in terms of relative standard deviation lower than 2% in intra-day assays and 5% in inter-day assays. The HPLC-EC method was applied to urine samples obtained from a healthy volunteer. Concentration levels of torasemide at different time intervals were monitorized and results were in agreement with the pharmacokinetic parameters of this diuretic. The determination of torasemide in urine required a liquid-liquid extraction prior to chromatographic analysis due to the interferences found in urine matrix. With this simple clean-up procedure, recoveries around 70% were achieved. Torasemide was directly determined in tablets after dissolution of the powder in a methanolic solution.